European Journal of Clinical Nutrition (1999) 52, 824±829
ß 1999 Stockton Press. All rights reserved 0954±3007/99 $15.00
http://www.stockton-press.co.uk/ejcn
Vitamin D levels in prepubertal children in Southern Tasmania:
prevalence and determinants
G Jones1*, CL Blizzard1, MD Riley1, V Parameswaran2, TM Greenaway2 and T Dwyer1
1
Menzies Centre for Population Health Research, Hobart; and 2Diabetes and Endocrine Services, Royal Hobart Hospital,
Liverpool St, Hobart, Tasmania
Objective: To describe the prevalence and determinants of 25-hydroxy D3 (25(OH)D) in children.
Design: Cross-sectional study.
Setting: Southern Tasmania between June and November 1997.
Subjects: Two hundred and one 8-y old male and female children taking part in a cohort study whose principal
endpoints were blood pressure and high-density lipoprotein (HDL) cholesterol.
Results: The mean 25(OH)D level was 79 nmol=l (s.d. 29.5, median 73, range 12±222). Boys had higher levels
than girls (82.1 vs 72.8 nmol=l, P ˆ 0:02). 25(OH)D was associated with sunlight exposure in winter school
holidays (r ˆ 0:20, P ˆ 0:005) and winter weekends (r ˆ 0:16, P ˆ 0:02), the month after school holidays (87.5
vs 69.5 nmol, P < 0:0001) and body mass index (r ˆ ÿ0:23, P ˆ 0:001). Dietary intake of vitamin D was low
(mean 40 IU=day, range 5.2±384) and was not associated with 25(OH)D levels (r ˆ 0:01, P ˆ 0:91). Variation in
skin melanin density was weakly associated with 25(OH)D (r ˆ 0:09, P ˆ 0:19).
Conclusions: Sunlight is the major determinant of vitamin D stores in our population. Neither variation in skin
type within Caucasians nor diet modi®ed this association to any signi®cant extent. Extrapolation of these ®ndings
to sunlight bone mass associations in a very similar population suggests that a minimum level of around
50 nmol=l in the population is required for optimal bone development in prepubertal children but this needs to be
con®rmed with further controlled trials of vitamin D supplementation and bone mass.
Sponsorship: Arthritis Foundation of Australia, Roche Pharmaceuticals.
Descriptors: vitamin D; bone mass; children; sunlight
Introduction
Increasing peak bone mass appears to be important in the
prevention of osteoporosis in later life (Hansen et al, 1991),
thus methods of increasing bone acquisition in childhood
are important. There is evidence to suggest that physical
activity and, to a lesser extent, diet (particularly calcium
intake) during the prepubertal period (Bass et al, l998;
Khan et al, 1998), adolescence (Valimaki et al, 1994;
Welten et al, 1994) and early adulthood (Valimaki et al,
1994) are determinants of peak bone mass. Apart from
physical activity and diet it is possible that other environmental factors are important in childhood. One potential
candidate is vitamin D. Overt vitamin D de®ciency in
children leads to rickets. Subclinical vitamin D de®ciency
may be associated with de®cits in bone mineralisation in
both adults and children but the exact de®nition of this
*Correspondence: Menzies Centre for Population Health Research, GPO
Box 252-23, Hobart, Tasmania, Australia, 7000.
E-mail:[email protected]
G Jones, Senior Research Fellow, is the guarantor, contribution±study
conception, data analysis, overall manuscript composition; CL Blizzard, is
the statistician, contribution±data analysis, manuscript review; MD Riley,
is the nutritional epidemiologist, contribution±processing and reporting of
dietary data, manuscript review; V Parameswaran, is the hospital scientist,
contribution±processing and reporting of laboratory data, manuscript
review. TM Greenaway, is the director, contribution±study conception,
manuscript review; and T Dwyer, is professor and head, contribution±
study conception, manuscript review.
Received 15 March 1999; revised 16 April 1999; accepted 9 May 1999
condition remains controversial. Recent studies have suggested that threshold levels between 50 and 110 nmol=l are
required in the elderly (McKenna & Freaney, 1998) and
30±50 nmol=l in childhood (Docio et al, 1998) for optimal
bone health although there are no studies in healthy
children directly relating 25-hydroxy vitamin D3
(25(OH)D) levels to bone mass or fracture.
Recent, we reported a dose response relationship
between winter sunlight exposure and bone mass in prepubertal children particularly girls (Jones & Dwyer, 1998).
