From the Pediatric Graduate School
Children’s Hospital, Helsinki University Central Hospital and University of Helsinki
and
National Institute for Health and Welfare
Helsinki, Finland
Nina Kaseva
Physical activity, nutrition
and stress response in young
adults born preterm –
determinants of health and
disease
The Helsinki Study of Very Low Birth
Weight Adults
ACADEMIC DISSERTATION
To be publicly discussed, with permission of the Faculty of Medicine,
University of Helsinki, in the Niilo Hallman Auditorium,
Children’s Hospital, on December 19th, 2014, at 12 noon
Helsinki 2014
Cover photo: Sunrise at Kytön selkä 21.9.2013 by Rasmus Kaseva
Layout: Nina Kaseva
ISBN (978-951-51-0351-2, printed)
ISBN (978-951-51-0352-9, online publication)
http://ethesis.helsinki.fi
Unigrafia Oy, Helsinki 2014.
Supervisors
Docent Eero Kajantie, M.D., Ph.D.
Department of Chronic Disease and
Diabetes Prevention, National Institute
for Health and Welfare
and
Children’s Hospital, University of
Helsinki and Helsinki University
Central Hospital
Reviewers
Docent Laure Morin-Papunen, M.D.,
Ph.D.
Institute of Clinical Medicine
Department
of
Obstetrics
and
Gynaecology
University and University Hospital of
Oulu
Oulu
Finland
Opponent
Professor Mikko Hallman, M.D., Ph.D.
Institute of Clinical Medicine
Department of Paediatrics
University and University Hospital of Oulu
Oulu
Finland
Docent Karoliina Wehkalampi, M.D.,
Ph.D.
Department of Chronic Disease and
Diabetes Prevention, National Institute
for Health and Welfare
and
Children’s Hospital, University of
Helsinki and Helsinki University
Central Hospital
Docent Marja Ojaniemi, M.D., Ph.D.
Institute of Clinical Medicine
Department of Paediatrics
University and University Hospital of
Oulu
Oulu
Finland
To my beloved husband Alek
Abstract
Background: Advancements in neonatal care during the past few decades are
resulting in increasing numbers of adult survivors after preterm birth at very low
birth weight (VLBW, ≤ 1500 g). VLBW is associated with risk factors of noncommunicable diseases, including cardiovascular disease, osteoporosis and diabetes.
The underlying mechanisms are unknown.
Aims: The mechanisms underlying the effects of preterm birth at VLBW on later
health in healthy VLBW young adults were investigated, with a focus on 1) physical
activity, 2) nutrition and 3) stress response.
Subjects and Methods: The participants were derived from a follow-up cohort
study, the Helsinki Study of Very Low Birth Weight Adults. They were all born
between 1978 and 1985 in the region of Uusimaa and treated and discharged alive
from the only tertiary neonatal intensive care unit in the region. The controls, born at
term, were group-matched for birth hospital, age and sex. From the original birth
cohort, different subgroups have, as young adults, participated in the studies of this
thesis. We evaluated physical activity by both self-report and objective
measurement. The participants (94 VLBW and 101 controls) completed a validated
30-item, 12-month physical activity questionnaire, the modified Kuopio Ischaemic
Heart Disease Risk Factor Study questionnaire, and the NEO-Personality Inventory.
For objective measurement, a subsample of 57 VLBW and 47 control participants
underwent wrist-worn accelerometer measurement. To assess dietary intake, 151
VLBW and 156 control participants completed a 3-day food record. For evaluation
of stress response, 54 VLBW and 40 control participants underwent a standardized
psychosocial stress test, the Trier Social Stress Test (TSST). In conjunction with
TSST, we measured heart rate, salivary cortisol, plasma ACTH, cortisol, glucose,
insulin, adrenalin and noradrenalin.
Results: 1) Based on self-report, healthy VLBW young adults undertake
approximately 50% less conditioning leisure-time physical activity, with lower
yearly frequency, total time, total volume and energy expenditure than controls born
at term. Differences in personality shown to exist between VLBW and controls
groups do not explain the association between VLBW and lower conditioning
leisure-time physical activity. We were unable to confirm our finding of lower
physical activity with wrist-worn accelerometer measurement. 2) Regarding dietary
habits, VLBW young adults had a lower consumption of vegetables, fruits, berries
and milk products. This was combined with a lower intake of calcium and vitamin
D. 3) Regarding stress response, VLBW adults showed a lower hypothalamic-
7
Physical activity, nutrition
and stress response in young
adults born preterm −
determinants of health and
disease
pituitary-adrenal axis response to stress than controls born at term. This was
accompanied by a lower insulin response. No evidence of a higher sympatheticadrenal-medullary system stress response was found. Furthermore, we observed a
lower noradrenalin response to stress in VLBW women.
Conclusions: This study showed that VLBW young adults undertake less
conditioning leisure-time physical activities and have unhealthier diets, both factors
that negatively affect future health in this high-risk population. They may in part
underlie the increased risk for chronic non-communicable diseases in VLBW
individuals.
Contrary to our expectations, a lower hypothalamic-pituitary-adrenal axis
response to stress was found in VLBW adults than in controls born at term. For the
sympatho-adrenal-medullary stress response, the results were similar in VLBW and
control groups, with a lower noradrenalin response to stress in VLBW women only.
These findings reinforce the supposition that stress response is programmed early in
life.
In sum, this study increased understanding of possible mechanisms linking
preterm birth and adult risk of disease.
Keywords: very low birth weight, preterm, infant, neonate, newborn, adult, physical
activity, accelerometer, actigraphy, diet, nutrition, food record, programming,
hypothalamic-pituitary-adrenal axis, cortisol, Trier Social Stress Test, sympatheticadrenal-medullary system, cortisol, adrenalin, noradrenalin, heart rate
8
Physical activity, nutrition
and stress response in young
adults born preterm −
determinants of health and
disease
Tiivistelmä
Taustaa: Viime vuosikymmenten kuluessa tapahtunut merkittävä kehitys
vastasyntyneiden tehohoidossa on johtanut yhä epäkypsempinä syntyneiden
keskosten eloonjäämiseen ja he saavuttavat myös aikuisiän. Pikkukeskosilla
(syntymäpaino ≤ 1500 g) on todettu lisääntyneitä riskitekijöitä aikuisiän
pitkäaikaissairauksiin, mm. osteoporoosiin, diabetekseen ja sydän- ja
verisuonisairauksiin. Syy-yhteys keskosuuden ja lisääntyneiden sairausriskien välillä
on tuntematon.
Tavoitteet: Tämän väitöskirjan päätavoitteena oli selvittää taustamekanismeja
pikkukeskosena syntymisen vaikutuksista aikuisiän terveyteen, keskittyen 1)
liikuntaan, 2) ravitsemukseen ja 3) stressivasteeseen.
Aineisto ja menetelmät: Tutkimusaineistoon kuuluu vuosina 1978–1985
pikkukeskosena syntyneiden seurantakohortti (the Helsinki Study of Very Low Birth
Weight Adults). Kaikki tutkittavat pikkukeskoset syntyivät Uudellamaalla ja heitä
hoidettiin samalla vastasyntyneiden teho-osastolla. Verrokeiksi on valittu samoissa
synnytyssairaaloissa täysiaikaisena syntyneitä, saman ikäisiä ja samaa sukupuolta
olevia henkilöitä. Alkuperäisestä kohortista valitut alaryhmät ovat nuorina aikuisina
osallistuneet tämän väitöskirjatyön eri osatutkimuksiin. Liikuntatottumuksia on
arvioitu sekä kyselylomakkeella että objektiivisesti. Tutkittavat, 94 pikkukeskosta ja
101 verrokkia, täyttivät laajan liikuntakyselylomakkeen, modifioidun Kuopio
Ischemic Heart Disease Risk Factor Study-lomakkeen, sekä persoonallisuuspiirteitä
mittaavan NEO-Personality Inventory-lomakkeen. Objektiivisena liikunnan
arvioinnin mittarina käytettiin rannemallista kiihtyvyysanturia (57 pikkukeskosta ja
47 verrokkia). Ruokavaliota ja ravitsemusta tutkittiin 3 päivän ruokapäiväkirjan
avulla (151 pikkukeskosta ja 156 verrokkia). Stressi-vasteita mitattiin
standardoidulla stressikokeella (Trier Social Stress Test, TSST). TSST:n yhteydessä
54 pikkukeskoselta ja 40 verrokilta seurattiin sykevastetta sekä kerättiin veri- ja
sylkinäytteitä kortisoli-, ACTH-, glukoosi-, insuliini-, adrenaliini- ja noradrenaliinivasteiden mittausta varten.
Tulokset: 1) Kyselylomakkeen perusteella pikkukeskosena syntyneet terveet
aikuiset harrastivat noin 50 % vähemmän kuntoliikuntaa; heillä oli vuositasolla
vähemmän liikuntakertoja, liikuntaan käytetty aika oli vähäisempää, liikunnan
kokonaisvolyymi oli pienempi ja liikunnan aiheuttama vuosittainen energiankulutus
oli merkittävästi verrokkiryhmää pienempi. Aiemmin havaitut erilaiset
persoonallisuuspiirteet keskosilla ja verrokeilla eivät selittäneet todettuja eroja
kuntoliikunnassa. Kyselylomakkeen perusteella todettuja eroja liikuntatottumuksissa
9
Physical activity, nutrition
and stress response in young
adults born preterm −
determinants of health and
disease
ei havaittu osalle tutkittavista tehdyllä rannemallisella kiihtyvyysanturilla. 2)
Ruokapäiväkirjan perusteella pikkukeskosena syntyneet nuoret aikuiset nauttivat
päivittäin merkittävästi vähemmän maitotuotteita, vihanneksia, hedelmiä ja marjoja.
Myös päivittäinen kalsiumin ja D-vitamiinin saanti oli pikkukeskosena syntyneillä
vähäisempää. 3) Pikkukeskosena syntyneiden aikuisten hypotalamus-aivolisäkelisämunuaisakselin stressireaktiot olivat vaimeammat täysaikaisena syntyneisiin
verrokkeihin nähden. Tähän liittyen myös insuliinivasteet stressille olivat
vaimeammat. Viitteitä korkeammasta sympaattisen hermoston vasteista stressille ei
todettu. Lisäksi pikkukeskosryhmän naisilla oli vaimeampi noradrenaliinivaste
stressille verrokkinaisiin nähden.
Päätelmät: Pikkukeskosena syntyneet nuoret aikuiset harrastavat vähemmän
kuntoliikuntaa ja heillä on joiltakin osin epäterveellisemmät ruokailutottumukset
verrokkeihin nähden. Nämä ovat molemmat tekijöitä jotka heikentävät terveyttä ja
ovat näin ollen erityisen merkittäviä tässä suuren riskin ryhmässä. Lisäksi
molemmat voivat osaltaan selittää lisääntynyttä pitkäaikaissairauksien riskiä
pikkukeskosilla.
Vasten odotuksiamme havaitsimme vaimeamman hypotalamus-aivolisäkelisämunuaisakselin vasteen stressille pikkukeskosryhmässä. Sympaattisen hermoston
stressivasteissa ei todettu merkittäviä eroja ryhmien välillä, poislukien
pikkukeskosnaisilla todettu matalampi noradrenaliinivaste stressille. Nämä tulokset
vahvistavat aiempia viitteitä stressivasteen ohjelmoitumisesta elämän alkuvaiheissa.
Yhteenvetona voidaan todeta että tämä tutkimus laajensi ymmärrystä
pikkukeskosena syntymisen ja aikuisiän sairausriskien välisistä mahdollisista
mekanismeista.
Avainsanat: pikkukeskonen, ennenaikainen, vastasyntynyt, aikuinen, liikunta,
kiihtyvyysanturi, ruokavalio, ravitsemus, ruokapäiväkirja, ohjelmoituminen,
hypotalamus-aivolisäke-lisämunuaisakseli, stressi, Trier Social Stress Test-koe,
sympaattinen hermosto, kortisoli, adrenaliini, noradrenaliini, syke
10
Physical activity, nutrition
and stress response in young
adults born preterm −
determinants of health and
disease
Sammandrag
Bakgrund: Stora framsteg har gjorts inom neonatalvården under de senaste
årtionden. Detta har lett till ett ökande antal överlevande unga vuxna bland dem som
fötts för tidigt med mycket liten födelsevikt (≤ 1500 g). Av okänd orsak har för
tidigt födda unga vuxna ökade riskfaktorer för kroniska sjukdomar, bl.a. hjärt- och
kärlsjukdomar, osteoporos och diabetes.
Mål: Syftet med detta avhandlingsprojekt var att utreda mekanismer bakom
sambandet mellan prematuritet och hälsa i vuxenåldern, med fokus på 1) fysisk
aktivitet, 2) näring och 3) stressrespons.
Deltagare och metoder: Forskningsmaterialet består av en kohort unga vuxna
födda som små prematurer (födelsevikt ≤ 1500 g) i Nyland under åren 1978-1985
(the Helsinki Study of Very Low Birth Weight Adults). Alla prematurer gavs
neonatalvård på samma sjukhus. Som kontrollgrupp valdes personer av samma kön
och ålder, födda på samma förlossningssjukhus efter fullgången graviditet. Från
kohorten har undergrupper som unga vuxna deltagit i denna doktorsavhandlings
olika delstudier. Vi mätte fysisk aktivitet med hjälp av frågeformulär samt objektivt
med accelerometer. Deltagarna (94 för tidigt födda och 101 kontrollpersoner) fyllde
i Kuopio Ischaemic Heart Disease-projektets standardiserade, detaljerade
frågeformulär samt NEO-Personality Inventory-formuläret. Accelerometerregistrering (armbandsmodell) utfördes åt 57 för tidigt födda och 47
kontrollpersoner. Kostvanor undersöktes genom att deltagarna (151 prematurer och
156 kontrollpersoner) antecknade allt de åt och drack under 3 dygn. För att mäta
stressrespons deltog 54 för tidigt födda och 40 kontrollpersoner i ett standardiserat
stress test (Trier Social Stress Test, TSST). I samband med TSST registrerades
pulsen och upprepade blod- och salivprov togs för ACTH-, kortisol-, glukos-,
insulin-, adrenalin och noradrenalin-bestämningar.
Resultat: 1) Baserat på frågeformuläret konditionstränade prematurgruppen
ungefär 50 % mindre: den årliga frekvensen, totaltiden, totalvolymen och
energikonsumtionen för konditionsträning var klart lägre. Skillnader i personlighet,
som tidigare konstaterats mellan för tidigt födda och individer födda efter fullgången
graviditet, förklarar inte denna stora skillnad i fysisk aktivitet grupper emellan. Vi
kunde inte påvisa dylika skillnader i fysisk aktivitet grupper emellan med
accelerometer-registrering. 2) Unga vuxna som fötts för tidigt konsumerade klart
mindre bär, frukt, grönsaker och mjölkprodukter. Deras dagliga kalcium och Dvitamin intag var märkbart mindre än kontrollgruppens. 3) De som fötts för tidigt
visade en lägre hypotalamus-hypofys-binjure-axel respons efter psykosocial stress
11
Physical activity, nutrition
and stress response in young
adults born preterm −
determinants of health and
disease
jämfört med vuxna födda efter fullgången graviditet. Detta åtföljdes av en lägre
insulinrespons. Vi fann ingen antydan på högre stressrespons förmedlad av det
sympatiska nervsystemet hos för tidigt födda. Hos för tidigt födda kvinnor var
noradrenalin-responsen efter stress lägre.
Konklusion För tidigt födda unga vuxna konditionstränar klart mindre och de
har ohälsosammare kostvanor jämfört med individer födda efter fullgången
graviditet. Dessa faktorer inverkar väsentligt på hälsa och välmående speciellt i
denna högrisk-grupp. Dessa resultat kan delvis förklara den förhöjda risken för
insjuknande i kroniska sjukdomar hos för tidigt födda individer.
Något oväntat fann vi en lägre hypotalamus-hypofys-binjure-axel respons efter
stress i prematurgruppen. Det sympatiska nervsystemets stressresponser var
likartade i båda grupperna, förutom den lägre noradrenalin-responsen hos för tidigt
födda kvinnor. Resultaten förstärker tidigare antydningar om att stressresponsen
programmeras i livets tidiga skeden.
Sammanfattningsvis har denna studie utökat kunskap kring eventuella
mekanismer bakom sambandet mellan för tidig födsel och sjukdomsrisker i vuxen
ålder.
Nyckelord: prematur, för tidigt född, nyfödd, vuxen, fysisk aktivitet, accelerometer,
diet, kostvanor, näring, programmering, hypotalamus-hypofys-binjure-axel, Trier
Social Stress Test, sympatiska nervsystemet, kortisol, adrenalin, noradrenalin, puls
12
Physical activity, nutrition
and stress response in young
adults born preterm −
determinants of health and
disease
Contents
Abstract ...................................................................................................................... 7
Tiivistelmä.................................................................................................................. 9
Sammandrag ............................................................................................................. 11
List of original publications ..................................................................................... 15
Abbreviations ........................................................................................................... 16
1 Introduction ........................................................................................................... 17
2 Review of the literature ......................................................................................... 18
2.1 Preterm birth ............................................................................................ 18
2.1.1 Definitions and epidemiology ......................................................... 18
2.1.2 Aetiology and risk factors of preterm birth ..................................... 19
2.1.3 Neonatal and childhood outcomes of preterm birth ........................ 21
2.1.4 Adult outcomes of preterm birth ..................................................... 23
2.2 Physical activity....................................................................................... 25
2.2.1 General recommendations ............................................................... 25
2.2.2 Health implications ......................................................................... 26
2.2.3 Preterm birth, very low birth weight (VLBW) and later physical
activity ........................................................................................................... 26
2.2.4 Measuring physical activity ............................................................ 28
2.3 Diet and nutrition..................................................................................... 29
2.3.1 General recommendations ............................................................... 29
2.3.2 Preterm birth, VLBW and later nutrition ........................................ 30
2.3.3 Measuring dietary habits and nutrient intake .................................. 31
2.4 Other lifestyle factors and VLBW ........................................................... 32
2.5 Psychosocial stress .................................................................................. 33
2.5.1 Stress response ................................................................................ 33
2.5.2 Preterm birth and stress response .................................................... 35
2.5.3 Measuring stress response ............................................................... 37
3 Aims of the study .................................................................................................. 40
4 Subjects and methods ............................................................................................ 41
4.1 Study population...................................................................................... 41
4.2 Measures.................................................................................................. 41
4.2.1 Background characteristics ............................................................. 41
4.2.2 Assessment of physical activity based on self-report (Study I) ...... 43
4.2.2.1
Adjustment for lean body mass................................44
4.2.2.2
Adjustment for personality…………..……….........44
4.2.3 Assessment of physical activity based on accelerometer
measurements (Study II) ................................................................................ 44
4.2.4 Assessment of nutrition and diet (Study III) ................................... 45
13
Physical activity, nutrition
and stress response in young
adults born preterm −
determinants of health and
disease
4.2.5 Psychosocial stress responses (Studies IV and V) .......................... 46
4.3 Statistical analyses ................................................................................... 48
4.4 Ethics ....................................................................................................... 49
5 Results ................................................................................................................... 50
5.1 Description of study participants ............................................................. 50
5.2 Conditioning leisure-time physical activity is lower in VLBW adults
(Study I) ......................................................................................................... 54
5.3 Lack of difference in physical activity by objective measurement in
VLBW adults and controls (Study II)............................................................ 56
5.4 Unhealthier dietary intake in VLBW adults (Study III) .......................... 56
5.5 Blunted hypothalamic-pituitary-adrenal axis and insulin response to stress
in VLBW adults (Study IV)........................................................................... 59
5.6 Similar or lower sympathetic-adrenal-medullary system response to stress
in VLBW adults (Study V) ............................................................................ 62
5.7 Non-participant analysis .......................................................................... 63
6 Discussion ............................................................................................................. 65
6.1 Physical activity....................................................................................... 65
6.2 Diet and nutrition..................................................................................... 67
6.3 Stress response......................................................................................... 70
6.4 Strengths and limitations ......................................................................... 72
6.5 Implications for future research ............................................................... 75
7 Conclusions ........................................................................................................... 76
8 Acknowledgements ............................................................................................... 77
9 References ............................................................................................................. 80
14
Physical activity, nutrition
and stress response in young
adults born preterm −
determinants of health and
disease
List of original publications
This thesis is based on five original publications. In the text, they are referred to by
their Roman numerals (I-V).