This association persisted after adjustment for physical
activity and is likely to be mediated through vitamin D
stores. However, we did not have biochemical measures on
these children and accurate data on the relative contribution
of sun exposure and diet to 25(OH)D stores in Australian
children is lacking. Furthermore, while it is clear that there
are well recognised racial differences in 25(OH)D photosynthesis there are limited data on whether variation in skin
colour within Caucasians modi®es the effect of sunlight. In
this study, therefore, we examined the associations between
sunlight exposure, season, gender, skin melanin density,
diet and 25(OH)D levels in a very similar population of 8-y
old male and female children to that in which we reported
the association between sunlight and bone mass.
Methods
Hobart (latitude 42 S) is the capital city of Tasmania, the
southernmost state in Australia. In 1989, there were 6813
25(OH)D in prepubertal children
G Jones et al
live births in Tasmania. Of these, 1411 were identi®ed as
being at high risk of Sudden Infant Death Syndrome by
previously published criteria (Dwyer et al, 1991) and were
invited to take part in a longitudinal study. In Southern
Tasmania, there were 765 births who met these criteria. Of
these, 739 (97%) agreed to an in-hospital interview and 666
(87%) agreed to the one month follow-up.
The 739 subjects who agreed to the in-hospital interview
were approached during 1997 to take part in a study of the
determinants of blood pressure and high-density lipoprotein
(HDL) cholesterol. After 8-y, we were able, through the use
of school lists, to de®nitely identify 605 of these subjects
(or 82% which is in close agreement to the Australian
Bureau of Statistics data on annual outward migration rates
from Tasmania of 2.5%). Subjects who provided informed
consent to take part underwent an extensive protocol
involving measurement of anthropometry, physical activity
and ®tness, diet, sunlight exposure, skin re¯ectance and
biochemical measures. The current study relates to vitamin
D levels only. Ethical approval for this study was obtained
from the University of Tasmania Ethics Committee
(Human experimentation).
Height was measured using a stadiometer with the
subject in bare feet. Weight was measured using bathroom
scales that were calibrated daily using known weights.
Body composition was assessed by skinfold measurement
at four sites as previously described (Durnin & Rahaman,
1967).
Physical activity measures included questionnaire items
regarding sports participation (de®ned as taking part in
organised sport for at least three months of the last twelve),
lunchtime activities (®ve point ordinal scale) and recess
school activities (three point ordinal scale). Objective
measures included measurement of muscle strength by
dynamometry at three sites: lower limb (involving both
legs simultaneously), upper limb push and upper limb pull
(Pyke, 1985). The child was instructed in each technique
prior to testing and each measure was performed twice.
Repeatability estimates (Cronbach's ) were as follows;
lower limb 0.91, upper limb pull 0.89, upper limb push
0.83. The devices were calibrated by suspending known
weights at regular intervals. Standing long jump distance
was also assessed on two occasions as previously described
(Pyke, 1985). Endurance ®tness was assessed by the use of
a progressive shuttle run test as previously described
(Ramsbottom et al, 1988). Shuttle level has been validated
against maximal oxygen uptake in adults and found to
correlate highly (r ˆ 0:92).
The usual dietary intake of each subject in the preceding
twelve months was measured using a semi-quantitative
food frequency questionnaire (FFQ) which was completed
by the child's mother or guardian. The FFQ consisted of
159 food categories, and associated standard serve sizes
appropriate for 8-y old children. The questionnaire also had
24 supplementary questions related to dietary practices,
including the type of milk consumed and whether margarine was consumed in preference to butter. Mean usual daily
nutrient intake was estimated using nutrient values for each
food category from Australian Tables of Food Composition
(NUTTAB95) and incorporating information from the
supplementary questions. In the absence of Australian
data, vitamin D content of each food category was assigned
using British tables of food composition (Paul & Southgate,
1988). Margarine is generally not forti®ed with vitamin D
in Australia, therefore no vitamin D was assigned to this
food in the FFQ. Subjects who reported consuming from
only 15 of the food categories or less were determined to
have inadequately completed the questionnaire and were
not included in further data analyses.
Sunlight exposure was assessed by questionnaire relating to the amount of daily exposure during school days,
weekends, and on school holidays in winter. Categories
were as follows: (1) less than 2 h;; (2) 2±3 h; (3) 3±4 h; and
(4) > 4 h. We have previously validated this measure of
exposure against actual exposure with polysulphone badges
in teenage children and found them to correlate well in
summer (ICC ˆ 0.62) (Dwyer et al, 1996).
Skin re¯ectance was assessed on three occasions on the
inner upper arm with a Minolta 508 spectrophotometer as
previously described (Dwyer et al, 1998). We used the
mean of the three measures of the re¯ectance at 420 nm less
the re¯ectance at 400 nm. We have previously found this to
be the best non-invasive measure of biopsy assessment of
melanin density in Caucasians (r ˆ 0:68) (Dwyer et al,
1998). In general the higher the score, the fairer the skin.
Repeatability estimates for this procedure were excellent
(Cronbach's a ˆ 0.997).
Serum samples were treated initially with acetronitrile to
rapidly extract 25(OH)D and other hydroxylated metabolites. 25(OH)D was then assayed utilising a two step double
antibody radioimmunoassay (INCSTAR Corp, Stillwater,
Minnesota). The intra- and inter-assay coef®cients of variations CVs in our hands are 6% and 10% respectively. The
sensitivity of the assay is 7 nmol=l. We did not measure
25(OH)D or spectrophotometry in all subjects due to
sample size considerations (see below). There were four
®eld days each week. All subjects studied on the Monday
and Tuesday of each week from the beginning of June until
the end of the study had assessment of 25(OH)D and skin
re¯ectance. The conversion factor for 25(OH)D for the
different units is 1,000 U ˆ 25 000 ng ˆ 62.5 nmol (Holick,
1988; Par®tt et al, 1982),
Statistics
Correlation coef®cients were utilised to examine the associations between study factors and log-transformed 25
(OH)D. Unpaired t-tests were utilised for comparison of
means. Linear modelling techniques were utilised to examine the relationship between log transformed 25(OH)D and
study factors of causal interest. Associations were ®rst
examined separately. All factors were then entered into a
multivariate model with the log transformed 25(OH)D as
the dependent variable. The exception to this was for winter
sunlight exposure where the three items were highly
correlated with each other. In this case, only the item
most strongly associated with 25(OH)D was included. In
the case of body size variables, we also chose the measure
most strongly correlated with 25(OH)D. A statistically
signi®cant result was regarded as P < 0.05 (two-tailed) or
a 95% con®dence limit not including the null point. Sample
size considerations revealed that 200 subjects would be
suf®cient to detect gender and seasonal differences as small
as 9 nmol=l, correlation coef®cients as small as 0.18 and to
detect overt vitamin D de®ciency in 20% of children. The
assumptions for these calculations were based on the
normal range of 25(OH)D in our institution (18±
114 nmol=l) as well as a ˆ 0.05 (two-tailed) and bˆ 0:20.
All statistical calculations were carried out using SPSS
version 6.1 for Windows.
825
25(OH)D in prepubertal children
G Jones et al
826
Results
signi®cant higher 25(OH)D levels as compared to the rest
of the subjects (88.2 vs 78.7 .nmol=l, P ˆ 0:25). As compared to those in the lowest quartile of skin re¯ectance
those in the highest quartile also had non statistically
signi®cantly higher levels of 25(OH)D (81.9 vs
77.4 nmol=l, P ˆ 0:52).
Log-transformed 25(OH)D was also correlated negatively with body mass index (r ˆ ÿ0:23, P ˆ 0:0011),
A total of 446 subjects took part (males n ˆ 302, females
n ˆ 144) representing an overall response rate of 74% of
those available in 1996 or 60% of those in the original
cohort. As per the Methods, 25(OH)D levels were available
in 201 of these subjects (45%). There were no signi®cant
differences between subjects who had 25(OH)D and those
who did not for current weight, height, endurance ®tness,
muscle strength and vitamin D intake. Children who had
25(OH)D measured were slightly older than children who
did not. Dietary vitamin D intake was low with no child
achieving a daily intake of 400 IU (l0 mg) (Table 1).
The mean 25(OH)D level in these children was
79 nmol=l (s.d. 29.5, median 73, range 12±222). One
child had a level below 25 mol=l. Males had higher levels
than females (82.1 vs 72.8 nmol=l, P ˆ 0:02). This did not
alter after adjustment for potential confounders (Table 2).