I
Nina Kaseva, Karoliina Wehkalampi, Sonja Strang-Karlsson, Minna Salonen,
Anu-Katriina Pesonen, Katri Räikkönen, Tuija Tammelin, Petteri Hovi, Jari
Lahti, Kati Heinonen, Anna-Liisa Järvenpää, Sture Andersson, Johan G.
Eriksson, Eero Kajantie. Lower conditioning leisure-time physical activity
in young adults born preterm at very low birth weight. PLoS One. 2012;
7(2):e32430.
II
Nina Kaseva, Silja Martikainen, Tuija Tammelin, Petteri Hovi, Anna-Liisa
Järvenpää, Sture Andersson, Johan G. Eriksson, Katri Räikkönen, AnuKatriina Pesonen, Karoliina Wehkalampi, Eero Kajantie. Objectively
measured physical activity in young adults born preterm at very low
birth weight. Accepted for publication in J Pediatr.
III
Nina Kaseva, Karoliina Wehkalampi, Katri Hemiö, Petteri Hovi, Anna-Liisa
Järvenpää, Sture Andersson, Johan G. Eriksson, Jaana Lindström, Eero
Kajantie. Diet and nutrient intake in young adults born preterm at very
low birth weight. J Pediatr. 2013 Jul; 163(1):43-8
IV
Nina Kaseva, Karoliina Wehkalampi, Riikka Pyhälä, Elena Moltchanova,
Kimmo Feldt, Anu-Katriina Pesonen, Kati Heinonen, Petteri Hovi, AnnaLiisa Järvenpää, Sture Andersson, Johan G. Eriksson, Katri Räikkönen, Eero
Kajantie. Blunted hypothalamic-pituitary-adrenal axis and insulin
response to psychosocial stress in young adults born preterm at very low
birth weight. Clin Endocrinol (Oxf). 2014 Jan; 80(1):101-6.
V
Nina Kaseva, Riikka Pyhälä, Karoliina Wehkalampi, Kimmo Feldt, AnuKatriina Pesonen, Kati Heinonen, Petteri Hovi, Anna-Liisa Järvenpää, Johan
G. Eriksson, Sture Andersson, Katri Räikkönen, Eero Kajantie. Adrenalin,
noradrenalin and heart rate responses to psychosocial stress in young
adults born preterm at very low birth weight. Clin Endocrinol (Oxf). 2014
Aug; 81(2):231-7.
These original publications are reprinted with permission from their copyright
holders. In addition, some unpublished material is included.
15
Physical activity, nutrition
and stress response in young
adults born preterm −
determinants of health and
disease
Abbreviations
A
ACTH
AGA
AUCg
BMI
BPD
CI
CP
cpm
CVD
DOHaD
E%
EE
ELBW
HeSVA
HPAA
HR
IQ
KIHD
LTPA
MET
NA
NCD
NEO-PI
NICU
SAM
SD
SES
SGA
TSST
TSST-C
VLBW
WHO
Adrenalin
Adrenocorticotropic hormone
Appropriate for gestational age
Area under the curve ground
Body mass index
Bronchopulmonary dysplasia
Confidence interval
Cerebral palsy
Counts per minute
Cardiovascular disease
Developmental origins of health and disease
Percentage of total energy intake
Energy expenditure
Extremely low birth weight
Helsinki Study of Very Low Birth Weight Adults
Hypothalamic-pituitary-adrenal axis
Heart rate
intelligence quotient
Kuopio Ischaemic Heart Disease Risk Factor Study questionnaire
Leisure-time physical activity
Metabolic equivalents
Noradrenalin
Non-communicable disease
NEO-Personality Inventory
Neonatal intensive care unit
Sympathetic-adrenal-medullary
Standard deviation
Socio-economic status
Small for gestational age
Trier Social Stress Test
TSST for Children
Very low birth weight
World Health Organization
16
Physical activity, nutrition
and stress response in young
adults born preterm −
determinants of health and
disease
Introduction
1 Introduction
Preterm birth is relatively common. In 2011-2012, 5.6% of all children born in
Finland were born preterm, by definition with a gestational age (GA) < 37 weeks
(Vuori and Gissler, 2013). Of these preterm children, 1060 children or 0.8% of all
newborns had a birth weight < 1500 g or a GA < 32 weeks (Vuori and Gissler,
2013). Of these children, 90.8% reached their first birthday (Vuori and Gissler,
2013). Thus, the number of survivors after preterm birth is considerable.
Since the late 1970s and early 1980s, when the participants of the studies
included in this thesis were born, great advancements have been made in neonatal
intensive care. However, preterm birth is similarly associated with pre- and postnatal
adversities today, as it was approximately 30 years ago. Compared with the natural
environment of the fetus, the mother’s womb, the preterm infant must continue to
grow and develop in a markedly different environment, the neonatal intensive care
unit (NICU) setting. Regardless of the many improvements in neonatal care, some
similarities remain between the preterm-born of today and those born some decades
ago. These include repeated stressful situations: painful treatments, administration of
medicines and increased risk of infections, among others. Such circumstances may
be assumed to similarly affect newborn infants undergoing intensive care, regardless
of changes or advancements in care.
An increasing body of evidence shows that pre-and postnatal events affect health
and disease later in life. As adults, people born at very low birth weight (VLBW, ≤
1500 g) have, for instance, impaired glucose regulation (Hofman et al., 2004; Hovi
et al., 2007), higher blood pressure (Doyle et al., 2003; Hack et al., 2005; de Jong et
al., 2012; Parkinson et al., 2013; Kajantie and Hovi, 2014), lower bone mineral
density (Hovi et al., 2009; Smith et al., 2011) and changes in lipid profile (Hovi et
al., 2013; Parkinson et al., 2013) compared with controls born at term. Previous data
also suggest that hypothalamic-pituitary-adrenal axis (HPAA) functioning and
endocrine stress responses (Wüst et al., 2005; Jones et al., 2006; Buske-Kirschbaum
et al., 2007; Grunau et al., 2007; Kajantie et al., 2007; Grunau et al., 2010; Kajantie
and Räikkönen, 2010; Brummelte et al., 2011) as well as feeding preferences
(Portella et al., 2012) may be affected by early life events. Physical activity and
fitness in adolescence and adulthood also seems to be affected by preterm birth
(Kajantie and Hovi, 2014). Some of the results of early life events may be reversed
or attenuated by a healthy lifestyle, including adequate physical activity and good
dietary habits. The focus of this thesis was to evaluate physical activity, nutrition
and dietary habits, and stress response in a cohort of healthy VLBW young adults.
17
Physical activity, nutrition
and stress response in young
adults born preterm −
determinants of health and
disease
Review of the literature
2 Review of the literature
2.1 Preterm birth
2.1.1 Definitions and epidemiology
Term birth refers to birth occurring between 37-42 weeks of gestation. The World
Health Organization (WHO) defines preterm birth as birth before 37 completed
weeks of gestation or 259 days since the first day of a woman’s last menstrual
period (Howson et al., 2012), while post-term birth refers to > 42 completed weeks
of gestation. Preterm birth can further be divided into extremely preterm (< 28
weeks), very preterm (28 - < 32 weeks) and moderate preterm (32 - < 37 weeks).
Late preterm refers to birth at 34 - < 37 weeks of gestation. In most high-income
countries there is a 50% chance for survival after birth at 24 weeks of gestation with
neonatal intensive care, while in low-income settings 50% of the children born at
even 32 weeks of gestation will not survive due to lack of adequate neonatal care
(Howson et al., 2012).
Based on birth weight, the newborn can be classified into low birth weight (LBW
< 2500 g), very low birth weight (VLBW < 1500 g) or extremely low birth weight
(ELBW, <1000 g). For comparison, the mean birth weight of all children born in
Finland in the year 2012 was 3498 g (Vuori and Gissler, 2013). Fetal growth
according to expectation, based on population mean and gestational age, i.e.
appropriate for gestational age (AGA), refers to birth weight within ± 2 standard
deviations (SDs). Small for gestational age (SGA) refers to birth weight < -2 SDs of
the population mean (Pihkala et al., 1989) and large for gestational age (LGA) to
birth weight > 2 SDs. Alternatively, 10th and 90th percentiles may be used as cut-off
points for SGA, AGA and LGA (Reeves and Bernstein, 2008).
Approximately 15 million preterm births occur yearly worldwide (Howson et al.,
2012), more than 10% of all births. Of these 15 million infants, 1.1 million die of
complications related to preterm birth (Howson et al., 2012). Preterm birth rates
vary across the world, ranging from 5% to 18%. The highest preterm birth rates
occur in the poorest countries (> 12%), with clearly lower rates (9%) in high-income
countries (Howson et al., 2012). In this respect, the United States is an exception,
with an 11.5% preterm birth rate in 2012. In Finland 5.6% of all children were born
preterm in 2012, in total 3387 children (Vuori and Gissler, 2013). Based on WHO
data, the global incidence of preterm birth is rising (Howson et al., 2012). Of the 65
countries with reliable data for the past 20 years, 14 countries have had stable rates
of preterm birth, only 3 countries (Croatia, Ecuador and Estonia) have had a
decrease in the rate of preterm birth and the majority of countries have shown
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increased rates. Factors such as assisted reproduction and increasing maternal age
account for only a minor portion in the rising numbers of preterm birth. These data
are alarming. In addition to neonatal deaths, short- and long-term effects on the
health and well-being of survivors and possible lifetime disability, the costs of
prematurity are overwhelming.
2.1.2 Aetiology and risk factors of preterm birth
A multitude of causes underlie preterm birth, many of them unknown. Commonly,
preterm birth may be grouped into a) spontaneous preterm birth and b) providerinitiated preterm birth. Spontaneous onset of labour and prelabour premature rupture
of membranes are included in the former, induction of labour or elective Caesarian
section before 37 weeks of gestation in the latter. Spontaneous onset of labour
occurs in approximately 45-50% of all preterm births, in 30% premature rupture of
membranes occurs and 15-20% are provider-initiated (Menon, 2008).
Reasons or mechanisms behind preterm birth are mostly unknown. However,
several risk factors, listed in Table 1, have been identified. Infection is associated
with 30-40% of all preterm births (Goldenberg et al., 2008) and often more than one
risk factor is present (Menon, 2008). This suggests aetiologic heterogeneity in
preterm birth. Ethnic and racial disparities that cannot fully be explained by
differences in socio-economic status (SES) or maternal risk-taking behaviour do
exist (Schaaf et al., 2013). There are marked differences between populations, for
example, in the United States the preterm birth rates in Caucasians are clearly lower
than those in African-Americans, 11.5 % versus 17.8 % (Menon, 2008). Severe
allergic reactions can produce uterine cramping, and such allergic reactions with
marked cytokine and antibody production have also been associated with preterm
birth (Garfield et al., 2006).
Activation of four main pathways that lead to preterm birth have been identified:
1. stress activates maternal or fetal HPAA
2. infection provokes inflammation with subsequent cytokine and prostaglandin
production
3. decidual haemorrhage
4. distension of the myometrium
How these pathways are activated is unclear. They overlap and interact and
involve complex biochemical pathways with, for example, prostaglandins,
metalloproteinases and cytokines. Pathological uterine distension caused by decidual
haemorrhage and infection, which causes overwhelming inflammation, both trigger
uterotonic activity. A contracting myometrium promotes cervical effacement and
dilatation, leading to rupture of fetal membranes with onset of preterm labour and
delivery (Menon, 2008).
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Table 1.
Known risk factors for preterm birth.
Risk factors for preterm birth
Cervical anomalies
Infection
Intrauterine infection
Fetal abnormalities
Extrauterine infection
Socio-economic status
Low socio-economic status
Pregnancy related maternal conditions
and maternal history
Previous preterm delivery
Social isolation
Repeated second trimester
Education
abortion
In-vitro fertilization
Income
Multiple pregnancy
Marital status
Parity
Inadequate prenatal care
Maternal medical complications
Strenuous physical workload
Malnutrition
Maternal behaviour
Gestational bleeding
Maternal weight gain
Abnormal placentation
Smoking
Low body mass index before
Alcohol abuse
conception
Age < 17 years or > 35 years
Drug abuse
Infertility
Sexual activities
Heredity
Stress
Ethnic and racial disparities
Stressful life events
Psychosocial stress
Abnormalities
Uterine anomalies
Severe allergic reactions
Garfield et al. 2006; Goldenberg et al. 2008; Menon 2008; Schaaf et al. 2013
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2.1.3 Neonatal and childhood outcomes of preterm birth
Prematurity is the leading cause of death in the newborn and the second leading
cause of death in children aged under 5 years (Howson et al., 2012). With
advancements in antenatal care, including prenatal corticosteroids, as well as
postnatal care with, for example, surfactant administration and improved ventilation
support systems, the numbers of extremely preterm infants surviving in high-income
countries are growing. In countries with modern care available, most extremely and
very preterm infants remain in hospital care close to term. As the focus of this thesis
is on adult outcomes of VLBW, the neonatal and childhood outcomes will only be
reflected on briefly, serving as an introduction to the adult outcomes of VLBW. The
sequelae of preterm birth vary, and the morbidity is inversely related to gestational
age (Moster et al., 2008; Saigal and Doyle, 2008).
After initial survival of preterm birth, respiratory distress syndrome, a patent
ductus arteriosus, infection or pulmonary barotrauma are common problems in the
early neonatal period. Tommiska et al. compared ELBW infants born in Finland
during 1996-1997 (n = 529) and 1999-2000 (n = 511) and reported an increase in the
incidence of respiratory distress syndrome and septicaemia (75% vs. 83% and 23%
vs. 31%) (Tommiska et al., 2007). However, no changes were seen in mortality rates
between these time periods, and the survival rates with stillborn infants included
were 40% and 44%, respectively (Tommiska et al., 2007).
Preterm-born babies are at risk for brain lesions (intraventricular haemorrhage,
periventricular haemorrhagic infarction and periventricular leucomalacia) associated
with adverse neurological outcomes (Ward and Beachy, 2003). In ELBW infants
born in Finland, a rise in the overall incidence of intraventricular haemorrhage was
seen between 1996-1997 and 1999-2000 (29% vs. 37%), while the rates of the most
severe grades (3-4) remained similar (16% vs. 17%) (Tommiska et al., 2007).
Hydrocephalus may develop after intraventricular haemorrhage, and to ensure
adequate cerebral fluid reabsorption a shunt may be required. In ELBW infants, 12% of those admitted to the NICU required a shunt before reaching the age of 40
weeks of gestation (Tommiska et al., 2007).
As for bowel complications, necrotizing enterocolitis is more common in infants
born preterm. In ELBW infants, the incidence of necrotizing enterocolitis with
bowel perforation was reported to be 4-8% (Tommiska et al., 2007).
Prolonged mechanical ventilation and oxygen supplementation may cause
damage to the lungs and eyes. Bronchopulmonary dysplasia (BPD) and retinopathy
of prematurity are complications that may develop somewhat later during NICU
care. In one study including 279 VLBW and 210 controls born during 1991-1992,
retinopathy of prematurity of any severity was reported in 51% of VLBW children,
and 13% of VLBW versus 4% of full-term children wore glasses at 7 years of age
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(Cooke et al., 2004). In that study, the VLBW children were more likely than the
controls to have strabismus, reduced stereopsis and poor visual acuity (Cooke et al.,
2004).
Serious health problems, such as cerebral palsy (CP), intellectual impairment,
cognitive problems, developmental delay, short bowel syndrome, growth reduction,
chronic lung disease, and impairment of motor development, vision and hearing may
follow the acute complications (Ward and Beachy, 2003). In a large cohort of people
born in Norway between 1967 and 1983, the prevalence of having CP was 9.1% for
those born between 23 and 27 weeks of gestation and 0.1% for those born full-term
(Moster et al., 2008), while in ELBW infants born in the late 1990s CP rates of 1114% have been reported (Tommiska et al., 2007).
At the time of discharge from hospital, suboptimal growth is common in VLBW
infants. This delay in growth often continues in early childhood. A recent study
comprising 181 VLBW children at age 5 years showed that in AGA infants, good
weight gain and growth of head circumference between birth and 2 years was
associated with better cognitive outcome at 5 years of age (Leppänen et al., 2014).
In SGA infants, the significant period for head circumference growth regarding
cognitive outcome was at term age (Leppänen et al., 2014).
As the above examples illustrate, there is a marked risk of serious health
problems in individuals born preterm. These preterm-born infants may face a
multitude of potential health problems with far-reaching effects, extending beyond
the neonatal period and throughout life. They thus require follow-up and specialized
care. Despite the increased survival rates due to improved neonatal care, there is not
a simultaneous increase in disability rates. On the contrary, morbidity rates in
VLBW survivors first decreased and then plateaued in the 1990s (Horbar et al.,
2002; Darlow et al., 2003; Fanaroff et al., 2007).
As for childhood, the level of immaturity at birth as well as the amount of
complications developed during NICU care will be reflected in growth and
development during infancy through school age and adolescence (Saigal and Doyle,
2008). Interestingly, VLBW has been shown to be associated with advanced
pubertal growth spurt compared with controls (Wehkalampi et al., 2011). This may
reflect advanced pubertal maturation in VLBW children. This is of importance since
advanced puberty and early maturation as such increase the risk of developing type
2 diabetes (Lakshman et al., 2008) and high blood pressure (Remsberg et al., 2005)
later in life. Within the limits of this thesis, other effects of preterm birth on
childhood and adolescence will be discussed in the following sections (entitled
physical activity, diet and nutrition, stress response).
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2.1.4 Adult outcomes of preterm birth
Implications of preterm birth extend throughout the lifespan of the individual. The
Developmental Origins of Health and Disease (DOHaD) theory suggests that
conditions at a specific sensitive period in early pre- or postnatal life may program
the functioning of cells, tissues and organs, altering their function throughout life
(Bateson et al., 2004; Gluckman et al., 2008). Preterm birth and low birth weight
mirror early life conditions that markedly differ from early life conditions in
individuals born at term with normal birth weight. An increasing body of evidence
suggests that adults who were born preterm have increased risk factors for later
cardiometabolic disease (Kajantie and Hovi, 2014). Regarding disease outcomes,
current data mostly describe increases in risk factors in young adult survivors of
preterm birth (in their twenties and thirties), and chronic non-communicable
diseases (NCDs), such as type 2 diabetes, stroke and osteoporosis, typically manifest
later in life.
Several neonatal centres have established follow-up cohorts of former pretermborn infants for investigating their later health. As for increased risk factors for
NCDs in preterm-born adults, reports on lower bone density (Hovi et al., 2009;
Smith et al., 2011), negative effects on glucose and insulin metabolism (Hofman et
al., 2004; Lawlor et al., 2006; Hovi et al., 2007; Rotteveel et al., 2008; Kaijser et al.,
2009; Kajantie et al., 2010; Pilgaard et al., 2010; Crump et al., 2011; Smith et al.,
2011), higher blood pressure (de Jong et al., 2012; Parkinson et al., 2013) and an
atherogenic lipid profile (Hovi et al., 2013; Parkinson et al., 2013) have been
published. In addition to lower bone density, adults born preterm are shorter than
their term-born counterparts (Roberts and Cheong, 2014). In a recent systematic
review and meta-analysis on markers of metabolic syndrome by Parkinson et al.