A total of 10% had levels below 50 nmol=l (similar in both
sexes). There was no evidence of the expected seasonal
variation. Indeed, the highest levels of 25(OH)D were seen
in July and October (combined vs other months 87.5 vs
69.5 nmol=l, P < 0.0001) (Figure 1). Log-transformed
25(OH)D levels were associated with sunlight exposure
in winter school holidays (r ˆ 0:20, P ˆ 0:005) and weekends (r ˆ 0:16, P ˆ 0:028) but not the other sunlight
category (Figure 2). The mean 25(OH)D level in category
4 of sunlight exposure in winter school holidays was
84.5 nmol=l (range 47±222) and winter weekends was
81.9 nmol=l (range 47±139).
Serum 25(OH)D did not correlate with either dietary
intake of vitamin D (r ˆ 0:001, P ˆ 0:91) or ®sh intake
(r ˆ ÿ0:07, P ˆ 0:31). 25(OH)D levels did not differ
between those in the highest quartile of vitamin D intake
and those in the lowest (81.1 vs 79.9 nmol=l, P ˆ 0:85).
Skin re¯ectance assessed by spectrophotometry correlated weakly with 25(OH)D levels (r ˆ 0:094, P ˆ 0:19).
Those with the lowest melanin density (de®ned as a
re¯ectance score greater than zero) had non statistically
Figure 1 Month after school holidays and 25(OH)D levels in prepubertal
chi1dren. 25(OH)D levels are signi®cantly higher in July and October as
compared to the other months (P < 0.0001). Results are presented as
antilog of mean log score s.e.m. In 1997, school holidays were in the
®rst two complete weeks of June and September. Mean daylight hours by
month are June±9.5, July±9.2, August±10, September±11, October±12.5,
NovemberÐ14. Mean daily sunlight hours by month in 1997 were June±
3.9, July±4.4, August±5.0, September±5.9, October±6.5, November±6.9
(source: Bureau of Meteorology).
Table 1 Demographic details of subjects strati®ed by measurement of vitamin D
Age (y)
Weight (kg)
Height (cm)
% Female
Endurance ®tness (mean shuttle level)
Lower limb muscle strength (kg)
Sunlight in winter school holidays (mean score)a
Vitamin D intake (U=d)
25(OH)D available (N=201)
25(OH)D not available (N=245)
P-value for difference
7.81 (0.40)
27.8 (4.8)
128.7 (6.2)
33
3.3 (1.0)
37.2 (11.4)
2.71 (1±4)
40.1 (25)
7.58 (0.50)
28.1 (5.7)
127.7 (6.1)
31
3.2 (1.0)
36.2 (10.4)
2.76 (1±4)
39.5 (35)
< 0.0001
0.56
0.09
0.67
0.58
0.31
0.62
0.83
All measurements are mean s.d. except for sunlight exposure where it is mean score (range) which approximately equates to mean daily hours of
exposure.
a
Tests of signi®cance are based on Mann ± Whitney U-test. All others are unpaired t-tests.
Table 2 Univariate and multivariate associations between study factors of interest and log- transformed 25(OH)D
Study factor
Body mass index (kg=m2 )
Sunlight in winter school holidays (per category)
Month after school holidays (1 ˆ N, 2 ˆ Y)
Gender (1 ˆ M, 2 ˆ F)
Skin re¯ectance (%)
Vitamin D Intake (U=d)
a
After adjustment for other factors in table.
Univariate (P-value)
Multivariatea (P- value)
70.044 (0.0011)
0.070 (0.0046)
0.22 ( < 0.0001)
70.12 (0.028)
0.025 (0.19)
0.0004 (0.69)
70.045 (0.0005)
0.066 (0.0054)
0.22 ( < 0.0001)
70.11 (0.032)
0.018 (0.31)
0.0001 (0.93)
25(OH)D in prepubertal children
G Jones et al
827
Figure 2 Sunlight exposure in winter and 25(OH)D levels. 25(OH)D levels are signi®cantly correlated with sunlight exposure during winter school
holidays and weekends but not school days (possibly re¯ecting the small numbers of children in the higher categories of exposure). Results are presented as
the antilog of mean log score and number in each category.
Table 3 Correlation between objective physical measures and logtransformed 25(OH)D
Variable
Endurance ®tness
Upper limb pull
Upper limb push
Lower limb strength
Standing jump
Correlation coef®cient (P-value)
0.18
70.02
0.04
70.06
0.02
(P ˆ 0.012)
(P ˆ 0.081)
(P ˆ 0.53)
(P ˆ 0.41)
(P ˆ 0.75)
P ˆ 0:51), duration of second stage of labour (r ˆ ÿ0:08,
P ˆ 0:31), maternal age (r ˆ 0:07, P ˆ 0:32) or season of
birth (P ˆ 0:83). Furthermore, both the parameter estimates and statistical signi®cance for each of the factors
of causal interest reported in Table 2 were not altered
signi®cantly by inclusion of the selection factors into the
model (data not shown).