(2013), no differences were found in preterm and term-born adults regarding body
mass index (BMI), waist-hip ratio, percentage fat mass, flow-mediated intima-media
thickness, fasting glucose and insulin profiles. However, the authors reported higher
plasma low-density lipoproteins in young adults born preterm (Parkinson et al.,
2013). Also in our Helsinki Study of Very Low Birth Weight Adults (HeSVA)
cohort, a more atherogenic lipid profile, with higher concentrations of triglycerides
in very-low-density and high-density lipoprotein subclasses, was found in the
VLBW adults (Hovi et al., 2013). Furthermore, in the same review, preterm birth
was associated with higher blood pressure, especially in women (Parkinson et al.,
2013). Similar findings of higher systolic blood pressure in preterm-born or VLBW
individuals were reported in another systematic review and meta-analysis focusing
entirely on blood pressure (de Jong et al., 2012). Modestly higher blood pressure
levels present already in early adulthood increase the risk of developing
hypertension and its sequelae later in life. A possible additional risk factor for
hypertension is elevated arterial stiffness, which has been reported in 11-year-old
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children born before 26 weeks of gestation (McEniery et al., 2011). The abovedescribed differences between adults born preterm and controls born at term may
represent a greater risk for atherosclerosis and cardiovascular diseases (CVDs) in
later adulthood.
When preterm birth occurs, many developmental stages that normally occur in
utero during the last trimester have to take place during hospitalization, in a NICU
environment. For example, normally, nephrogenesis in humans occurs during 9-36
weeks of gestation and new nephrons are not formed after birth, with the exception
of extremely preterm-born infants. In one study, smaller kidney size and impaired
renal function were found in very preterm SGA participants compared with termborn controls (Keijzer-Veen et al., 2007). VLBW and prematurity may also
predispose to secondary focal segmental glomerulosclerosis (Hodgin et al., 2009).
Also cardiac development seems to be affected by preterm birth. Birth is associated
with a change in cardiomyocyte phenotype from the fetal hyperplastic form to the
neonatal hypertrophic form as the cardiomyocytes are exposed to the change from a
low-resistance fetal blood circulation to the high-resistance arterial system that
normally occurs after birth (Gessner et al., 1965). When this shift in blood
circulation occurs in the preterm infant, the cardiomyocytes are still immature and
this has been associated with significantly higher left ventricular mass in adulthood
(Lewandowski et al., 2013). The more premature the birth, the greater the increase in
left ventricular mass in the 102 25-year-old adults born preterm compared with
term-born controls (Lewandowski et al., 2013). This finding of greater left
ventricular mass in preterm-born adults was independent of the higher blood
pressure also reported in the same preterm-born group. Increased left ventricular
mass is a known risk factor for cardiovascular morbidity and mortality (Lorell and
Carabello, 2000).
Preterm birth is associated with chronic respiratory morbidity in infancy (Been et
al., 2014; Greenough, 2013). Long-term respiratory consequences seem to last, as
evidenced by subtle lung function abnormalities and diminished lung function in
former preterms at 19 years of age (Vrijlandt et al., 2006). The development of
immune system responses and immunologic pathways are likely affected by preterm
birth. This is supported by the finding of a significantly lower incidence of atopy in
young VLBW adults, defined as a positive skin prick test reaction, than in controls
(Siltanen et al., 2011).
Preterm birth, especially when combined with SGA, is a risk factor for a range of
psychiatric disorders, including depression, non-affective psychosis, bipolar
affective disorder and eating disorders (Räikkönen et al., 2008; Nosarti et al., 2012).
Differences in personality traits have been reported. The VLBW subjects seem more
conscientious and agreeable and show less neuroticism than controls (Pesonen et al.,
2008). VLBW adults also show more inhibition, internalizing behaviour problems
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and risk avoidance (Hack et al., 2002; Hille et al., 2008; Pesonen et al., 2008;
Schmidt et al., 2008; Schmidt et al., 2010). For instance, VLBW adults report less
smoking, less alcohol and drug use, have lower pregnancy rates and engage in less
criminal activities as young adults (Hack et al., 2002; Hille et al., 2008). They also
seem to have more difficulties in establishing social contacts (Hille et al., 2008),
perhaps caused by more prominent internalizing behaviour. Some outcomes may be
different in subgroups of VLBW individuals. For instance, in one study, VLBW
adults born at AGA were shown to be at reduced risk for some psychiatric disorders,
including depression (Räikkönen et al., 2008).
At 8 years of age, VLBW children showed higher rates of neurosensory
impairment, e.g. CP, shunt-dependent hydrocephalus, blindness or deafness, and
subnormal intelligence quotient (IQ) compared with normal birth weight children
(Hack et al., 2009). Many of these findings persist into young adulthood, as shown
in a cohort of 242 20-year-old VLBW survivors born between 1977 and 1979, which
is approximately the same time period that the participants of our studies were born.
Fewer VLBW adults graduated from high school, and they had lower mean IQ (87
vs. 92) and academic achievement scores than controls (Hack et al., 2002).
Furthermore, the rates of neurosensory impairments were higher (10% vs. < 1%),
and VLBW men were less likely to continue post-secondary studies (Hack et al.,
2002). Among survivors of preterm birth without medical disabilities, gestational
age at birth was associated with attained educational level, income, receiving social
security benefits and establishment of a family, but not with criminal activity or
rates of unemployment (Moster et al., 2008). In a meta-analysis on neurobehavioural
outcomes, moderate to severe deficits in academic achievement, attention problems,
internalizing behavioural problems and poor executive function were found to
strongly correlate with the degree of immaturity at birth and to persist into adulthood
in former preterm or VLBW individuals (Aarnoudse-Moens et al., 2009).
2.2 Physical activity
2.2.1 General recommendations
Regular physical activity is associated with the reduction of many chronic diseases,
including coronary heart disease, hypertension, stroke, metabolic syndrome, breast
and colon cancer, type 2 diabetes, osteoporosis, and depression, as well as all-cause
mortality (Physical Activity Guidelines Advisory Committee, 2008; Warburton et al.,
2010; World Health Organization, 2010). Guidelines on physical activity for adults
aged 18-64 years recommend at least 150 minutes of moderate-intensity aerobic
physical activity per week (Physical Activity Guidelines Advisory Committee, 2008;
World Health Organization, 2010). Alternatively, at least 75 minutes of weekly
vigorous-intensity aerobic physical activity is recommended to improve
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cardiorespiratory fitness and bone health and to reduce the risk of NCDs and
depression (Physical Activity Guidelines Advisory Committee, 2008; World Health
Organization, 2010). The aerobic activity should last at least 10 minutes each time in
order to achieve health benefits. Guideline recommendations are similar for adults
aged 65 years and above. For children aged 5-17 years, the aim is at least 60 minutes
of moderate to vigorous-intensity physical activity every day to improve
cardiorespiratory and muscular fitness, bone health, and cardiovascular and
metabolic health biomarkers and to reduce symptoms of depression and anxiety
(World Health Organization, 2010).
2.2.2 Health implications
Physical inactivity is identified as the 4th leading risk factor for global mortality,
accounting for 5.5% of deaths globally (World Health Organization, 2009). Only
high blood pressure (12.8% of deaths globally), tobacco use (8.7%) and high blood
glucose (5.8%) precede physical inactivity as risk factors (World Health
Organization, 2009). Overweight and obesity come next, accounting for 4.8% of
global mortality (World Health Organization, 2009). Adults tend to spend
considerable time daily in sedentary behaviour, and increasing evidence shows time
spent sedentary to be an important, independent risk factor for health (Rhodes et al.,
2012). In a large review including 109 papers, sedentary behaviour (divided into e.g.
computer use, TV viewing, sitting, socializing) and its correlates were evaluated,
and several socio-demographic and health factors were linked to sedentary
behaviour in addition to the simple absence of physical activity (Rhodes et al., 2012).
The impact of physical inactivity has even been characterized similarly to that of
smoking as being related to risk for NCDs (Lee et al., 2012). For example, in the
Finnish population, physical inactivity has been calculated to alone contribute to
6.3% of all coronary heart disease, 7.8% of type 2 diabetes, 9.1% of breast cancer,
11.2% of colon cancer and 10.1% of all-cause mortality (Lee et al., 2012).
2.2.3 Preterm birth, very low birth weight (VLBW) and later physical
activity
Survivors of preterm birth are at risk for developing long-term pulmonary sequelae,
especially after BPD. Former preterm-born tend to have more bronchial obstruction
and lower diffusion capacity, leading to diminished lung function in young
adulthood (Vrijlandt et al., 2006). In a recent review, preterm birth was associated
with a 1.7-fold (95% CI: 1.57-1.87) increased risk of childhood wheezing disorders
(Been et al., 2014). Evidence suggests that lung function in infants born very
prematurely may even deteriorate over the first year of life (Greenough, 2013). Even
mild impairment in lung function in infancy, childhood and adulthood may have a
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negative effect on physical activity. Subtle abnormalities that persist into adulthood
may be of importance with aging, especially in smokers.
Motor problems are frequently seen after preterm birth. In a large meta-analysis
of 9653 children aged 0-15 years, VLBW and birth at ≤ 32 weeks of gestation were
associated with significant motor impairment that persisted from infancy through
childhood and into late adolescence (de Kieviet et al., 2009). The different types of
motor impairment reported included balance skills, ball skills, manual dexterity and
gross motor development (de Kieviet et al., 2009). For comparison, in 9009 healthy
14-year-olds born at ≥ 36 weeks of gestation, earlier infant motor development was
associated with higher frequency of participation in leisure-time physical activity
(LTPA) and participation in a larger number of different types of sports (Ridgway et
al., 2009). In 54 ELBW 12-year-olds, a significant fitness deficit was shown relative
to 55 controls (Burns et al., 2009). In that study, motor coordination was the most
powerful predictor of physical fitness as measured by maximal oxygen uptake
(Burns et al., 2009). Complications of preterm birth at VLBW also include visual
impairments; for example, poor visual acuity and strabismus are more frequent in
VLBW adolescents than in controls (Evensen et al., 2009). Such visual impairments
have been shown to influence definite and borderline motor problems, balance and
manual dexterity in VLBW adolescents (Evensen et al., 2009). Poor motor
coordination (Rogers et al., 2005), impaired lung function (Vrijlandt et al., 2006),
visual impairment (Evensen et al., 2009), motor impairment (de Kieviet et al., 2009)
combined with lower physical self-confidence (Hack et al., 2007) and exercise
capacity (Rogers et al., 2005) and lower perceived physical ability (Saigal et al.,
2007) are all factors that may lead to undertaking less physical activity in former
preterm adolescents and young adults.
Previous studies, based on a relatively small number of questionnaire items,
suggest that adolescents or young adults born severely preterm participate less in
sports (Rogers et al., 2005; Saigal et al., 2007; Hille et al., 2008) and undertake less
LTPA (Rogers et al., 2005; Kajantie et al., 2010) than their term-born peers. They
also perceive their physical abilities as poorer in young adulthood (Hack et al.,
2007). ELBW individuals have weaker hand grip strength, lower scores for physical
self-efficacy, lower perceived physical ability and lower physical self-confidence, all
factors affecting the willingness to engage in physical activity (Saigal et al., 2007).
A limitation of these studies is, however, that they are based on self-report. Only in
one study has accelerometry been used to objectively measure physical activity in 31
11-year-olds born before 25 weeks of gestation and 30 controls born at term (Welsh
et al., 2010). This study used hip-worn accelerometers, and similarly to Study II, the
authors found no difference in physical activity between extremely preterm-born and
control groups. However, in the same study, the preterm group reported more
breathing difficulties and found physical activity to be more difficult than controls (p
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for difference 0.01) (Welsh et al., 2010). They also showed reduced peak oxygen
consumption and alternations in ventilatory adaptations during peak exercise as
measured by cycle ergometer and spirometry (Welsh et al., 2010).
A large meta-analysis including Nordic cohorts with birth weights ranging from
1.26 kg to 5.25 kg found that adolescents and adults at the extreme ends of the birth
weight spectrum were the least physically active. The lower end of the distribution is
likely to represent preterm-born subjects.
2.2.4 Measuring physical activity
Methods for measuring physical activity can simply be divided into a) self-report
and b) objective measurement.
Self-report methods on physical activity are cost-effective and participantfriendly. Furthermore, a large sample size can be handled. Detailed, prospective data
on daily physical activity can be gathered using a physical activity diary or record.
In the physical activity record, all bouts of activity during the day are reported.
Physical activity logs are used to capture time spent in different activity categories;
in this type of self-report, also sedentary activities (such as inactive periods and
sitting) are reported. Specific physical activity questionnaires are widely used in
research and vary in detail (Strath et al., 2013). Recall questionnaires typically
contain 5-15 items, while quantitative history questionnaires may contain 15-75
items. The data may be obtained by either interview or a record maintained by the
participant. Using a compendium of physical activities, data acquired from both
questionnaires and diaries can be transferred into metabolic equivalents (METs)
(Ainsworth et al., 1993; Ainsworth et al., 2000; Ainsworth et al. 2011) for
evaluation of energy expenditure (EE) from physical activity. For children and
adolescents, a separate compendium for calculating EE has been published (Ridley
et al., 2008). One MET equals the ratio of metabolic rate during exercise to
metabolic rate at rest and corresponds to an EE of about 1 kcal/kg/hour. For
classification of physical activities by intensity, the following cut-off points using
METs are recommended: moderate-intensity activity 3.0 to 5.9 METs and vigorous
intensity ≥ 6 METs (Physical Activity Guidelines Advisory Committee, 2008).
Thus, the recommended 150 minutes of moderate-intensity aerobic physical activity
per week for adults (Physical Activity Guidelines Advisory Committee, 2008;
World Health Organization, 2010) equals 450-885 MET-minutes weekly (150
minutes *3 METs, 150 minutes * 5.9 METs).
Among the objective methods assessing physical activity, the doubly labelled
water method (Lifson et al., 1955) is regarded as the gold standard for evaluating
total EE. This method is not suitable for large or epidemiologic studies as it is
expensive, time-consuming, burdensome on the participant and does not provide
qualitative data. In brief, the participant is to drink a known dose of water enriched
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with 2H and 18O. Repeated samples of blood, saliva or urine are collected during the
next 5-14 days for calculating the elimination rates of the isotopes to estimate CO2
production. The 2H is lost by the body only as water, while the 18O is lost as both
water and CO2. Thus, the difference between the turnovers of these isotopes provides
a measure of CO2 production.
The most commonly used objective measure of physical activity is accelerometry.
Depending on its location, the accelerometer measures acceleration and movement
of the body or its segments. It is usually placed on the wrist, hip, thigh or ankle. The
accelerometer can be used for relatively long time periods, typically 3-28 days. It is
easy to wear and a multitude of validation studies have been done for separate
models (Strath et al., 2013). Another type of motion sensor is the pedometer, which
is usually belt-worn. The pedometer measures steps and it is used to estimate the
distance walked over a specific time period.
Other objective measures of assessing physical activity are indirect calorimetry
performed in a laboratory setting, direct observation by a trained observer (more
often used in children) and heart rate monitoring. One of the weaknesses of heart
rate monitoring is that it is affected by several factors besides physical activity, e.g.
medications, caffeine and emotions. Sometimes also combinations of the abovedescribed methods are used for a more exact evaluation of physical activity. Heart
rate monitoring, for example, may be combined with accelerometry to differentiate
active and inactive periods and different intensity levels of physical activity.
All of the forementioned methods for measuring physical activity have their
strengths and limitations. Many of these are related to cost-effectiveness, data
processing, participant burden, measurable time period, type of activity investigated
and possible need for a laboratory setting.
2.3 Diet and nutrition
2.3.1 General recommendations
An unhealthy diet is a major contributing factor to many NCDs such as type 2
diabetes and CVD. In high-income countries (defined by WHO as gross national
income per capita US$ ≥ 10 066 or 7344 €), a low intake of fruits and vegetables
was ranked as the 7th leading risk factor for mortality, accounting for 2.5% of deaths
globally (World Health Organization, 2009). Compared with omnivorous diets,
consumption of vegetarian diets is associated with lower blood pressure; in adults, 5
mmHg lower mean systolic and 2-7 mmHg lower mean diastolic blood pressure
have been reported (Yokoyama et al., 2014). Based on a recent meta-analysis, even
small actions count; Briggs et al. (2013) calculated that daily consumption of an
apple weighing 100 g would reduce cardiac-related deaths almost equally efficiently
to daily simvastatin (40 mg) on a population level.
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Slight variations in recommended daily nutritional intakes exist countrywise. The
Finnish nutrition recommendations are based on the Nordic nutrition
recommendations (Nordic Council of Ministers, 2014) and the new, updated Finnish
recommendations have been published in 2014 (National Nutrition Council, 2014).
One specific difference between the recommendations is noteworthy; the
recommended daily salt intake for Finns is 5 g/day, while the Nordic
recommendation is 6 g/day. The new recommended Finnish macro- and
micronutrient intakes for adults aged 18-64 years are shown in detail in Table 6. In
brief, the recommended percentages of total energy intake (E%) of daily
macronutrient intake is as follows: carbohydrates 45-60%, protein 10-20% and fat
25-40%. Of carbohydrates, the intake of fibres should be at least 25-35 g/day and the
E% of sucrose < 10%. The quality of fat is important; the recommendation is that
saturated and transfatty acids comprise < 10 E%. Children and adults aged 65 years
and above have separate nutrition recommendations (National Nutrition Council,
2014).
2.3.2 Preterm birth, VLBW and later nutrition
As already pointed out in the previous sections, people born preterm have increased
risk factors for many NCDs. Food and nutrient intakes are also associated with
NCDs, e.g. hypertension (salt), type 2 diabetes (excess energy intake) and
osteoporosis (calcium and vitamin D). Although dietary habits and nutrient intake
are closely related to NCDs, previous data on VLBW subjects’ food and nutrient
intake are scarce. VLBW subjects often experience adverse environmental
conditions, such as inadequate nutrition and growth, during both prenatal and early
postnatal life. Such conditions predict dietary habits later in life (Lussana et al., 2008;
Barbieri et al., 2009; Portella et al., 2012). For example, severe maternal
undernutrition during the fetal period has been linked to preference for fatty food in
later life (Lussana et al., 2008). Poor fetal growth has been found to correlate with
altered hedonic response to sweet taste in preterm SGA infants at birth (Ayres et al.,
2012). At 3 years of age, girls born SGA were more responsive towards a sweet
reward and the impulsivity predicted fat preference and BMI at 4 years of age
(Silveira et al., 2012). Also in young women, birth with SGA has been associated
with higher carbohydrate intake compared with AGA women (Barbieri et al., 2009).
In young children born at term, AGA children with lower birth weight had higher fat
intake, especially in boys (Stafford and Lucas, 1998; Shultis et al., 2005). In a study
with 1797 term-born Finnish adults aged 56-70 years, daily intake of fruits and
berries increased with birth weight: + 83 g per 1 kg higher birth weight (Perälä et al.,
2012). In addition, a lower birth weight was also associated with higher E% of daily
fat and lower E% of daily carbohydrate intake (Perälä et al., 2012). In that study, a
validated food-frequency questionnaire was used for assessing dietary intake.
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Early prenatal and postnatal flavour experiences may explain differences in food
choices or food preferences. This seems related to fetal growth or size at birth. In a
study of infants aged approximately 6 months, maternal diet during pregnancy
affected postnatal preference for the same diet (Mennella et al., 2001). This is likely
the way culture-specific flavour preferences are initiated in early life (Mennella et
al., 2001). Furthermore, in another study, birth weight was inversely associated with
both salty taste acceptance and salt intake in children at 2 months as well as at 3-4
years of age (Stein et al., 2006). In this study, only participants with a birth weight >
2500 g were included. This is of note, as salt intake is directly related to blood
pressure (Chobanian and Hill, 2000; Kaplan, 2000), and at least in one study those
born with a low birth weight were especially sensitive to the blood pressure-raising
effect of salt as elderly adults (Perälä et al., 2011).
2.3.3 Measuring dietary habits and nutrient intake
The most commonly used methods for evaluating dietary intake are based on selfreport. The weighed food record (or diary) is considered to be the most precise selfreport method (Livingstone et al., 1990). All foods and drinks consumed are
weighed by the participant, who then records this information. This method does not
rely on an individual’s memory as everything consumed is recorded at the time of
consumption. The weighed food record is more labour-intensive for the participant
than the estimated food record (or diary), which is also a prospective dietary
assessment tool. Similarly, the participant records everything eaten and drunk at the
time of consumption. The amount of food or drink consumed is estimated based on
photographs, household measures or unit sizes (e.g. one slice of bread). Dietary
habits vary over the week and a 7-day diary takes this into account. Also foods
consumed only once or twice weekly may be underestimated or overestimated with
shorter evaluation periods. However, for practical reasons, shorter measurement
periods are also used, as was the case in Study III.
For large, epidemiologic studies, dietary recall may be the method of choice.
This retrospective method of dietary assessment is mostly used to gather information
on food and drink consumption only over the previous day or a 24-hour period, as it
relies on memory. Detailed information is gathered by interview, often by telephone.
Also widely used in epidemiologic studies is the food frequency questionnaire.
The food frequency questionnaire is based on a list of food items and assesses the
frequency of use over a specific time. Frequency categories ranging from “never” or
“less than once monthly” to “6+ per day” are used. The aim is to assess habitual diet.
Thus, foods commonly consumed in the study population, major sources of specific
nutrients or foods that contribute to variability in intake between the study
participants, are of interest. The food frequency questionnaires can be selfadministered or interviewer-administered. For this type of questionnaire, bias for
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overestimating foods perceived as healthy and underestimating unhealthy foods is
common. Data support that measurement error with food frequency questionnaires is
greater than with 7-day estimated food records (Bingham et al., 1997; Day et al.,
2001).
The dietary checklist, observation, diet history (i.e. a structured interview
enquiring about dietary habits during a past time period) and duplicate diets are
methods seldom used in epidemiologic studies. The dietary checklist is a
combination of an estimated food record and a food frequency questionnaire. The
participant receives a list of foods to report or check, and also reports portion sizes at
the time of consumption. The duplicate method refers to retaining duplicate portions
of everything consumed during the day. These are weighed and later chemically
analysed.
Many dietary surveys report lower than expected intakes of energy and nutrients.
This may to some extent be explained by inaccurate estimates of consumed foods.
The participant may also consciously or subconsciously make changes in normal
dietary habits during the follow-up period (Livingstone et al., 1990). Also
underreporting may occur, consciously or subconsciously (Livingstone et al., 1990).
Objective methods for evaluation of dietary intake are expensive and
burdensome for the participant. Typically, nutritional biomarkers are measured from
blood or urine to evaluate nutrient intakes (Arab and Akbar, 2002; Bingham, 2002).
Doubly labelled water (measures EE), urinary potassium (intake of e.g. vegetables
and fruits) and urinary nitrogen (protein intake) are regarded as gold standards for
objective measurement of nutrient intake. Plasma carotenoids and vitamin C
correspond to nutrient intake, although these measures do not reflect total nutrient
intake. In addition, sodium, phytoestrogens, polyphenols and aflatoxin can be used
as proxies for measuring intake. The above-mentioned biomarker-based objective
measurements are mostly used for validation studies on dietary self-report methods
(Bingham, 2002).
2.4 Other lifestyle factors and VLBW
Lifestyle factors are closely linked to health and disease. Knowledge on lifestyle
factors in the increased-risk VLBW population is especially important, as many of
these factors are modifiable and therefore possible targets for interventions.
According to the latest WHO report (World Health Organization, 2009), the top 10
leading risk factors causing death in high-income countries are as follows: tobacco
use (17.9%), high blood pressure (16.8%), overweight and obesity (8.4%), physical
inactivity (7.7%), high blood glucose (7.0%), high cholesterol (5.8%), low fruit and
vegetable intake (2.5%), urban outdoor air pollution (2.5%), alcohol use (1.6%) and
occupational risks (1.1%). Of these risk factors, physical inactivity and poor dietary
habits are central themes of this thesis, both of which are associated with high blood
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pressure, overweight, obesity and elevated glucose levels. Dietary habits are more
closely associated with cholesterol levels and food choices.
Previous studies show differences in lifestyle between young VLBW and control
adults (Kajantie and Hovi, 2014). As for tobacco, alcohol and drug use, the parents
of young VLBW adults have reported lower rates of use than controls (Hack et al.,
2002). In two other studies, VLBW participants smoked less cannabis and consumed
less alcohol than normal birth weight controls (Cooke, 2004; Strang-Karlsson et al.,
2008b). In addition, Hille et al. (2008) reported lower use of alcohol and drugs in
VLBW adults, while in a slightly older study similar use of drugs and alcohol was
seen in VLBW and control adults (Bjerager et al., 1995). Regarding tobacco use, the
results are mixed. In some studies, VLBW subjects report less smoking (Hille et al.,
2008; Strang-Karlsson et al., 2008b), whereas other studies show no difference in
smoking between VLBW and control groups (Cooke, 2004; Hack et al., 2002).
Of other lifestyle factors, sleep duration of less than 6 hours daily is associated
with weight gain (Xiao et al., 2013), and both the amount and quality of sleep are
associated with cardiometabolic risk factors (Merikanto et al., 2013). Based on
accelerometer measurement, VLBW adults have similar amounts and quality of
sleep as term-born controls (Strang-Karlsson et al., 2008a). Furthermore, night owl
behaviour is related to an increased risk for type 2 diabetes and higher blood
pressure (Merikanto et al., 2013). Thus, the finding of a trend towards early bird
behaviour in VLBW adults, based on both questionnaire (Strang-Karlsson et al.,
2010) and accelerometer measurement (Strang-Karlsson et al., 2008a), may be
protective.
2.5 Psychosocial stress
2.5.1 Stress response
Two interrelated pathways constitute the main stress response mechanisms in
humans: 1) the HPAA and 2) the sympathetic-adrenal-medullary (SAM) system
(Figure 1). In response to stress, the hypothalamus releases corticotropin-releasing
hormone, which stimulates the pituitary to release adrenocorticotropic hormone
(ACTH). ACTH in turn stimulates the adrenal glands. Thus, the HPAA is activated
and cortisol, the end-product, is released from the adrenal cortex. Simultaneously,
perceived stress activates the hypothalamus to stimulate the sympathetic nervous
system, which then stimulates the adrenal medulla to release two end-products:
adrenalin (A) and noradrenalin (NA). These two stress response pathways are
important determinants of health and disease. The intensity of the stress responses
varies between individuals and altered stress response is linked to several adverse
outcomes. A hyperactive HPAA increases the risk of type 2 diabetes (McEwen,
1998), CVD (McEwen, 1998), depression (Bjorntorp, 1996) and metabolic
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syndrome with its components (Chrousos, 2000). Furthermore, a hyporesponsive
HPAA has been linked to several conditions, including fibromyalgia, post-traumatic
stress disorder, burnout and depression (Fries et al., 2005). Regarding autonomic
stress response and blood pressure reactivity, evidence shows that high
cardiovascular reactivity to stress predicts later development of hypertension
(Treiber et al., 2003; Matthews et al., 2004; Chida and Steptoe, 2010; ), carotid
atherosclerosis (Treiber et al., 2003; Matthews et al., 2006;) and increased left
ventricular mass (Treiber et al., 2003). In a meta-analysis including 36 trials, the
association between greater stress reactivity and poor cardiovascular risk status
(including a myocardic ischaemic event, hypertension and elevated systolic or
diastolic blood pressure) was clearer in men, in populations aged under 18 years and
in follow-ups ≥ 3 years (Chida and Steptoe, 2010).
Later HPAA function is affected by pre- and postnatal events. Examples from
animal studies show that prenatal events may program the HPAA; maternal stress
during pregnancy (Weinstock, 2001; Welberg and Seckl, 2001), exposure to
synthetic glucocorticoids (Matthews, 2000; Liu et al., 2001) and nutrient restriction
(Hoet and Hanson, 1999; Lesage et al., 2002) all seem to affect HPAA functioning
in offspring. Likewise, maternal undernutrition is a stressor for the human fetus.
Based on animal studies on the effects of reduced nutrition in pregnancy on HPAA
function of offspring (Hoet and Hanson, 1999; Lesage et al., 2002), it has been
speculated that also in humans, maternal stress and undernutrition during pregnancy
might lead to fetal programming of later HPAA function. In animal models maternal
or fetal undernutrition affects the development and responses of the HPAA, blunted
responses in utero swich to enhanced responses postnatally (Hoet and Hanson,
1999). The mechanisms behind these changes are unknown.
After birth, neonatal handling (Meaney et al., 2000), maternal behaviour
(Meaney, 2001), exposure to synthetic glucocorticoids (Bakker et al., 2001) and
infection (Nilsson et al., 2002) have effects on future HPAA functioning.
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-
Figure 1. Two main pathways of stress response in humans: the sympathetic-adrenalmedullary system and the hypothalamic-pituitary-adrenal axis.
2.5.2 Preterm birth and stress response
The DOHaD theory implies that early life events affect later health and disease
through programming; this also seems to be true for stress responses (Kajantie and
Räikkönen, 2010). It has been postulated that a hyporeactive HPAA might develop
after prolonged periods of stress (Hellhammer and Wade, 1993). Prolonged stress
with constant activation of the HPAA leads to long periods of elevated cortisol
levels. The prolonged elevation of cortisol levels in turn lead to suppression of the
HPAA through negative feedback. Pre- and postnatal stress in the VLBW neonate
would thus link preterm birth with later stress response. Therefore, all types of
stressful events that fetuses and preterm neonates undergo may be risk factors
adversely affecting future HPAA functioning. This is supported by previously
published data suggesting that fetal growth, gestational age and birth weight
influence lifelong functioning of both HPAA and SAM stress response (Wüst et al.,
2005; Jones et al., 2006; Buske-Kirschbaum et al., 2007; Grunau et al., 2007;
Johansson et al., 2007; Kajantie et al., 2007; Grunau et al., 2010; Kajantie and
Räikkönen, 2010; Brummelte et al., 2011). Neonatal intensive care after preterm
birth is one example of such stress (Carbajal et al., 2008). In addition to postnatal
stress, also prenatal stress may affect later HPAA function, as was shown in a study
of 12 adult men and 48 women whose mothers were exposed to severe stress in the
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form of a major life event during pregnancy. These young adults showed higher
cortisol responses to the Trier Social Stress Test (TSST) than controls (Entringer et
al., 2009).
More specifically, also the different types of treatment given during neonatal
intensive care may alter later stress response. For example, dexamethasone was
widely used to treat or prevent chronic lung disease of prematurity in the 1990s. In a
study comparing children born before 32 weeks of gestation, cortisol, NA and
cardiovascular responses to Trier Social Stress Test for Children (TSST-C) at school
age were all lower in those children exposed to dexamethasone treatment than in
children receiving hydrocortisone treatment or in former preterm children who did
not receive any corticosteroid treatment (Karemaker et al., 2008a; Karemaker et al.,
2008b).
As for HPAA response to stress, previous studies have indicated lower cortisol
responses in infants born preterm. For example, immunization, a stressor to the
infant, provoked lower cortisol responses in 4-month-old infants born preterm
(divided into subgroups according to GA: 24-28 weeks and 29-32 weeks) than in
full-term infants (Grunau et al., 2010). This lower cortisol response to stress in the
preterm infants was statistically significant only in boys. Grunau et al. (2007) also
reported lower salivary cortisol levels in extremely preterm infants at 3 months of
corrected age. In these same infants, at 8 and 18 months of corrected age the authors
reported a shifting to higher cortisol levels, perhaps indicative of long-term effects
of prematurity on HPAA (Grunau et al., 2007). Also among 8- to 12-year-old
children born at any degree of prematurity, the cortisol response to psychosocial
stress was attenuated, but not statistically significantly, in those born preterm
(Buske-Kirschbaum et al., 2007). In that study, the TSST-C was used as a stressor
and the participants comprised 18 preterm children and 18 same-sex friends.
Recently published data showed lower hair cortisol levels in 83 healthy children
born ≤ 32 of weeks gestation compared with 45 term-born children at age 7 years (p
= 0.018) (Grunau et al., 2013). Hair cortisol represents a measure of chronic HPAA
activity, and also this finding suggests persisting effects of early HPAA
programming.
Mechanically ventilated neonates often undergo procedures and treatments
causing both pain and stress (Carbajal et al., 2008). In a follow-up study of 5-yearold children born at 24-42 weeks of gestation who were mechanically ventilated as
neonates, higher cortisol levels were measured across the day than in full-term
control children (Joke de Graaf et al., 2014). This finding supports the early-life
programming concept of the HPAA, as production of cortisol follows a diurnal
rhythm, with the highest concentrations occurring in the morning (Sherman et al.,
1985). This diurnal rhythm is an important index of HPAA regulation (Rosmalen et
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al., 2005), and it is established during the first year of life (Tollenaar et al., 2010), in
some infants even at 2 months of age (de Weerth et al., 2003).
As for birth weight, the association with HPAA stress response has been studied
previously (Kajantie and Räikkönen, 2010). Lower birth weight was associated with
greater salivary cortisol response in 7- to 9-year-old boys (Jones et al., 2006) as well
as young, healthy male twins (Wüst et al., 2005). In the twin study, a trend for
higher cortisol responses was seen in participants born at an earlier GA. Contrary to
those two studies, in 60- to 70-year-old adults TSST provoked lower cortisol and
ACTH response in those born with a lower birth weight, mostly after term
pregnancy (Kajantie et al., 2007). However, data on HPAA stress reactivity in adults
born preterm are scarce, and the results have been mixed (Kajantie and Räikkönen,
2010).
Some data on SAM response to stress in preterm children have been published.
For example, after immunization, heart rate (HR) responses were similar in 4month-old infants born at GA 24-28 weeks, at GA 29-32 weeks and at term (Grunau
et al., 2010). Furthermore, in 9- to 10-year-old children, both preterm birth (n=39)
and fetal growth restriction (n=29) were associated with increased SAM activity, as
indicated by urinary catecholamine and HR responses after a mathematical mental
stress (Johansson et al., 2007). After TSST-C, HR responses to stress in 8- to 12year-old children were similar in former preterm infants and controls infants (BuskeKirschbaum et al., 2007).
Previously reported from our HeSVA cohort, diastolic blood pressure responses
to psychosocial stress were higher in young adult VLBW participants (Pyhälä et al.,
2009). Also slightly higher resting HR was found in the HeSVA cohort VLBW
participants in conjunction with a clinic visit (Hovi et al., 2007). Previously
published data on SAM response to stress in adults born preterm are scarce.
2.5.3 Measuring stress response
Stress response tests can be divided into a) psychosocial stressors and b)
physiological stressors. The psychosocial stressors include mental arithmetics,
mirror drawing or other cognitive tasks and public speaking (Dickerson and Kemeny,
2004). Of the different psychosocial stressors, the TSST has been reported to most
consistently stimulate the HPAA (Kirschbaum et al., 1993). This test measures
response to moderate psychosocial stress in a laboratory setting. It includes a 5minute anticipation period and a 10-minute test period (Kirschbaum et al., 1993).
During the test the participant delivers an unprepared speech and performs mental
arithmetic tasks in front of an evaluation committee (Evaluation Committee, Figure
2). A modified version of the test has been developed specifically for children, the
TSST-C (Buske-Kirschbaum et al., 1997). Key factors of the TSST, as well as other
psychosocial stressors, are the participant’s feelings of uncontrollability and social-
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evaluative threat or judgement (Dickerson and Kemeny, 2004). This stimulates the
participant to undergo strong feelings of discomfort and stress and triggers the
release of ACTH, cortisol, A and NA.
Figure 2. The TSST evaluation committee at work.
The cold pressor test, exercise testing and nutritional challenge (e.g. intake of
standardized meals) are examples of physiological stressors (Kudielka et al., 2009).
The cold pressor test is performed by having the participant’s hand immersed in cold
water (0-5°C) for 1-6 minutes. This procedure provokes a vascular sympathetic
response with increased peripheral resistance. As a result, elevations in blood
pressure (Greene et al., 1965; Victor et al., 1987) and changes in HR (Mourot et al.,
2009) are observed. Another type of physiological stressor, intense physical exercise,
elicits significant cortisol responses, and in contrast to psychological stressors, no
strong habituation effect is seen in exercise-induced cortisol responses (Kudielka et
al., 2009). Cortisol also increases after meals, and the level of cortisol response
varies with the time of day, with higher rises in lunchtime than in evening responses
(Brandenberger et al., 1982; Follenius et al., 1982; Quigley and Yen, 1979).
A variety of pharmacological stimulants, such as vasopressin, synthetic ACTH or
corticotropin-releasing hormone, are also used to stimulate HPAA stress response
(Kudielka et al., 2009). Such pharmacological challenges measure HPAA response
at different levels of the HPAA system, and they are dose-dependent in assessing,
for example, adrenal cortex sensitivity (low dose of synthetic ACTH) or capacity
(high dose of synthetic ACTH) (Kudielka et al., 2009).
Exposure to the different types of stressors results in different physiological
responses. As a result of habituation, the stress response also tends to attenuate after
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repeated exposure to the same psychosocial stressor (Dickerson and Kemeny, 2004;
Kudielka et al., 2009). Furthermore a multitude of factors, including age, gender,
menstrual cycle phase, medications (e.g. hormonal contraception and hormone
replacement therapy), pregnancy, lactation, breast-feeding, nicotine, coffee, alcohol,
dietary energy supply, time of testing, early life stress experiences and genetic and
personality factors, affects especially cortisol responses to stress (Kudielka et al.,
2009).
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Aims of the study
3 Aims of the study
The general aim of this study was to evaluate mechanisms underlying the link
between preterm birth and later health, focusing on stress response and
lifestyle, more specifically physical activity, dietary habits and nutrition. The
general hypothesis was that differences between former preterm and termborn adults exist, and these may in part explain the increased risk factors for
chronic non-communicable diseases in former preterm, very low birth weight
adults.
Specific aims were as follows:
1. To investigate whether preterm birth at very low birth weight affects
physical activity in young adulthood (Studies I and II). Special
attention was directed to conditioning leisure-time physical activity.
2. To explore dietary habits and nutrition intake in young adults born
preterm at very low birth weight (Study III).
3. To determine whether very low birth weight is associated with
changes in hypothalamic-pituitary-adrenal axis (Study IV) and
sympathetic-adrenal-medullary system (Study V) responses to
psychosocial stress in young adulthood.
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Subjects and methods
4 Subjects and methods
4.1 Study population
All participants of the studies were derived from the HeSVA, a follow-up, casecontrol cohort (Figure 3). During 1978-1985 a total of 474 VLBW infants were
admitted to the NICU of Children’s Hospital, Helsinki University Central Hospital.
All tertiary neonatal care in the province of Uusimaa in southern Finland is
centralized to this NICU. Of the 474 VLBW infants admitted to the NICU, 71%
(335) survived. They were discharged from the hospital and followed up during
infancy and childhood (Järvenpää and Granström, 1987).
These VLBW subjects were traced using data provided by the Population
Register Centre of Finland. For each VLBW subject, a control was selected based on
hospital records. From the same birth hospital in which the VLBW participant was
born, the next person who was born full-term, singleton, not SGA and of the same
sex as the VLBW participant was chosen as a control. During 2004-2005 those 255
VLBW and 314 control subjects living in the greater Helsinki area (within 110 km
of the study clinic) were invited to participate in a clinical study. Of those invited,
166 VLBW (65%) and 172 controls (55%) chose to participate (Study III). A
follow-up study was carried out during 2007-2008. To the follow-up, we invited
participants of the first visit, excluding 25 individuals for the following reasons:
developmental delay (n =1), earlier refusal of future contact (n =4), living abroad (n
=11), being untraceable (n= 2) and being ineligible for the glucose metabolism
studies included in the follow-up visit (n= 7; pregnancy, medication, type 1
diabetes). Thus, 159 VLBW and 154 control subjects were invited. Of these, 113
VLBW (71%) and 105 controls (68%) chose to participate (Studies I, II, IV, V).