Discussion
weight (r ˆ ÿ0:23, P ˆ 0:0012), height (r ˆ ÿ0:13,
P ˆ 0:057), fat mass (r ˆ ÿ0:20, P ˆ 0:004), lean mass
(r ˆ ÿ0:21, P ˆ 0:003) as well as endurance ®tness
(r ˆ 0:18, P ˆ 0:012) but not with measures of muscle
strength (Table 3). Those who participated in sport had
similar 25(OH)D levels to those who did not (79.1 vs
78.5 nmol=l, P ˆ 0:88).
In multivariate analysis, log transformed 25(OH)D was
signi®cantly associated with the month after school holidays, winter sunlight exposure in school holidays, body
mass index and gender but not skin re¯ectance or diet
(Table 2). The proportion of the variance in log transformed 25(OH)D explained by these variables was 18%
(P < 0.00001). The association between endurance ®tness
and log transformed 25(OH)D became non-signi®cant after
adjustment for sunlight exposure, body mass index and
gender (P ˆ 0:31).
With regard to the selection factors for the 1989 study,
current levels of 25(OH)D were associated with gender (see
above) but not with birth weight (r ˆ 0:03, P ˆ 0:68),
breastfeeding intention (difference yes vs no 2.8 nmol=l,
In our population of prepubertal children, sunlight exposure
in winter, particularly in school holidays, was the major
predictor of 25(OH)D. In addition, the expected seasonal
variation in 25(OH)D was not observed with higher mean
levels in July and October. This is consistent with the
school holiday observation as these are the months directly
following the two mid year holiday periods in Tasmania in
June and September. Boys had higher 25(OH)D levels than
girls even after adjustment for other factors such as
reported sunlight exposure. The most likely explanation
for this is that boys expose more skin to the sun than girls
and thus synthesise more 25(OH)D for the same level of
exposure. We also found a negative association between
body mass index and 25(OH)D. This is likely to re¯ect
decreased physical activity with resultant lower sun exposure. Indeed, adjustment for fat mass negated the association between endurance ®tness and 25(OH)D. This also
suggests that the accuracy of questionnaire assessment of
sunlight can be improved by assessment of body composition as the two factors remained independently associated
with 25(OH)D.
25(OH)D in prepubertal children
G Jones et al
828
Our data suggest that 3 or 4 h or more sunlight each day
on either winter school holidays or winter weekends is
desirable in Hobart (latitude 42 S) to maximise vitamin D
stores. Vitamin D photosynthesis depends on the action of
ultraviolet radiation on skin thus can be affected by subject
activity during peak exposure periods, amount of clothing
worn, sun angle on skin, skin colour, humidity, temperature, wind speed, sunscreens and sweating (Elwood &
Diffey, 1993). Interestingly, the relationship between sun
exposure and 25(OH)D was not signi®cantly modi®ed by
skin re¯ectance which is an indirect, but accurate, measure
of skin melanin density (Dywer et al, 1998). This perhaps
re¯ects the narrow variation in skin type in our predominantly Caucasian population. Hobart has comparable levels
of ultraviolet exposure to other regions within Australia
during summer at 20 minimal erythemal doses. In winter,
however, the levels drop markedly to 1.6 minimal erythemal doses (Gies, 1994). Comparative data indicate that
Northern and Southern latitudes are not equivalent. In New
Zealand, it has been estimated that ultraviolet levels in
Summer are 40±80% higher than equivalent latitudes in
Germany due mainly to lower ozone levels (Seckmeyer &
McKenzie, 1992). This may explain the apparent contradiction with observations in Boston (at the same latitude as
Hobart in the northern hemisphere) where sunlight intensity
during the winter months is insuf®cient to lead to photosynthesis of vitamin D (Webb et al, 1988). Alternatively,
25(OH)D synthesis may be more ef®cient in young children as compared to adults.
In Australia, dietary supplementation with vitamin D is
not considered necessary due to our abundant sunlight.
Dietary intake of vitamin D was very low in our sample at
40 U=d. This is considerably below the current adequate
intake of 200 U and recommended intake of 400 U (Holick,
1998). Indeed, no child reached a daily intake of 400 U.
Dietary reporting of nutrients from food frequency questionnaires is often subject to signi®cant measurement error
but the combination of the reported intakes and the lack of
multivariate association between vitamin D intake and
25(OH)D indicate that that dietary intake is unlikely to
contribute to vitamin D stores in our population.
There is consensus that a level of 25(OH)D necessary to
prevent osteomalacia is of the order of 25 nmol=l (Par®tt et
al, 1982). Optimum levels to prevent hypovitaminosis D
(de®ned as a threshold below which abnormalities in bone
homeostasis may occur) are more controversial. In adults,
studies have suggested thresholds of between 50 and
110 nmol=l (McKenna & Freaney, 1998) which is much
higher than levels indicative of overt de®ciency. Studies in
children are lacking and there have been no studies in
normal children relating 25(OH)D directly to bone mineralisation. Docio et al reported that a level above 30±
50 nmol=l was necessary to maintain 1,25(OH)2 D synthesis
in children in Spain (Docio et al, 1998). Recently, we
reported a dose response relationship between winter sunlight exposure and bone mass in 8-y old children in a cohort
of children born in 1988. In all respects, apart from year of
birth, their recruitment as well as dietary and sunlight
assessment were identical to the current study. Boys in
the highest category of sun exposure had bone mass up to
4% higher than those in the lowest category while girls in
the highest category had bone mass up to 10% higher than
those in the lowest category (Jones & Dwyer, 1998).
Within the limitation of variation in climatic conditions
from year to year, the minimum levels of 25(OH)D in the
highest (that is, optimal) category of winter sunlight exposure on winter weekends and school holidays was
47 nmol=l (with only one child less than 50 nmol=l) suggesting a threshold for optimal bone mineralisation at the
upper limit of that reported by Docio et al above. Ninety
percent of children in our study were above this threshold
during that part of the year when levels would be expected
to be lowest which is somewhat reassuring. However, the
results of this study combined with our previous study
suggest that the normal range concept needs to be revised
in children as variation in bone mass with sun exposure was
seen in children who were likely to be predominantly
within the normal range. It may be dif®cult to modify the
level of exposure as the amount of sunlight in the highest
category closely parallels the average daily sunlight hours
which are 3.9 in June as compared to 7.4 in January (Jones
& Dwyer, 1998). Furthermore, current public health messages to both avoid excess sun exposure and use sunscreens
to prevent skin cancer may lead to some confusion suggesting dietary supplementation, as practised in the United
States, may be more appropriate as an intervention. The
above points strongly indicate the need to test these
observations in a controlled trial of vitamin D supplementation in children incorporating both biochemical and bone
density assessments.
This study has a number of potential limitations. The
ideal study to describe prevalence is a randomly selected
population-based sample with high response rates which is
dif®cult to obtain in children due to the lack of suitable
databases and reluctance to consent to venipuncture. Our
sample of prepubertal children is a convenience sample and
some caution is necessary with extrapolation of our results
to the general Tasmanian population of prepubertal children. However, there was no relationship between any of
the original selection factors and current 25(OH)D levels
suggesting that the selection bias that clearly applied to this
cohort in terms of lower maternal age, low birth weight,
under-representation of breastfeeding and over-representation of smokers (Jones et al, 1999) does not materially
in¯uence the results of the current study. In addition, there
were no signi®cant differences between those in our sample
who had 25(OH)D measured and those who didn't in a
number of important study factors with the exception of age
which can be related to the protocol whereby 25(OH)D
measurement commenced in June 1997 as compared to
March for the overall study. While the apparent selection
bias may in¯uence inferences about prevalence in a population, this is less likely to apply to the etiologic associations we report in this study. These observations provide
some reassurance that our results may be generalisable to
Hobart children and other populations of children in comparable southern latitudes where dietary vitamin D supplementation is not practised.
Conclusions
Sunlight is the major determinant of vitamin D stores in our
population. Neither variation in skin type within Caucasians nor diet modi®ed this association to any signi®cant
extent. Extrapolation to our previous ®ndings suggests a
minimum level around 50 nmol=l in the population is
required for optimal bone development in prepubertal
children but this needs to be con®rmed with further controlled trials of vitamin D supplementation and bone mass.
25(OH)D in prepubertal children
G Jones et al
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