The clinical study visits were carried out in collaboration with the Children’s
Hospital, Helsinki University Central Hospital, the National Institute for Health and
Welfare, and the Institute of Behavioural Sciences, University of Helsinki.
4.2 Measures
4.2.1 Background characteristics
Maternal and perinatal data were collected from maternal welfare clinics, hospital
records and well-baby clinic records. During the clinical study visits the participants
completed questionnaires covering medical history, current illnesses, use of
medications and smoking habits. Information regarding schooling and family socioeconomic status was gathered by self-report. In Studies I-V, the highest parental
education of either parent is used as a proxy of childhood socio-economic status.
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Figure 3. Flow chart of the Helsinki Study of Very Low Birth Weight cohort. Subgroups of
the original cohort participated in Studies I-V.
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4.2.2 Assessment of physical activity based on self-report (Study I)
In the two studies on physical activity, we assessed physical activity by both selfreport (Study I) and objective measurement (Study II). In Study I, we used the
modified Kuopio Ischaemic Heart Disease Risk Factor Study (KIHD) questionnaire
for assessing physical activity habits. The KIHD questionnaire has been shown to be
reproducible and valid in several different populations (De Backer et al., 1981;
Jacobs et al., 1993; Lakka and Salonen, 1992; Lakka and Salonen, 1993). This
questionnaire includes a 30-item list of different physical activity types, and it
collects data on the previous 12-month period. The participant could also add
physical activity types not included in the 30-item list. The participant was to report
monthly frequency and duration of each physical activity during the previous 12
months. The self-reported monthly data were then transformed into units of
times/year (frequency), and the duration of each physical activity session was
summed and transformed into units of minutes/year (total time). The participant was
also asked to report the intensity of each physical activity on a scale from 0-3 (0 =
light, 1= moderate, 2 = strenuous and 3 = very strenuous). These self-rated physical
activity intensities were transformed into METs by use of standardized activityspecific tables (Ainsworth et al., 1993; Ainsworth et al., 2000). By definition, a
MET equals the ratio of metabolic rate during exercise to metabolic rate at rest.
Roughly, 1 MET corresponds to an EE of approximately 1 kcal/kg/hour. This is
approximately equivalent to the energy cost of sitting quietly. We then used the
MET values to calculate the total volume of physical activity (METh/year) as
follows: MET x hours of physical activity / year. Yearly EE (kcal/year) was
calculated as total time of physical activity (min/year) x MET (kcal/kg/min/year) x
weight (kg).
The KIHD questionnaire further divides physical activity into commuting to
work (walking and cycling), conditioning LTPA and non-conditioning LTPA. For
conditioning LTPA, 20 different types of sports participation, e.g. running,
swimming and skiing, are included in the questionnaire. For non-conditioning
LTPA, 8 activities other than sports participation requiring physical efforts, e.g.
shoveling snow, gardening and household chores, are included. Based on MET
values, physical activity was additionally categorized into vigorous physical activity
if MET ≥ 5. We separately looked at vigorous conditioning LTPA, vigorous nonconditioning LTPA and vigorous commuting activity.
The KIHD questionnaire was completed by the participants during the second
clinical visit in 2007-2008. Of the 113 VLBW and 105 control study visit
participants, 12 and 3, respectively, did not complete the KIHD questionnaire. Of
the 101 VLBW and 102 control participants with data available, 7 and 1 participant,
respectively, were excluded due to CP, developmental delay, hearing deficit,
43
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Subjects and methods
blindness or another condition affecting mobility. Thus, 94 VLBW and 101 controls
were included in the analyses of Study I.
4.2.2.1
Adjustment for lean body mass
We had information on lean body mass (total body mass – fat and bone mass) in a
subset of the participants of Study I (91 VLBW and 88 control participants). During
the first clinical study visit, in 2004-2005, these subjects had their body composition
measured by dual-energy x-ray absorptiometry (DXA, Discovery A, Hologic,
Hologic Inc., Bedford, MA, USA). The DXA scan required a separate visit, and not
all study participants were willing to undergo DXA. This information on lean body
mass was used in additional analyses as body composition is an important covariate
in analyses of physical activity.
4.2.2.2
Adjustment for personality
Individual personality traits are linked to physical activity. In the analyses of Study
I, we utilized information on personality traits of the participants assessed at the first
clinical study visit in 2004-2005. At that time, the participants completed the NEOPersonality Inventory (Costa and McCrae, 1985). This is a 180-item inventory based
on the Big Five taxonomy. The Big Five is the most commonly used classification
of personality traits (agreeableness, conscientiousness, extraversion, neuroticism and
openness to experience). The 180 items are rated on a 5-point scale (0 = very untrue,
4 = very true). Data on personality traits were available for 90 VLBW and 98 control
participants.
4.2.3 Assessment of physical activity based on accelerometer
measurements (Study II)
To objectively assess physical activity, we used a wrist-worn accelerometer, the
Actiwatch AW4 model (Cambridge Neurotechnology Ltd., UK). The AW4 registers
body movements as minute-by-minute activity counts (cpm, counts per minute) and
is a validated tool for measuring physical activity in free living subjects (Heil et al.,
2009).
The participants were instructed to wear the accelerometer on the non-dominant
wrist for at least three days. It was to be removed only when showering, bathing or
swimming. Simultaneously with the accelerometer recording, the participants kept a
sleep log for reporting bedtimes and awakening times. They also registered getting
up and bedtimes on the accelerometer by pressing a button. We used 1-min epochs
in the scoring and analysed data between 8:00 and 24:00, excluding times in bed and
times not wearing the accelerometer. Analysis started upon getting up and ended at
bedtime. To be included in the analyses, we required a minimum of 600 min/day of
wearing the accelerometer and ≥ 3 days of recording. We further separately analysed
44
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Subjects and methods
data on physical activity during weekend days and weekdays. We also compared
differences in the intensity of physical activity by dividing physical activity into
sedentary time (< 1.5 METs, < 320 cpm), moderate physical activity (3-6 METs,
1048 -1624 cpm), vigorous physical activity (> 6 METs, > 1624 cpm) and their sum,
moderate-to-vigorous physical activity (MVPA, ≥ 3 METs, ≥ 1048 cpm). These cutoff points are based on a validation study in 8- to 10-year-old children as to date
such validation studies have not been done in adults (Ekblom et al., 2012). Initially,
the accelerometer recordings were performed for gathering sleep data on the
participants (Strang-Karlsson et al., 2008a), and the data were now also used for
separate analyses of daily physical activity.
To analyse physical activity data, we used the Actiwatch Activity & Sleep
Analysis version 7.43 software (Cambridge Neurotechnology Ltd., UK).
During the second clinical study visit in 2007-2008 all attending cohort
participants were offered an accelerometer if one was available. Of the attending 113
VLBW and 105 control participants, a subsample of 57 VLBW and 47 controls
underwent accelerometer measurements.
4.2.4 Assessment of nutrition and diet (Study III)
To assess dietary intake we used the 3-day food record. Among the various selfreport methods for estimating dietary intake, the 3-day food record is considered one
of the most reliable (Prentice et al., 2011). During the clinical study visit a study
nurse instructed the participants on completion of the food record. The participants
were instructed to report everything they ate and drank during 3 consecutive days.
This 3-day period was to include 2 workdays and 1 day off. The participants used a
picture booklet of portion sizes for estimating the amounts of food consumed.
Estimating portion size with the aid of a picture booklet gives more accurate
estimations of the amounts of food consumed (Ovaskainen et al., 2008). At the time
of returning the food record, a trained nutritionist interviewed the participant to
ensure completeness of the 3-day food record.
From the 3-day food record, information regarding all intakes of foods and
drinks was collected. Using a dietary analysis program based on the FINELI
database (Finnish Food Composition Database) (Ovaskainen et al., 1996), mean
daily energy intake, amounts of foods consumed and macronutrient and
micronutrient intakes were calculated. The FINELI database was developed at the
National Public Health Institute, and it includes 987 food items and 1622 composite
dishes.
Of the 338 individuals who participated in the first clinical study during 20042005, 155 VLBW and 156 controls completed the food record data. Of these, 4
VLBW participants were excluded from the analyses due to incomplete food record
data. Thus, Study III included 151 VLBW and 156 control participants.
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Subjects and methods
4.2.5 Psychosocial stress responses (Studies IV and V)
To assess stress response, we used a standardized stressor, the TSST. The TSST
measures response to moderate psychosocial stress in a laboratory setting
(Kirschbaum et al., 1993). The test includes a 5-min anticipation period and a 10min test period. During the test the participant delivers an unprepared speech to
apply for a job and performs arithmetic tasks in front of a two-person evaluation
committee (Figure 2). The arithmetic task starts with calculating 2023-17-17-17…
and, if needed, the difficultness of the task is modified according to the mathematic
abilities of the participant. To maximize the unpleasantness of the situation, the
committee minimizes all verbal and non-verbal communication with the participant.
The TSST was performed between 10:45 and 16:10, and in the analyses we
adjusted for the time of day (dummy-coded based on when the TSST began: 10:50,
12:05 or 13:20). The participants were to refrain from eating and drinking 2 h before
the test. An intravenous line was inserted 45 min prior to the beginning of the TSST
for collection of blood samples. Blood was drawn and salivary samples were
collected at baseline (15-30 min before the test) and at 0, 10, 20, 30, 45, 60 and 90
min after the TSST (Figure 4). HPAA and SAM stress response was evaluated by
repeated biomarker measurements. Plasma and salivary cortisol were measured at all
eight time-points. Plasma ACTH was measured from the four first time-points, as it
returns faster to pre-stress levels. Plasma insulin and glucose were measured at
baseline, 0, 20 and 90 min, while A and NA samples were collected at baseline, 0,
10 and 90 min. All samples were immediately frozen to -20 °C and transferred once
a week to -70° C for storage until the time of analysis.
Plasma cortisol concentrations were determined by ELISA (ImmunoBiological
Laboratories, Hamburg, Germany) and salivary cortisol concentrations by
competitive solid-phase, time-resolved fluorescence immunoassay with fluorometric
endpoint detection (DELFIA, Wallac, Turku, Finland). ACTH was determined by
chemiluminescence immunofluorometric assay (Nichols Institute Diagnostics, San
Clemente, CA, USA). Cortisol and ACTH measurements were assayed in duplicate
at the University of Trier, Germany. Plasma insulin concentrations were determined
by Immunotech Insulin(e) IRMA kit (Beckman Coulter Inc., Prague, Czech
Republic). Glucose concentrations were determined by an enzymatic hexokinase
method (Gluco-quant®, Roche Diagnostics, Tokyo, Japan). Plasma A and NA
concentrations were determined by high-performance liquid chromatography
(HPLC, ChromSystems diagnostics by HPLC, München, Germany).
46
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Subjects and methods
Figure 4. The TSST was performed between 10:45 and 16:10 applying the above
schedule.
In addition to the above-described biomarker measurements, we also measured
HR response to stress in conjunction with the TSST. A continuous beat-to-beat
monitoring of HR during baseline and task was conducted by ECG. We averaged all
HR readings, as mental stress measures for cardiovascular activity are more
reproducible when responses to individual tasks are aggregated (Kamarck and
Lovallo, 2003). In line with a previous report on blood pressure response to TSST in
the same cohort of participants (Pyhälä et al., 2009), mean HR was reported 1)
during the 5-min baseline, 2) during the 10-min task period (speech and arithmetic)
and 3) as HR reactivity (task response, i.e. task average minus baseline average).
The recording of HR data was performed with Biopac AcqKnowledge 3.8.1
software (Santa Barbara, CA, USA).
The TSST was performed on a subset of the original HeSVA cohort. A random
sample among the participants of the first clinical visit in 2004-2005 was invited to
the TSST in 2005. From the randomization process, we excluded subjects who 1)
were unable to stand or to manage without an assistant, 2) used glucocorticoids or 3)
had a nightshift during the previous week. A total of 54 VLBW (28 women, 52%)
and 40 controls (23 women, 56%) attended the TSST and were included in the
analyses of Study IV. Of these TSST participants, 50 VLBW (28 women, 56%) and
39 controls (22 women, 56%) had adequate blood samples for also analysing A and
NA responses to stress and were thus included in the analyses of Study V.
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4.3 Statistical analyses
Statistical analyses were conducted with SPSS for Windows versions 17, 19 or 21
(SPSS Inc., Chicago, IL, USA) and R 2.8.1 (R Foundation for Statistical Computing,
Vienna, Austria).
In all studies, descriptive characteristics between VLBW and control groups
were compared with independent samples t-test (continuous variables) and χ2-test
(categorical variables). In all analyses, a p-value < 0.05 was considered significant.
We explored interactions between gender and birth status by entering an interaction
term (sex*VLBW/control) into the statistical models. If no significant interaction
was found, the results were presented with men and women combined.
In the self-report-based study on physical activity (Study I), all outcome
variables were log-transformed [10log(variable+1)] to attain normality. We used
multiple linear regression to compare yearly frequency, total time, total volume and
EE of conditioning, non-conditioning commuting and vigorous physical activity
(MET ≥ 5). Adjustment was made for common confounders (age, sex, BMI, daily
smoking, highest parental education). In further analyses, adjustment for personality
based on the mean scores of the NEO-Personality Inventory was performed. In the
subgroup with data available, we reran all analyses after replacing BMI with body
composition (lean body mass, measured by DEXA). The results are presented as
mean differences (%) and 95% confidence intervals (CIs) between VLBW and
control groups. In addition, the influence of each personality trait on conditioning
LTPA was analysed by incorporating each trait one at a time into the linear
regression model. The influence of each personality trait is presented as a correlation
coefficient.
In the physical activity study based on accelerometer measurement (Study II), a
linear regression model adjusted for common confounders (age, sex, season, BMI,
smoking, SES) was used to compare physical activity between VLBW and control
groups. Season was dummy-coded as follows: December-February, March-May,
June-August and September-November. The results are presented as mean
differences in activity cpm registered by the accelerometer, with 95% CI between
VLBW and control participants. We also compared differences in the intensity of
physical activity after dividing physical activity into sedentary time (< 1.5 METs, <
320 cpm), moderate physical activity (3-6 METs, 1048 -1624 cpm), vigorous
physical activity (> 6 METs, > 1624 cpm) and moderate-to-vigorous physical
activity (MVPA, ≥ 3 METs, ≥ 1048 cpm). These cut-off points are based on a
validation study in 8- to 10-year-old children as to date such validation studies have
not been conducted in adults (Ekblom et al., 2012).
In the study on dietary habits and nutrition (Study III), a linear regression model
was used to compare differences in daily food and nutrient intakes. For easier
48
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interpretation, the results are presented as mean daily food and nutrient intakes in
VLBW and control groups.
In Studies IV and V, all biomarker concentrations were log-transformed to attain
normality. A mixed-effects model was used to analyse the biomarker responses to
stress. We also used a linear regression model to compare differences between
VLBW and control groups in commonly used indicators of stress response, i.e.
baseline, peak, relative increment (peak after stress / baseline value) and areas under
the curve ground (AUCg), calculated as described elsewhere (Kajantie et al., 2007).
Age, sex, BMI, hormonal contraception, time of day (dummy-coded based on time
when TSST began: 10:50, 12:05 or 13:20) and highest parental education were
adjusted for in all analyses. In addition, we further adjusted for menstrual cycle
phase. We divided the women with data available on menstrual cycle phase into two
groups (cycle phase days 1-8 and days ≥9). Only the women not using hormonal
contraception were included in the two groups. In addition to examining gender
interactions (sex*VLBW/control group), we assessed interactions by including an
interaction term with sampling time (time*VLBW/control group) and
(sex*time*VLBW/control group).
4.4 Ethics
This study was performed according to the Declaration of Helsinki guidelines. The
study protocol was approved by the Ethics Committee of the Helsinki and Uusimaa
Hospital District. Written informed consent was obtained from each participant.
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Results
5 Results
5.1 Description of study participants
Different subgroups from the HeSVA cohort participated in the five studies included
in this thesis. For comparison, characteristics of the different subgroups in Studies IV are presented in Table 2. As adults, the VLBW participants were shorter and had
lower lean body mass than controls. Daily smoking was more common among
controls.
A summary of the main results is shown in Table 3.
50
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disease
Table 2.
Descriptive characteristics of the subgroups participating in Studies I-V.
Characteristic
VLBW
(n = 94)
Birth
Gestational age, mean (SD),
weeks
Birth weight, mean (SD), grams
Birth weight SDS, mean (SD)
Women, n (%)
Men, n (%)
SGA b, n (%)
Preeclampsia, n (%)
Twin, n (%)
Triplet, n (%)
Current
Age, mean (SD), years
Height, mean (SD), cm
Women
Men
Study I
Term
(n =101)
VLBW
(n =57)
Study IIa
Term
(n =47 )
Study III
VLBW
Term
(n =151)
(n=156)
Study IV-V
VLBW
Term
(n = 54)
(n = 40)
29.5 (2.3)
40.1 (1.1)
29.3 (2.5)
40.1 (1.1)
29.2 (2.3)
40.1 (1.2)
29.4 (2.3)
40.2 (1.1)
1157
(208.7)
-1.3 (1.5)
57 (60.6)
37 (39.4)
35 (37.2)
24 (25.5)
14 (14.9)
2 (2.1)
3608
(492.0)
0.1 (1.1)
59 (58.4)
42 (41.6)
0
9 (8.9)
0
0
1116
(215.5)
-1.4 (1.6)
35 (61.4)
22 (38.6)
21 (36.8)
10 (18.2)
6 (10.9)
2 (3.6)
3648
(499.0)
0.2 (1.1)
32 (68.16)
15 (31.9)
0
3 (6.5)
0
0
1111
(221)
-1.4 (1.5)
91 (60.3)
60 (39.7)
54 (35.8)
32 (21.2)
21 (13.9)
5 (3.3)
3595
(472)
0.1 (1.0)
96 (61.5)
60 (38.5)
0
11 (7.1)
0
0
1103
(212.4)
-1.5 (1.5)
28 (51.9)
26 (48.1)
23 (42.6)
12 (22.2)
7 (13.0)
1 (1.9)
3639
(455.6)
0.1 (1.0)
23 (57.5)
17 (42.5)
0
4 (10.0)
0
0
24.9 (2.1)
25.1 (2.2)
24.6 (2.0)
24.9 (2.2)
22.4 (2.1)
22.5 (2.2)
23.1 (2.2)
23.5 (2.0)
163.0 (7.4)
166.4 (6.2)
163.9 (7.2)
166.5 (6.5)
165.7 (6.7)
180.8 (6.1)
173.7 (6.7)
180.9 (5.0)
167.4
(6.6)
180.6
(6.4)
161.5 (6.9)
176.2 (7.2)
162.0
(7.6)
174.1
(8.1)
174.3 (8.2)
180.2 (4.9)
Footnotes are on the page following Table 2.
Table 2 continued.
Study I
Study IIa
Study III
VLBW
Term
VLBW
Term
VLBW
Term
(n = 94)
(n =101)
(n =57)
(n =47 )
(n =151)
(n = 156)
2
Body mass index, mean (SD), kg/m
Women
21.8 (3.7)
22.9 (4.3)
22.7 (4.5)
23.9 (5.5)
22.2 (4.0) 22.8 (3.8)
Men
22.4 (3.5)
23.0 (2.9)
22.8 (4.0)
24.2 (3.2)
21.9 (3.7) 23.5 (3.2)
Daily smoking, n (%)
15 (16.0)
31 (30.7)
7 (12.3)
12 (25.5)
31 (20.8)
46 (29.5)
Parental education, n (%)
Elementary
8 (8.5)
5 (5.0)
5 (8.8)
3 (6.4)
15 (10.1)
11 (7.1)
High school
21 (22.3)
19 (18.8)
17 (29.8)
9 (19.1)
30 (20.3)
28 (18.1)
Intermediate
32 (34.0)
33 (32.7)
14 (24.6)
15 (31.9)
62 (41.9)
51 (32.9)
University
33 (35.1)
44 (43.6)
21 (36.8)
20 (42.6)
41 (27.7)
65 (41.9)
a
Of the participants in Study I 50 VLBW (53%) and 46 controls (46%) also participated in Study II.
b
small for gestational age, birth weight < -2SD
Characteristic
Study IV-V
VLBW
(n =54)
Term
(n =40)
21.7 (3.3)
22.3 (4.2)
19 (35.2)
23.7 (4.6)
23.9 (3.1)
10 (25.0)
5 (9.3)
14 (25.9)
19 (35.2)
16 (29.6)
3 (7.5)
4 (10.0)
15 (37.5)
18 (45.0)
Table 3.
Study
Study I
Summary of the main results for Studies I-V.
VLBW / Term (n)
94 / 101
Main outcome
Method
Main results
Physical activity
Questionnaire (Kuopio
Markedly lower yearly frequency, total time, total
Ischemic Heart Disease
volume and energy expenditure of conditioning
Risk Factor Study
leisure-time physical activity in VLBW adults.
Questionnaire)
Study II
Study III
57 / 47
151 / 156
Physical activity
Diet and nutrition
Accelerometer (Actiwatch
No statistically significant difference in physical
AW4)
activity between VLBW and control groups.
3-day food record
Lower consumption of milk products, vegetables,
fruits and berries in VLBW adults, accompanied by a
lower intake of calcium and vitamin D.
Study IV
54 / 40
Psychosocial stress
Trier Social Stress Test
response
Study V
50 / 39
Psychosocial stress
response
Overall lower plasma cortisol reactivity and lower
cortisol peak after stress in VLBW adults
Trier Social Stress Test
Overall adrenalin, noradrenalin and heart rate
responses to stress were not significantly different
between VLBW and control adults. For VLBWwomen, noradrenalin reactivity after stress was lower.
Results
5.2 Conditioning leisure-time physical activity is lower in
VLBW adults (Study I)
Based on the modified KIHD questionnaire, we evaluated yearly frequency, total
time, total volume and EE of conditioning LTPA, non-conditioning LTPA, vigorous
physical activity and commuting physical activity. All dimensions of conditioning
LTPA were lower in VLBW participants (Table 4). For non-conditioning LTPA,
vigorous physical activity or commuting physical activity, this difference was not
statistically significant in the fully adjusted model (Table 4). Further adjusting for
personality traits slightly increased the difference in yearly frequency [-48.1% (95%
CI: -64.8, -23.6)], total time [-60.5% (95% CI: -77.7, -30.2)], total volume [-54.9%
(95% CI: -71.6, -28.4)] and EE [-68.4% (95% CI: -84.0, -37.2)] of conditioning
LTPA.
No significant difference in physical activity was present between those 19
VLBW participants with a history of BPD and the remaining 75 VLBW participants
without BPD. Furthermore, all results remained similar after we re-analysed the data
after excluding those 16 VLBW and 6 control participants with a history of asthma.
We evaluated the effects of personality on physical activity by incorporating
personality scores one by one into the linear regression model. Extraversion and
agreeableness had a positive correlation and neuroticism a negative correlation with
all dimensions of conditioning LTPA (yearly frequency, total time, total volume and
EE). Openness to experience had a positive correlation and extraversion a negative
correlation with commuting physical activity. The five personality traits did not have
any significant correlations with non-conditioning LTPA. Furthermore,
conscientiousness was not correlated with any measured type of physical activity. A
significant interaction was found between extraversion and birth status on total
volume and EE of conditioning LTPA. No other interactions were found between
personality traits and birth status on conditioning LTPA.
Of the VLBW participants, 35 (37%) were born SGA and 59 (63%) AGA. We
reran all analyses comparing the SGA (Pihkala et al., 1989) and AGA VLBW
participants. No significant differences in physical activity emerged.
Guidelines on physical activity for adults recommend at least 150 min/week of
moderate PA (Physical Activity Guidelines Advisory Committee, 2008; World
Health Organization, 2010). This criteria was fulfilled by 55.3% of VLBW and
57.4% of control participants (p = 0.77 and p = 0.73, adjusted, unpublished results).
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Physical activity, nutrition
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disease
Table 4. Mean difference (%) and 95% CI between VLBW (n = 94) and control (n = 101) participants.
Frequency
(times/year)
Total time
(min/year)
Total volume
(METh/yeara)
Energy
expenditure
(kcal/year)
-38.5 (-58.9, -7.7)c
-47.4 (-71.2, -4.1)c
-44.3 (-65.8, -9.2)c
-55.9 (-78.6, -9.4)c
Conditioning leisuretime physical activity
-4.5 (-34.5, 39.6)
-4.3 (-49.2, 79.9)
-8.4(-41.3, 43.2)
-12.1 (-57.0, 79.9)
Non-conditioning
leisure-time physical
activity
6.2 (-48.7, 119.8)
16.9 (-60.5, 246.7)
6.7 (-56.6, 162.4)
14.3 (-68.2, 314.0)
Commuting physical
activity
-34.7 (-58.0, 1.9)
-8.8 (-21.3, 5.7)
-9.2 (-21.7, 5.4)
-55.3 (-80.1,0.5)
Vigorous physical
activityb
a
MET x hours of physical activity / year (MET = metabolic equivalents; ratio of metabolic rate during exercise and estimated metabolic rate
at rest; 1 MET corresponds to energy expenditure of approximately 1 kcal/ kg x hour)
b
physical activity with MET ≥ 5
c
p < 0.05
Predicted by linear regression and adjusted for age, sex, body mass index, daily smoking of the participant and highest education of either
parent.
Results
5.3 Lack of difference in physical activity by objective
measurement in VLBW adults and controls (Study II)
Both VLBW and control participants spent the majority of the day in sedentary
activity (521 min/day vs. 508 min/day, p for difference 0.6). Time spent in sedentary
or moderate activity was not significantly different between VLBW and control
groups (Table 5). Neither did daily MVPA significantly differ between groups (37
min vs. 45 min, p = 0.4). The VLBW participants undertook slightly less vigorous
physical activity [-5.8 min/day (95% CI: -13.1, 1.5)] and MVPA [-8.3 min/day (95%
CI:-25.7, 9.1)], although these differences did not attain statistical significance.
As expected, daily physical activity varied with season. During wintertime both
groups were least active. Daily total physical activity was not significantly different
between VLBW and control participants [-18.9 cpm (95% CI: -77.3, 39.5)] after
adjustment for age, sex, season, BMI, daily smoking and highest parental education
(Table 5). No significant difference between groups was seen in physical activity
when analysed separately for weekdays [-15.5 cpm (95% CI: -76.8, 45.9)] and
weekends [-40.4 cpm (95% CI: -109.3, 28.6)] (Table 5).
5.4 Unhealthier dietary intake in VLBW adults (Study III)
We found lower intakes of all milk products, low-fat (≤2%) dairy products and
vegetables, fruits and berries in VLBW adults than in controls. High-fat (> 2%)
dairy product intake and intake of pasta, potatoes and rice were not significantly
different in the fully adjusted model. Consumption of meat, fish, fat spreads, sugared
beverages, bread, cereals, sugar and sweets was similar in both groups. Detailed
information on food intake is shown in Table II, Study III (Kaseva et al., 2013).
The macro- and micronutrient intakes of participants are shown in detail in Table
6. For comparison, the new 2014 Finnish nutrition recommendations are also
provided (National Nutrition Council, 2014). In brief, no significant differences
were seen in carbohydrate, protein or fat intakes between VLBW and control
participants. VLBW participants showed lower intakes of calcium, vitamin D,
vitamin C, zinc, iodide and magnesium. The acquisition of polyunsaturated fatty
acids was higher in VLBW participants, while the daily intake of cholesterol was
lower in VLBW participants [189 mg (74) vs. 227 mg (105), p = 0.002]. The daily
cholesterol dose recommended by the Finnish Heart Association is ≤ 300 mg, while
the National Nutrition Council no longer provides daily recommendations on intake
of cholesterol. In both VLBW and control participants, intakes of carbohydrates,
protein and fat were within recommended daily levels (National Nutrition Council,
2014). Daily intakes of sucrose, saturated and transfatty acids and salt were higher
than recommended, while the intakes of fiber, magnesium, vitamin D and folate
were lower than recommended in both groups (National Nutrition Council, 2014).
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Results
The daily intake of polysaturated fatty acids was slightly lower than recommended
in controls only.
Of the VLBW participants, 54 (36%) were born SGA (Pihkala et al., 1989). In
line with our previous studies, we reran all analyses comparing the SGA and AGA
VLBW participants. The daily intake of cholesterol was higher in SGA participants
[204 (80) mg vs. 180 (70) mg, p = 0.03)]. No other significant differences in macroor micronutrient intake emerged.
Table 5. Daily physical activity presented as mean differences (95% CI) between
VLBW and control participants. Predicted by linear regression and adjusted for
covariates in models 1 and 2.
Physical activity
Model 1
-16.5 (-77.5, 44.5)
Daily physical activity, mean cpm
Daily physical activity on weekdays
only, mean cpm
Daily physical activity on weekends
only, mean cpm
Sedentary time, min/day
Moderate physical activity, min/day
Moderate-to-vigorous physical
activity, min/day
Vigorous physical activity, min/day
Model 2
-18.9 (-77.3, 39.5)
Model 1
-15.4 (-79.3, 48.6)
Model 2
-15.5 (-76.8, 45.9)
Model 1
-31.8 (-98.5, 34.9)
Model 2
-40.4 (-109.3, 28.6)
Model 1
16.4 (-39.4, 72.2)
Model 2
14.1 (-40.4, 68.5)
Model 1
-1.6 (-13.5, 10.4)
Model 2
-2.5 (-14.6, 9.6)
Model 1
-6.9 (-24.2, 10.4)
Model 2
-8.3 (-25.7, 9.1)
Model 1
-5.3 (-12.6, 2.0)
Model 2
-5.8 (-13.1, 1.5)
Model 1: adjusted for age, sex and season
Model 2: adjusted for age, sex, season, body mass index, daily smoking of the
participant and highest education of either parent.
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Physical activity, nutrition
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Results
Table 6. Daily nutrient intake in preterm-born VLBW young adults and term-born controls.
Recommended daily values for the adult Finnish population are also shown.
Total energy, kcal
Alcohol, E%
Macronutrients
Carbohydrates, E%
Sucrose, E%
Fiber, g
Fiber, g/1000kcal
Protein, E%
Fat, E%
Saturated and
transfatty acids, E%
Monounsaturated
fatty acids, E%
Polyunsaturated fatty
acids, E%
Essential fatty
acids, E%
α-Linolenic
fatty acid (FA
18:3 n-3), E%
Micronutrients
Calcium, mg
Salt (NaCl), g
Iron, mg
Zinc, mg
Iodide, µg
Magnesium, mg
Vitamin D, µg
Vitamin A, µg
Vitamin C, mg
Thiamin, mg
Folate, µg
VLBW
Term
(n = 151 )
(n = 156 )
Mean (SD)
1800 (563)
1994 (613)
3.0 (6.5)
3.2 (4.9)
Recommended
daily intake a
P
―
≤5
0.2
0.6
46.3 (7.4)
10.5 (5.2)
47.2 (8.4)
10.5 (5.1)
45-60
< 10
0.3
0.8
14.6 (8.1)
8.4 (4.4)
16.1 (3.8)
34.7 (7.0)
13.4 (3.9)
16.3 (7.1)
8.7 (4.1)
16.2 (3.9)
33.4 (7.1)
13.6 (4.0)
25-35
12.6
10-20
25-40
< 10
0.3
0.3
0.2
0.2
0.8
11.7 (2.9)
11.1 (2.8)
10-20
0.1
5.3 (1.8)
4.8 (1.6)
5-10
0.04
4.3 (1.5)
4.0 (1.5)
≥3
0.01
0.9 (.4)
0.8 (.3)
0.5
0.007
858 (389)
1080 (514)
800 / 900 b
6.2 (2.2)
6.9 (2.5)
<5
9.8 (3.4)
11.1 (5.2)
9 / 15 c
9.6 (3.1)
10.9 (4.9)
9/7
195 (71)
228 (98)
150
278 (100)
315 (103)
350/ 280 c
3.7 (2.6)
4.4 (3.6)
10
771 (986)
726 (368)
900 / 700 c
76 (66)
88 (62)
75
1.0 (.5)
1.2 (.6)
1.4/1.1 c
189 (87)
211 (74)
300 / 400 c
Footnotes are on the page following Table 6.
58
<0.001
0.2
0.2
0.02
0.004
0.02
0.02
0.3
0.04
0.08
0.09
Physical activity, nutrition
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determinants of health and
disease
Results
P-values for the difference in means obtained by linear regression and adjusted for
age, sex, body mass index, height, living at parental home, daily smoking and
highest parental education
a
Finnish nutrition recommendations for ages 18-30 years (National Nutrition
Council, 2014)
b
ages 21-30 years / 18-20 years
c
men / women
5.5 Blunted hypothalamic-pituitary-adrenal axis and insulin
response to stress in VLBW adults (Study IV)
As we found no gender differences between the effects of preterm birth at VLBW on
stress response measured as changes in cortisol, ACTH, glucose and insulin
concentrations, these results are reported with men and women combined (p for
interaction sex*birth status > 0.3 and time* sex*birth status > 0.4). Overall, ACTH
and cortisol reactivities to TSST were lower in VLBW participants; this result was
statistically significant for plasma cortisol only (Table 7). Similarly, no significant
differences were found in overall glucose or insulin reactivity to TSST. Results of
the mixed effects model are shown in Table 7.
Table 7. Mixed model results are presented as mean differences (95% CI). The analyses
are adjusted for age, sex, BMI, hormonal contraception use, menstrual cycle phase, time
of day and highest parental education.
Mean difference
(95% CI)
P for
difference
P for
interaction
time*birth
status
Plasma ACTH
-24.2 % (-43.8 to 2.4)
0.07
0.5
Plasma cortisol
-17.2 % (-28.9 to -3.5)
0.02
0.02
Salivary cortisol
-15.0 % (95% CI; -33.4 to 8.4)
0.2
0.04
Plasma glucose
-2.8 % (-6.2 to 0.8)
0.1
0.9
Plasma insulin
-6.8 % (-28.5 to 21.5)
0.6
0.2
Plasma adrenalin
-3.0 % (-15.7 to 9.6)
0.6
0.07
Plasma noradrenalin
-19.1 % (-42.5 to 4.4)
0.1
0.4
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Physical activity, nutrition
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Results
Baseline concentrations, peak after stress, increments and AUCg of the measured
biomarkers are shown in Table 8. In line with the lower overall plasma cortisol
reactivity to TSST in VLBW participants, also the cortisol peak after stress was
lower in the VLBW group (Table 8). Furthermore, the VLBW participants had on
average a lower increase in plasma and salivary cortisol (p for time*birth status 0.02
and 0.04, respectively).
Of the VLBW participants, 23 (43%) were born SGA (Pihkala et al., 1989). With
a mixed model, the responses to TSST in all overall concentrations (cortisol, ACTH,
glucose and insulin) were similar in SGA and AGA participants. Glucose and
insulin responsiveness after TSST was higher in the SGA participants (p for the
interaction time*SGA status < 0.001 for glucose and 0.01 for insulin).
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Physical activity, nutrition
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Results
Table 8. ACTH, cortisol, glucose, insulin, A and NA concentrations of participants.
Mean a (SD b)
VLBW
Term
Pc
Baseline concentrations
ACTH, pmol/l
3.5 (1.8)
3.6 (2.5)
.6
Plasma cortisol, nmol/l
325.3 (1.4)
302.3 (1.8)
.5
Salivary cortisol, nmol/l
7.0 (1.7)
6.1 (2.0)
1.0
Insulin, mU/l
6.8 (2.1)
7.3 (2.1)
.7
Glucose, mmol/l
4.6 (1.1)
4.7 (1.1)
.3
Adrenalin, pmol/l
25.6 (1.4)
24.9 (1.4)
.7
Noradrenalin, pmol/l
178.1 (1.6)
180.4 (1.5)
.6
Peak after stress
ACTH, pmol/l
8.5 (2.7)
11.3 (2.7)
.3
Plasma cortisol, nmol/l
471.3 (1.5)
509.4 (1.5)
.03
Salivary cortisol, nmol/l
12.7 (1.9)
13.7 (2.0)
.3
Insulin, mU/l
9.7 (1.9)
11.5 (1.9)
.6
Glucose, mmol/l
5.0 (1.1)
5.1 (1.1)
.2
Adrenalin, pmol/l
40.9 (1.8)
40.9 (1.6)
.2
Noradrenalin, pmol/l
461.3 (1.6)
494.5 (1.5)
.09
Increments d
ACTH
2.5 (1.9)
3.1 (2.6)
.5
Plasma cortisol
1.4 (1.4)
1.7 (1.5)
.1
Salivary cortisol
1.7 (1.9)
2.3 (2.2)
.2
Insulin
1.3 (1.7)
1.5 (1.5)
.08
Glucose
1.1 (1.1)
1.1 (1.1)
.3
Adrenalin
1.6 (1.6)
1.6 (1.6)
.3
Noradrenalin
2.7 (1.4)
2.8 (1.4)
.2
Time-weighted area under
curve e
ACTH
5.9 (1.9)
7.8 (2.2)
.2
Plasma cortisol
349.4 (1.4)
352.8 (1.6)
.2
Salivary cortisol
7.5 (1.6)
8.4 (1.9)
.1
Insulin
9.7 (2.0)
12.0 (2.3)
.6
Glucose
6.3 (1.1)
6.6 (1.1)
.3
Adrenalin
30.1 (1.5)
28.3 (1.4)
.4
Noradrenalin
328.1 (1.5)
365.0 (1.4)
.2
a
th
Geometric mean, denotes the n root of the product of n individual values
b
Geometric standard deviation, denotes the relative increase in a variable corresponding to
one standard deviation unit change in the logarithm of the variable
c
All analyses are adjusted for age, sex, BMI, hormonal contraception, menstrual cycle
phase, time of day and highest parental education
d
Log (peak after stress/baseline value)
e
Area under the curve with respect to ground (above zero)
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Results
5.6 Similar or lower sympathetic-adrenal-medullary system
response to stress in VLBW adults (Study V)
Overall A [-3.0% (-15.7 to 9.6)] and NA [-19.1% (-42.5 to 4.4)] reactivity to TSST
did not significantly differ between VLBW and control participants (Table 7). In
line with Section 5.5, which presents results of Study IV, these results are shown
after additionally adjusting for menstrual cycle phase (unpublished results). The
effects of preterm birth at VLBW on A and NA stress response were similar in both
sexes. P-values for interaction terms for A were as follows: sex*birth status 0.36 and
time*sex*birth status 0.41, and for NA 0.48 and 0.92, respectively. We reran all
analyses for men and women separately, and the results remained unchanged
regarding A. For women, we found a significantly lower overall NA reactivity to
TSST [-27.7% (-52.2 to -3.1)]. Among men, this difference was not statistically
significant [-15.1% (-38.9 to 8.8)].
Baseline concentrations, peak after stress, increments and AUCg of A and NA
with men and women combined are shown in Table 8 (unpublished results). In Table
2 of Study V (Kaseva et al., 2014), the results are presented separately for men and
women.
We found no differences in HR at baseline [1.8 beats/min (-7.3 to 10.8)], during
the task [1.6 beats (-11.7 to 14.8)] or in HR task reactivity [0.4 beats/min (-10.1 to
10.8)] between VLBW and control women. Similarly, the mean difference in HR at
baseline [2.6 beats/min (-7.6 to 12.8)], during the task [9.9 beats/min (-4.2 to 24)]
and in HR task reactivity [7.3 beats/min (-2.6 to 16.9)] was not significantly
different in VLBW and control men. The mean values for HR at baseline, during the
task and in HR task reactivity are shown in Table 9.
We also compared those 22 VLBW participants (44%) born SGA (Pihkala et al.,
1989) with the remaining 28 AGA VLBW participants. With a mixed model, the
overall concentrations for A and NA were similar in SGA and AGA VLBW
participants. Also baseline, peak after stress, increment and AUCg for A and NA
were similar.
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Physical activity, nutrition
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Results
Table 9. Changes in heart rate provoked by the Trier Social Stress Test in study
participants.
Characteristic
MEN, mean b (SD c)
Heart rate
(beats/min)
Baseline d
Task e
Reactivity f
VLBW
(n = 22)
Term
(n = 17)
Pa
83.9 (15.3)
101.0 (18.1)
17.2 (12.0)
77.9 (9.4)
92.0 (15.6)
14.1 (11.6)
0.61
0.16
0.13
WOMEN, mean b(SD c)
(n=28)
(n= 22)
Heart rate
(beats/min)
Baseline d
85.1 (12.3)
82.9 (14.3)
0.70
Task e
109.1 (19.1)
106.9 (20.9)
0.81
Reactivity f
24.7 (15.2)
24.0 (16.7)
0.94
a
P for linear regression model, analyses are adjusted for age, BMI, time of day
and highest parental education in men, the same covariates plus hormonal
contraception in women
b
Geometric mean, denotes the nth root of the product of n individual values
c
Geometric standard deviation, denotes the relative increase in a variable
corresponding to one standard deviation unit change in the logarithm of the variable
d
Mean heart rate during a 5-min baseline measurement
e
Mean heart rate during a 10-min task period
f
Mean heart rate during task minus mean heart rate at baseline
5.7 Non-participant analysis
We performed non-participant analyses separately for all studies included in this
thesis (I-V), as different subgroups of the original HeSVA cohort participated in
these five studies.
For Study I, we compared the adult characteristics of the participants (94 VLBW
and 101 controls) with those 72 VLBW and 71 controls who were invited and chose
not to participate or met an exclusion criterion. They had all participated in the first
clinical examination in 2004-2005 (Figure 3), and data on adult measurements were
therefore available. We found no differences in height, BMI or SES, as indicated by
highest parental education (all p-values ≥ 0.1). We further compared perinatal
characteristics between the participants of Study I and the remaining original cohort
with data available (161 VLBW and 213 controls). No significant differences were
found regarding gestational age, birth weight, sex, pre-eclampsia or multiple
pregnancy (all p-values ≥ 0.1).
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Physical activity, nutrition
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Results
In line with the non-participant analyses performed in Study I, we compared the
adult characteristics of the participants of Study II (57 VLBW and 47 controls) with
those 109 VLBW and 125 controls who were invited and chose not to participate.
Since all of them participated in the first clinical examination in 2004-2005 (Figure
3), data on adult measurements were available. We found no differences in height,
BMI or SES (all p-values ≥ 0.2). Daily smoking was more common in nonparticipant VLBW subjects (28% vs. 12%, p = 0.02). In the controls, no difference
in daily smoking was seen (34% vs. 27%, p = 0.4). We further compared perinatal
characteristics between the participants of Study II and the remaining original cohort
with data available (198 VLBW and 267 controls). No significant differences were
found regarding gestational age, birth weight, or the incidence of SGA (Pihkala et
al., 1989), pre-eclampsia or multiple pregnancy (all p-values ≥ 0.4). In both VLBW
and control groups, there were more men among the non-participants (VLBW 48%
vs. 35%, p for difference 0.1 and controls 49% vs. 29%, p = 0.02); this finding was
significant for controls only.
We compared perinatal characteristics of the participants of Study III (151
VLBW and 156 controls) with the remaining original cohort subjects who were
invited to the first clinical examination in 2004-2005 (Figure 3) and were without
food record data (104 VLBW and 158 controls). We separately compared VLBW
and control participants and found no differences in gestational age, birth weight,
type of birth, incidence of pre-eclampsia or multiple pregnancy between the
participants of Study III and those who were not included in this study (all p-values
≥ 0.1). In both VLBW and control groups, there were more men among the nonparticipants (VLBW 54% vs. 40%, p for difference 0.03 and controls 52% vs. 45%,
p = 0.01). Furthermore, maternal smoking was more common among VLBW nonparticipants (32% vs. 17%, p = 0.004).
In Studies IV and V, perinatal and current descriptive characteristics were
compared between the participants of the TSST (54 VLBW and 40 controls) and the
group from which they were recruited (Figure 3), i.e. the remaining cohort subjects
who had previously participated in the clinical examination (112 VLBW and 132
controls). VLBW and control groups were separately compared, and we found no
differences in gestational age, birth weight, sex, incidence of pre-eclampsia, multiple
pregnancy or SGA (Pihkala et al., 1989) between the participants of Studies IV and
V and the group from which they were recruited (all p-values ≥ 0.1). Age, daily
smoking, hormonal contraception use, parental education, adult height and BMI
were also similar between the participants of Studies IV and V and the group from
which they were recruited (all p-values ≥ 0.1).
64
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Discussion
6 Discussion
Preterm birth at VLBW is associated with increased risk factors for adulthood
NCDs; impaired glucose and insulin metabolism (Hofman et al., 2004; Lawlor et al.,
2006; Hovi et al., 2007; Rotteveel et al., 2008; Kaijser et al., 2009; Kajantie et al.,
2010; Pilgaard et al., 2010;; Crump et al., 2011; Smith et al., 2011), higher blood
pressure (de Jong et al., 2012; Parkinson et al., 2013), an atherogenic lipid profile
(Hovi et al., 2013; Parkinson et al., 2013) and lower bone density (Hovi et al., 2009;
Smith et al., 2011) have been reported. These are all factors that may be modified by
lifestyle. In Studies I, II and III, we focused on physical activity and dietary habits of
young VLBW adults.
Individual differences in stress response are to some extent programmed during
the fetal period and early childhood. Studies IV and V assessed psychosocial stress
responses in VLBW adults.
6.1 Physical activity
The most important finding of questionnaire-based Study I was the markedly lower
conditioning LTPA in young adults born at VLBW relative to the term-born controls.
Based on the detailed, validated KIHD questionnaire, we found that the healthy
VLBW adults showed 48% lower yearly frequency, 61% lower total time, 55%
lower total volume and 68% lower EE of conditioning LTPA. This was not
explained by age, sex, BMI, daily smoking, socio-economic status or personality
traits. No significant differences in non-conditioning LTPA or commuting physical
activity were observed. Despite clear differences in conditioning LTPA in Study I,
we were unable to confirm the finding of lower physical activity in a subgroup of
VLBW adults by accelerometer measurement in Study II.
Guidelines on physical activity for adults recommend at least 150 minutes
weekly of moderate intensity physical activity (Physical Activity Guidelines
Advisory Committee, 2008; World Health Organization, 2010). This
recommendation was met by 55% of VLBW and 57% of control participants
(unadjusted p for difference = 0.8 and adjusted p = 0.7, unpublished results). For
comparison, approximately half of the general adult population in Finland meets this
recommendation (Husu et al., 2011). The participants of Study I were healthy young
adults, and thus, a slightly higher physical activity level than in the general adult
population is not surprising.
Earlier studies, by us and others, using self-report methods consistently show
lower sports participation and LTPA in young VLBW adults (Vrijlandt et al., 2006;
Hack et al., 2007; Kajantie et al., 2010). Also ELBW adolecents have reported lower
65
Physical activity, nutrition
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adults born preterm −
determinants of health and
disease
Discussion
physical activity, muscle strength and flexibility (Rogers et al., 2005). In line with
these findings, young ELBW adults reported lower physical efficacy, self-perceived
physical ability and physical self-confidence than controls (Saigal et al., 2007).
Objective measurement of physical activity has not previously been performed in
VLBW adults. Only one study in extremely preterm children has used accelerometry
for objective evaluation of physical activity. This was done in 11-year-old children
using hip-worn accelerometers, and, as in Study II, no difference was found in
physical activity between the 31 former extremely preterm-born subjects and the 30
controls (Welsh et al., 2010). Measuring physical activity and sedentary behaviour is
complex, and self-report probably captures different aspects of overall physical
activity than accelerometry during a relatively short period of measurement.
Personality traits are linked to physical activity. Differences in personality offer
one potential explanation for the lower conditioning LTPA in VLBW adults.
Conscientiousness and extraversion are personality traits associated with higher
levels of physical activity, while neuroticism is related to lower levels of physical
activity (Rhodes and Smith, 2006). Based on earlier published data, VLBW
individuals are more conscientious, agreeable and show less neuroticism and
openness to experience than their peers born at term (Pesonen et al., 2008). Also in
ELBW adults, higher shyness, behavioural inhibition and socialization (a measure of
prosocial behaviour) and lower sociability and emotional well-being have been
reported (Schmidt et al., 2008). However, these differences in personality traits did
not explain the lower conditioning LTPA in VLBW adults that we found in Study I.
Other factors possibly accounting for the lower conditioning LTPA in VLBW
adults may include visual impairment (Evensen et al., 2009), impaired lung function
(Vrijlandt et al., 2006), poor motor coordination (Rogers et al., 2005), lower muscle
strength (Keller et al., 2000; Rogers et al., 2005), lower physical self-confidence
(Hack et al., 2007), lower exercise capacity (Rogers et al., 2005) and lower
perceived physical ability (Saigal et al., 2007), all reported from childhood onwards
in the former preterm-born. However, the participants of Studies I and II were all
healthy VLBW adults. It is therefore not likely that impairment in functioning would
explain the lower conditioning LTPA that we reported in Study I.
Adults born preterm have in some studies shown lower BMI (Hack et al., 2003;
Hovi et al., 2007) and been found to be shorter (Roberts and Cheong, 2014) and
have lower lean body mass (Weiler et al., 2002; Hovi et al., 2007) than controls.
Such differences in body composition may lead to or reflect the lower physical
activity levels reported in preterm-born subjects. This was also seen in Study I.
When we adjusted for lean body mass instead of BMI in the subgroup with data
available, the differences in frequency, total time, total volume and EE of
conditioning LTPA were attenuated.
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Discussion
Specific dietary and lifestyle factors, e.g. low physical activity, alcohol use,
smoking, watching television and sleep (< 6 h or > 8 h), are all factors independently
associated with long-term weight gain. In non-obese populations, gradual weight
gain tends to accumulate over time; in a large study including 120877 men and
women in the United States, the rate was approximately 0.36 kg/year (Mozaffarian
et al., 2011). Even moderate weight gain increases the risk of NCDs, e.g. type 2
diabetes (Chan et al., 1994; Colditz et al., 1995) and CVD (Willett et al., 1995).
Small lifestyle changes have an impact; reducing daily energy intake by 2-3% or
walking an extra 10-15 minutes daily could offset weight gain in approximately 90%
of the population (Zhai et al., 2008). Commonly, weight gain results from a
relatively small energy imbalance or gap; 46-72 kcal/day in children (PlachtaDanielzik et al., 2008) and 45-100 kcal/day in adults (Zhai et al., 2008).
6.2 Diet and nutrition
We observed that young adults born preterm at VLBW consume markedly less milk
products, vegetables, fruits and berries than their term-born counterparts. Lower
daily intakes of vitamin D and calcium were also seen. Lower consumption of
vegetables, fruits and berries reflects an unhealthy diet and poses a risk factor for
CVD (Joshipura et al., 1999; Dauchet et al., 2009; Mirmiran et al., 2009). The
observed differences in dietary habits may to some extent underlie the VLBW
adults’ increased risk factors for NCDs, including CVD, type 2 diabetes and
osteoporosis.
Dietary quality, i.e. the types of foods and beverages consumed, influences
dietary quantity, which affects total calorie intake. Food fat content, energy density
and added sugars are factors to be emphasized when evaluating food and nutrient
intake. Energy intakes of fat, carbohydrates and protein were similar in VLBW and
control groups. High intake of protein, especially animal protein, is associated with
an elevated risk for type 2 diabetes (van Nielen et al., 2014). Thus, maintaining
protein intake at recommended levels is important in a high-risk population such as
VLBW adults. The total daily energy intake was slightly lower in VLBW adults.
After adjusting for body size (BMI and height), daily energy intake was similar to
that of controls. Naturally food consumption reflects body size, and those born
preterm at VLBW tend to be smaller (Roberts and Cheong, 2014).
The difference in daily consumption of vegetables, fruits and berries was 58
g/day, corresponding to 0.3 SDs. The daily recommendation of 500 g/day was not
reached by the VLBW or control participants (183 g vs. 241 g, p = 0.002). However,
this may be of greater importance to the VLBW participants, as they have higher
levels of other risk factors for NCDs. A low consumption of vegetables and fruits is
an important, modifiable risk factor of CVD (Joshipura et al., 1999; Dauchet et al.,
2009; Mirmiran et al., 2009). This is emphasized by the WHO ranking low
67
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disease
Discussion
consumption of fruits and vegetables as the 7th leading risk factor for mortality
(World Health Organization, 2009).
Data are lacking on dietary habits and nutrient intake in VLBW adults, while
associations between birth size and food intake in term-born children and adults
have been reported. In line with our results, term-born adults, aged 56 to 70 years,
showed higher daily intake of fruits and berries with increasing birth weight (83 g
per 1 kg higher birth weight) (Perälä et al., 2012). Those adults with smaller birth
size also had higher intake of fat and lower intake of carbohydrates, sucrose,
fructose and fibre (Perälä et al., 2012). Also in young children born at term, an
inverse association between fat intake and birth weight has been described (Stafford
and Lucas, 1998; Shultis et al., 2005). These data suggest that intrauterine growth
may modify food intake later in life.
We also observed lower daily intake of milk products in VLBW participants. In
the Finnish diet, milk products are a central source of vitamin D and calcium.
Accordingly, the VLBW participants had significantly lower intake of both vitamin
D and calcium than controls. Both protective and predisposing associations have
been reported regarding the impact of dairy product use on the risks for CVD
(Larsson et al., 2009; Goldbohm et al., 2011; Phelan and Kerins, 2011; SoedamahMuthu et al., 2011; Sonestedt et al., 2011; Astrup, 2014). In a recent review,
consumption of dairy products had a beneficial effect on dyslipidaemia, insulin
resistance, blood pressure and body fat (Astrup, 2014). The consumption of high-fat
(> 2%) dairy products, expected to be more harmful for health, was not significantly
different between VLBW and control groups, while intake of low-fat (≤ 2%) dairy
products was lower in VLBW adults. Previously, lower bone mineral density, a risk
factor for osteoporosis, has been reported in the HeSVA cohort’s VLBW
participants (Hovi et al., 2009). This finding of lower bone mineral density may to
some extent be explained by the lower intake of calcium and vitamin D relative to
the controls. Low levels of vitamin D are also associated with increased risk for
diabetes (Forouhi et al., 2012), as is preterm birth (Hofman et al., 2004; Lawlor et al.,
2006; Hovi et al., 2007; Rotteveel et al., 2008; Kaijser et al., 2009; Kajantie et al.,
2010; Pilgaard et al., 2010; Crump et al., 2011; Smith et al., 2011). Furthermore, low
intake of calcium is associated with higher blood pressure at least in children
(Gillman et al., 1992; Gillman et al., 1995).
In previous studies of VLBW adults, one of the most consistent findings is high
blood pressure (Irving et al., 2000; Doyle et al., 2003; Hacket al., 2005; Pyhälä et al.,
2009; Hovi et al., 2010; Norman, 2010; de Jong et al., 2012; Parkinson et al., 2013).
High salt intake is directly related to higher blood pressure in infants (Pomeranz et
al., 2002), children (Simons-Morton and Obarzanek, 1997) and adults (Chobanian
and Hill, 2000; Kaplan, 2000). Furthermore, high blood pressure is a known risk
factor for cardiovascular complications such as stroke and coronary heart disease
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and stress response in young
adults born preterm −
determinants of health and
disease
Discussion
(Campbell et al., 2011). Thus, our finding of similar salt intake in VLBW and
control groups when adjusting for body size (BMI and height) is especially
important, as already young VLBW adults tend have higher blood pressure than
term-born controls (de Jong et al., 2012; Parkinson et al., 2013). The effects of
dietary intake may also be different in individuals born at low birth weight. For
example, Perälä et al. (2011) found that individuals born at a low birth weight were
more sensitive to the blood pressure-raising effect of salt as elderly adults.
Compared with controls, the VLBW group had a higher intake of essential fatty
acids, including α-linoleic fatty acid. However, seafood intake, which is a central
source of omega-3 polyunsaturated fatty acids and vitamins, was similar in both
groups, as was intake of other fatty acids. The intake of fibre was lower than
recommended in both VLBW and control groups. Obtaining a satisfactory dietary
intake may be more important to VLBW individuals since they are at greater risk for
NCDs.
A possible explanation for the differences in dietary habits and nutrient intake
between VLBW and control groups may be found in the eating difficulties that
VLBW individuals often experience in early infancy and childhood (Samara et al.,
2010). Evidence also suggests that inadequate nutrition pre- or postnatally may
program individual food preferences later in life. Epidemiological data shows that
exposure to the Dutch famine in 1944-1945 during early pregnancy (defined as first
16 weeks of gestation) is associated with a preference for fatty foods in the offspring
later in life (Lussana et al., 2008). It is likely that during postnatal care in 19781985, when the participants of the HeSVA cohort were born, NICU care offered
inadequate nutrition. In addition to early programming of food preferences (Lussana
et al., 2008; Barbieri et al., 2009; Portella et al., 2012), differences in oral motor
abilities, socio-economic factors and family eating habits may play a role in later
dietary habits (Portella et al., 2012). The effects of early under-nutrition on adult diet
could also in part be mediated through psychological functioning; early childhood
under-nutrition predicts depressive symptoms (Galler et al., 2010), anxiety and
lower self-esteem (Walker et al., 2007), all of which could be related to an unhealthy
diet. Another possible explanation for our findings is that low birth weight may
affect appetite. Parents might also feed their infants more to compensate for the low
birth weight.
Other possible mechanisms behind food choices or food preferences include
prenatal and early postnatal flavour experiences. Maternal diet during pregnancy has
been shown to affect the infant’s postnatal preference for the same diet (Mennella et
al., 2001). Culture-specific flavour preferences are likely initiated this way in early
life (Mennella et al., 2001). Furthermore, both salty taste acceptance and salt intake
have been demonstrated to be inversely associated with birth weight in small
69
Physical activity, nutrition
and stress response in young
adults born preterm −
determinants of health and
disease
Discussion
children (Stein et al., 2006). That study included only participants with a birth
weight > 2500 g.
6.3 Stress response
We measured HPAA (Study IV) and SAM (Study V) responses to psychosocial
stress and, contrary to our hypothesis, found that healthy VLBW adults had lower
HPAA response after stress. This was followed by a lower insulin response. Also
contrary to our hypothesis, we found no evidence of higher SAM response to
psychosocial stress in VLBW adults. In VLBW women, the rise in NA was lower
than in controls. The results were not explained by age, sex (Study IV), BMI,
hormonal contraception, menstrual cycle phase, timing of TSST or childhood socioeconomic status.
Both animal (Weinstock, 2001; Welberg and Seckl, 2001) and human studies
(Kajantie and Räikkönen, 2010) give us reason to believe that stressful events preand postnatally influence later stress response. Preterm birth at VLBW is likely to
involve both pre- and postnatal stress. The mechanisms involved in programming of
stress response are not well understood. Our finding of lower HPAA response to
stress in young VLBW adults supports the concept of programming of the HPAA.
The lack of difference in HPAA response after TSST between AGA and SGA
VLBW subgroups indicates that our finding of lower HPAA response was caused by
immaturity rather than intrauterine growth retardation. Although HPAA stress
response after TSST in VLBW adults has not been studied previously, data on
preterm children have been published. In line with our results, a study of 18 children
aged 8-12 years with any degree of prematurity and 18 term-born controls showed
attenuated salivary cortisol responses to TSST-C (Buske-Kirschbaum et al., 2007).
This finding did not, however, reach statistical significance, possibly due to the
small sample size. Also in 4-month-old preterm infants, a lower cortisol response to
immunization than in term-born controls has been reported (Grunau et al., 2010).
Based on these data, preterm birth can be speculated to be associated with blunted
HPAA response to stress. Data on lower salivary cortisol levels in extremely
preterm infants at 3 months of corrected age compared with term infants have also
been published (Grunau et al., 2007). In that study, a shift to higher cortisol levels in
the former preterm children was seen at 8 and 18 months.
Somewhat different results on the effects of birth weight and HPAA response to
TSST have been reported after term pregnancy relative to preterm birth (Kajantie
and Räikkönen, 2010). Lower birth weight has been found to be associated with
higher salivary cortisol response after TSST in both 7- to 9-year-old boys (Jones et
al., 2006) and young adult male twins (Wüst et al., 2005). In line with our results,
TSST provoked lower cortisol and ACTH responses in adults aged 60-70 years who
were born with lower birth weight, mostly after term birth (Kajantie et al., 2007).
70
Physical activity, nutrition
and stress response in young
adults born preterm −
determinants of health and
disease
Discussion
These mixed results of the effects of birth weight on HPAA function may to some
extent be explained by a variety of mechanisms causing low birth weight.
In addition to the blunted HPAA response to stress in VLBW adults, we found
that insulin concentrations increased 1.3-fold in VLBW participants, while the
increase in the term group was 1.5-fold. This in in contrast to the insulin response
after a 75-g oral glucose load, which was higher in the VLBW participants: 6.1-fold
versus 5.1-fold in controls (Hovi et al., 2007). A lower rise in insulin after stress
may be a potential protective factor. There is a dearth of knowledge about the effects
of psychosocial stress on glucose and insulin responses in individuals born preterm.
Thus, the finding of a blunted insulin response to stress in former preterm
individuals is new.
Early life predictors of the SAM stress response in later life have been
investigated previously (Kajantie and Räikkönen, 2010). After immunization, 4month-old preterm-born and term-born infants showed similar HR responses
(Grunau et al., 2010). This is in line with our results in Study V in which HR
responses to TSST did not differ between VLBW and control groups. Also
consistent with our results, HR responses after TSST-C in 8- to 12-year-old children
were found to be similar in preterm and control groups (Buske-Kirschbaum et al.,
2007). In contrast to our findings, higher urinary catecholamines and higher HR at
both rest and after mental stress were seen in former preterm children at 9-10 years
of age (Johansson et al., 2007). This may be due to methodological differences, as in
that study a single morning urine sample was collected for evaluation of
catecholamines the day after the stress test. The analysis of one urinary sample
provides a summary measure of stress response, while we used repeated
measurements at the time of performing the TSST. Repeated measurements at the
time of undergoing stress can be assumed to yield more precise data in terms of
acute stress response, as other incidents occurring later the same day will not affect
these measurements.
It is noteworthy that data on the effects of preterm birth on SAM stress response
in VLBW adults is scarce. Provoked by TSST, the same VLBW participants, in who
we found similar A and HR responses and lower NA after stress in the VLBW
women (Study V), Pyhälä et al. (2009) reported higher diastolic blood pressure
responses than controls. A higher blood pressure response to stress, not seen in HR,
A or NA levels, may in part be explained by differences in arterial stiffness,
endothelial function or renal function. Also somewhat higher HR was seen in the
HeSVA cohort VLBW participants in conjunction with a clinic visit (Hovi et al.,
2007). This may be due to different mechanisms underlying HR and catecholamine
stress responses. The circulating A and NA levels reflect release from both the
adrenal medulla and noradrenergic nerve terminals, while HR reacts to sympathetic
and parasympathetic nervous system activity. The finding of lower NA after stress
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Physical activity, nutrition
and stress response in young
adults born preterm −
determinants of health and
disease
Discussion
in VLBW women could be a protective characteristic for CVD in this high-risk
population (Parkinson et al., 2013). SAM overactivity is associated with
hypertension (Julius, 1996), and increased HR predicts cardiovascular mortality
(Palatini et al., 2006).
Abnormal stress responses, mediated by both the HPAA and SAM pathways, are
also associated with increased risk for NCDs, including type 2 diabetes (McEwen,
1998), CVD (McEwen, 1998), depression (Bjorntorp, 1996), metabolic syndrome
(Chrousos, 2000), hypertension (Treiber et al., 2003; Matthews et al., 2004; Chida
and Steptoe, 2010) and carotid atherosclerosis (Treiber et al., 2003; Matthews et al.,
2006). It is not known whether these changes in stress response can be modified
later in life.
6.4 Strengths and limitations
An important strength of the study population (the HeSVA cohort) is the age-, sexand birth hospital-matched control participants. The study sample is relatively small,
but still comparable to or larger than previous studies with related outcomes on
physical activity (Keller et al., 2000; Rogers et al., 2005; Vrijlandt et al., 2006;
Saigal et al., 2007; Welsh et al., 2010) and stress response (Wüst et al., 2005; Jones
et al., 2006; Buske-Kirschbaum et al., 2007; Johansson et al., 2007; Brummelte et al.,
2011). The sample size in Study III on dietary habits can be considered relatively
large, as well. We were also able to adjust for the most important confounding
factors in the five different studies of this thesis. One limitation of the HeSVA
cohort is that the participants were born in the late 1970s and 1980s. The current
treatment of prematurely born infants differs from when the HeSVA participants
received neonatal intensive care. Today, we have more to offer, which hopefully will
lead to a healthier next generation of VLBW individuals. Thus, all of the results in
this thesis may not be directly applied to the present.
Subgroups of the original cohort were used in Studies I-V. It is possible that
these subgroups are not representative of the original cohort. This is unlikely since
non-participant analyses showed only slight differences between participants and
non-participants in Studies II and III. There were more men among the nonparticipants in both VLBW and control groups in Studies II and III. Daily smoking
was more common among the non-participant VLBW subjects in Study II. Maternal
smoking during pregnancy was more common among VLBW non-participants in
Study III. No significant differences regarding perinatal or current characteristics
were found in non-participant analyses for Studies I, IV and V.
Data on physical activity (Study I) and dietary habits (Study III) were gathered
by self-report. A limitation of all data obtained by self-report is potential bias arising
from underreporting or overreporting. Questionnaires concerning physical activity
have been found to be fairly reliable and not all types of physical activity can be
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and stress response in young
adults born preterm −
determinants of health and
disease
Discussion
measured by objective measurement. The reproducibility and validity of the detailed
KIHD questionnaire (Study I) have been confirmed in different populations in
Finland (Lakka and Salonen, 1992; Lakka and Salonen, 1993), Belgium (De Backer
et al., 1981) and the United States (Jacobs et al., 1993). Using one of the most
reliable self-report methods for estimating dietary intake, the prospective 3-day food
record, is a major strength (Prentice et al., 2011) of Study III. In addition, the 3-day
food record was combined with use of a picture booklet for estimating portion sizes,
as this improves the accuracy of self-reported food intake (Ovaskainen et al., 2008).
People tend to underestimate their food intake and this was probably also the case in
Study III, as overall energy intake was somewhat low in both VLBW and control
groups. Also of importance, the intake of rarely consumed food items, such as fish,
might be overestimated or underestimated when only a 3-day period is assessed.
Another weakness of the 3-day food record is the difficulty in gathering reliable
information on micronutrient intake.
Former preterm-born individuals may also respond differently to questionnaires
than those born at term. Preterm-born individuals show a tendency to give more
false answers to appear more socially acceptable (Allin et al., 2006). However,
regarding physical activity as well as dietary habits, the VLBW group showed
findings that may be perceived as less acceptable outcomes, i.e. they were less
physically active and their diets were unhealthier.
Furthermore, as the aim of this thesis was to explore the effects of preterm birth
on healthy VLBW individuals, the full extent of physical inactivity in all individuals
born at VLBW may be underestimated since having an impairment affecting
physical activity was an exclusion criterion in Studies I and II.
In Study II, accelerometer measurement was used to objectively measure
physical activity and sedentary behaviour. For this, the wrist-worn Actiwatch AW4
model (Cambridge Neurotechnology Ltd., UK), validated for measuring physical
activity in free-living people (Heil et al., 2009), was used. However, there are some
limitations in the use of accelerometers for measuring physical activity. The
accelerometer must be removed when showering, swimming or bathing, and the
wrist-worn model is not well suited for all types of physical activity, e.g. cycling is
not captured reliably by this model. However, participant compliance is important
and the wrist-worn accelerometer is easier to wear than the hip-worn models.
Comparable data can be obtained by both models (Heil et al., 2009). All participants
were asked to wear the accelerometer on the non-dominant wrist continuously and to
record the bedtimes and awakening times by both pressing an event marker and
simultaneously recording this in a sleep log. For inclusion in the analyses, a
minimum of 600 min/day of wearing the accelerometer and at least 3 days of
recording were required. A 3-day record of physical activity is relatively short, and
this may lead to overestimating or underestimating an individual’s physical activity.
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adults born preterm −
determinants of health and
disease
Discussion
Furthermore, for evaluating the intensity of physical activity, we had to use cut-off
points validated for 8- to 10-year-old children (Ekblom et al., 2012) since no
validation studies have been done in adults. Children tend to be more physically
active than adults, and therefore, using cut-off points validated for children is
suboptimal. Also of note is that we did not assess cardiorespiratory fitness in any of
the studies. Lower cardiorespiratory fitness would be expected with lower physical
activity levels. A further limitation of Study II is the relatively small sample size. In
sum, measuring physical activity as well as sedentary behaviour is complex, and
regardless of the above limitations, a major strength of Study II is the use of an
objective measure.
A major strength of Studies IV and V, regarding HPAA and SAM stress
response, is the use of a standardized stressor, the TSST (Kirschbaum et al., 1993).
We used a wide range of measurements, including repeated salivary and blood
samples for the different biomarkers and recording of HR. We were also able to
adjust for important confounders, including menstrual cycle phase, use of hormonal
contraception and time of day when the TSST was performed. The participants
underwent TSST between 10:45 and 16:10; for this time interval, the responses to
TSST are regarded as comparable (Kudielka et al., 2004). A limitation of these two
studies evaluating stress response is the small sample size, which may diminish the
possibilities of finding associations in subgroups, i.e. when evaluating men and
women separately. This should be noted when interpreting the lack of sex
interactions in HPAA / cortisol response to stress, as well as the lower NA response
in VLBW women. Sex differences have previously been reported in several HPAA
stress response studies (Kajantie and Phillips, 2006). The TSST is a labour-intensive
test and our group sizes are similar or larger than in previous studies in the field
(Wüst et al., 2005; Jones et al., 2006; Buske-Kirschbaum et al., 2007; Johansson et
al., 2007; Brummelte et al., 2011). Preterm birth is caused by a variety of factors,
with varying impacts on adult HPAA or SAM stress response. Of the VLBW
participants in Studies IV and V, some were born SGA (43% in Study IV, 44% in
Study V) from pre-eclamptic (IV: 22%, V: 26%) or multiple pregnancy (IV: 15%, V:
16%). This reflects the reality of the VLBW population and may limit the extent to
which these data can be generalized to all individuals born preterm. However, in
previous studies the cardiovascular outcomes of VLBW infants have been quite
similar regardless of maternal pregnancy conditions (Kajantie and Hovi, 2014). This
suggests that the programming of cardiovascular outcomes may occur after birth.
The time spent in neonatal intensive care, with the painful and stressful treatments
neonates routinely undergo (Carbajal et al., 2008), could represent a time period
when programming of the stress response occurs. Lifetime stressful experiences and
current perceived stress are factors affecting stress response that we were unable to
control for in Studies IV and V.
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Physical activity, nutrition
and stress response in young
adults born preterm −
determinants of health and
disease
Discussion
6.5 Implications for future research
The finding of lower conditioning LTPA in VLBW adults, based on self-report,
offers a potential mechanism linking preterm birth and increased risk factors for
chronic NCDs later in life. This finding was not confirmed in a subgroup of VLBW
participants with objective measurement (accelerometry). To date, no other studies
have objectively assessed physical activity in VLBW adults. Thus, larger studies
using objective measurement are warranted to confirm this hypothesis. Also
evaluation of cardiorespiratory fitness in VLBW adults, not only studies focusing on
behaviour, would give additional information on individual risk for NCDs.
Our finding of unhealthier dietary habits in a specific risk group, VLBW adults,
is important. However, the 3-day food record is suboptimal for gathering detailed
data on, for example, micronutrient, salt and vitamin intake. To obtain more reliable
data, studies using objective measurement of nutrient intake should be conducted.
The blunted HPAA response to stress in VLBW adults in a laboratory setting
strengthens the association between early life events and programming of later stress
response. Of relevance would be additional information on stress responses in
everyday life. Are the stress responses different also in real life, not only in
laboratory settings? Another important direction for future research is the lower NA
in VLBW women after stress. Our study group size is somewhat small for
evaluating sex differences in stress response. If replicated, this may be a protective
finding regarding hypertension and CVD.
Finally, it would be very interesting to combine the topics of Studies I-V, and
measure cortisol responses after a) intense physical exercise and b) a nutritional
challenge.
75
Physical activity, nutrition
and stress response in young
adults born preterm −
determinants of health and
disease
Conclusions
7 Conclusions
1. Based on self-report, healthy adults born preterm at VLBW undertake
markedly less conditioning LTPA than controls born at term. The lower
conditioning LTPA is not explained by differences in personality
characteristics. This difference in physical activity was not captured by
accelerometer measurement. Accelerometer measurement probably captures
different aspects of physical activity than self-report; all types of physical
activity cannot be measured by accelerometer measurement.
2. VLBW adults have unhealthier dietary habits than controls, with lower
intake of fruits, vegetables, berries and dairy products. This is accompanied
by lower intake of calcium and vitamin D.
3. A sedentary lifestyle and an unhealthy diet are modifiable features and offer
a target for prevention of chronic NCDs, including osteoporosis, diabetes
and CVD, in this high-risk population.
4. Our findings suggest that VLBW adults have a blunted HPAA response to
psychosocial stress. This is accompanied by a lower insulin response.
5. We found no evidence of higher SAM stress response in VLBW adults. If
replicated, our result of lower NA in VLBW women may be a protective
finding.
6. Our results on stress response in VLBW adults reinforce previous
suggestions that stress response is programmed early in life.
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Physical activity, nutrition
and stress response in young
adults born preterm −
determinants of health and
disease
Acknowledgements
8 Acknowledgements
This study was carried out in 2010-2014 at the Department of Chronic Disease
Prevention, Diabetes Prevention Unit, National Institute for Health and Welfare
(THL), in collaboration with Children’s Hospital, Helsinki University Central
Hospital. I express my gratitude to the Director of THL, Professor Juhani Eskola, the
former head of THL, Professor Pekka Puska, the Assistant Director General,
Professor Erkki Vartiainen, the Head of the Department of Chronic Disease
Prevention, Professor Markku Peltonen, and the Head of the Diabetes Prevention
Unit, Docent Jaana Lindström, for providing excellent research facilities.
I sincerely thank the Director of the Department of Gynaecology and Paediatrics
of Helsinki University Central Hospital, Docent Jari Petäjä, and the Chair of
Children’s Hospital, University of Helsinki, Professor Mikael Knip, for research
infrastructure. I acknowledge the Head of the Institute of Clinical Medicine,
University of Helsinki, and the former Head of the Paediatric Graduate School
(TKO) at Children’s Hospital, Professor Markku Heikinheimo, and the Head of the
Paediatric Graduate School (TKO) at the Children’s Hospital, Docent Jussi
Merenmies.
My deepest gratitude is owed to my two supervisors, Docent Eero Kajantie and
Docent Karoliina Wehkalampi, for providing me with endless support. No question
was too trivial, no issue too insignificant; I could always count on your help and
guidance. You have encouraged me along the way and you have shared with me
something precious – your time and knowledge.
I warmly thank my reviewers, Docent Laure Morin-Papunen and Docent Marja
Ojaniemi, for critical and constructive comments and Professor Mikko Hallman for
accepting the role of Opponent at my thesis defense. I also thank Professor Sture
Andersson for agreeing to act as Custos.
I am indebted to my follow-up group, Docent Iiris Hovatta and Päivi Luukkainen,
for providing guidance and support during the past years.
Thank you, Docent Petteri Hovi, for both statistical advice and all other
assistance. As an exemplary researcher, you have set the bar high.
I am grateful to all of my co-authors for valuable comments. Without Docent
Anna-Liisa Järvenpää, there would be no HeSVA cohort, and thus, no thesis today. I
want to bring up the prompt tempo of response, always with deepness of thought
77
Physical activity, nutrition
and stress response in young
adults born preterm −
determinants of health and
disease
Acknowledgements
included, of Professor Johan G. Eriksson. For someone slightly impatient, this was
greatly appreciated. Only Professor Sture Andersson can compete with you in speed
of response. Elena Moltchanova gave an important contribution to this thesis with
her vast knowledge of statistical methods. Research Director Tuija Tammelin
provided advice and guidance on physical activity, and Docent Jaana Lindström and
Katri Hemiö on aspects of nutrition. Professor Katri Räikkönen, Docent AnuKatriina Pesonen, Docent Kati Heinonen-Tuomaala, Kimmo Feldt and Docent Jari
Lahti have given me insight into the world of psychology. Riikka Pyhälä-Neuvonen
and Silja Martikainen are thanked for effective teamwork. I especially want to
mention Minna Salonen and Sonja Strang-Karlsson; you were there at the beginning
and showed me the way.
I thank all of my other research colleagues, current and former, at the Diabetes
Prevention Unit at THL and at the University of Helsinki. I am thankful to Maiju
Mikkonen, Mia-Maria Perälä, Marika Sipola-Leppänen, Marjaana Tikanmäki, Suvi
Alenius, Pia Villa, Tia Aalto-Viljakainen, Pieta Näsänen-Gilmore, Maarit Olkkola
and Sara Sammallahti for peer support; you know how important it can be.
I am also grateful to research nurses Paula Nyholm and Leena Järvinen and data
manager Sigrid Rostén. Pirjo Saastamoinen, thank you for invaluable help in all
kinds of details and paperwork.
Outside the world of science, I thank my friends and relatives for support and for
just being there.
My parents, you gave me something to build on. My sister, I can always count on
your support and help. My brothers – your turn will come. My parents-in-law, thank
you for your role in my family’s everyday life, for helping with the children in times
of need and for shared moments.
Finally, my family deserves the greatest thanks. Rasmus and Rebecka – I love
you, and I am thankful and grateful to you for letting me sometimes concentrate on
my work, while at other times you made me forget it for a moment. Alek – the love
of my life – thank you for your support and understanding! What we have, many
others only dream of!
This research project was funded by grants from the Academy of Finland, the
Finnish Foundation for Paediatric Research, the Finnish Special Governmental
Subsidy for Health Sciences, the Jalmari and Rauha Ahokas Foundation, the Juho
Vainio Foundation, the Novo Nordisk Foundation, the Päivikki and Sakari Sohlberg
Foundation, the University of Helsinki, the Perklén Foundation, the Research
78
Physical activity, nutrition
and stress response in young
adults born preterm −
determinants of health and
disease
Acknowledgements
Foundation for the Orion Corporation, the Signe and Ane Gyllenberg Foundation,
the Sigrid Jusélius Foundation, the Waldemar von Frenckell Foundation, the Yrjö
Jahnsson Foundation, the Finnish Medical Foundation, the Otto A. Malm Donation
Fund, the Emil Aaltonen Foundation, the Medical Society of Finland (Finska
Läkaresällskapet) and Victoriastiftelsen, all of which are gratefully acknowledged.
Espoo, December 2014
Nina Kaseva
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disease
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Physical activity, nutrition
and stress response in young
adults born preterm −
determinants of health and
disease