Estimating
body composition
of young children
by using bioelectrical
resistance
MICHAEL
I. GORAN,
MARY C. KASKOUN,
WILLIAM
H. CARPENTER,
ERIC T. POEHLMAN,
ERIC RAVUSSIN,
AND ANNE-MARIE
FONTVIEILLE
Division of Endocrinology, Metabolism and Nutrition, Department of Medicine, and The Sims Obesity1
Nutrition Research Center, University of Vermont, Burlington, Vermont 05405; and The Clinical Diabetes
and Nutrition Section, National Institutes of Health, Phoenix, Arizona 85016.
I., MARY C. KASKOUN,
WILLIAM
H. CARPOEHLMAN, ERIC RAVUSSIN, AND ANNEEstimating
body composition
of young
children by using bioelectrical resistance. J. Appl. Physiol. 75(4):
1776-1780, 1993.-It
is currently unclear whether age-specific
equations should be used for assessing body composition
from
bioelectrical
resistance. Kushner
et al. (Am. J. Clin. Nutr.
56: 835-839, 1992) showed that the relationship
between
height2/resistance
and total body water (TBW) is robust across
a wide age range, although uncertainty
remained over the relationship in preschool children. We therefore cross-validated
the Kushner equation for predicting total body water in 4- to
6-yr-old children in two independent
laboratories.
TBW was
measured from H2180 dilution, and bioelectrical
resistance and
reactance were measured using an RJL 10lA analyzer in 31
children (15 females, 16 males; 5 t 0.8 yr) studied in Burlington, Vermont, and 30 children (14 females, 16 males; 5 t 0.2 yr)
studied in Phoenix, Arizona. There was no significant
difference between TBW predicted from the Kushner equation and
that measured in children in Burlington
(11.76 t 2.00 vs.
11.91 t 2.46 k g; r = 0.94) or in Phoenix (11.53 k 1.64 vs. 11.66 k
1.90 kg; r = 0.94). The Kushner equation for TBW can be
transformed into an equation for fat-free mass (FFM) by using
published age- and gender-specific
constants for the hydration
of FFM: hydration of FFM = 76.9 - 0.25 age (yr) - 1.9 gender
where female equals 0 and male equals 1. The intraclass reliability for estimates of fat mass and FFM with the use of bioelectrical resistance in an independent
group of 26 children (5.0 k 0.8
yr, 20.2 t 3.0 kg) was >0.99 for duplicate observations
performed 2 wk apart. We conclude that the relationship
between
TBW and height”/resistance
in young children is robust across
independent
laboratories and that the Kushner equation relating these two variables is viable in young children.
GORAN,
MICHAEL
PENTER,
ERIC T.
MARIE
FONTVIEILLE.
fat-free mass; fat mass; total body water
RESISTANCE
is an appealing tool for in
vivo assessmentof body composition because it is simple,
quick, and inexpensive to perform. Theoretically, the
technique of bioelectrical resistance is based on the principle that the body’s electrical resistance is a function of
the distribution of water and electrolytes among the
various compartments in the body. The relationship between the body’s resistance to an imperceivable electrical
current and the body’s composition has been examined
using a number of other independent and more elaborate
techniques (1, 14-16, 19, 22). Those studies have led to
the general conclusion that, under most conditions, bio-
BIOELECTRICAL
1776
0161-7567193
$2.00 Copyright
electrical resistance is an accurate and moderately precise technique for assessingbody composition, although
its validity to detect change in body composition during
weight change remains unclear (17, 21, 23).
The technique of bioelectrical resistance is of particular appeal for use in children because other available
body composition methods either require complicated
test procedures that are impractical for young children to
perform (e.g., underwater weight) or involve radiation
exposure (e.g., in vivo neutron activation). Bioelectrical
resistance is also an attractive alternative to other available techniques (e.g., total body water, total body potassium, dual-energy X-ray absorptiometry) because of its
potential application to epidemiologic and field studies of
body composition.
Bioelectrical resistance has been examined specifically
in children and youths at several laboratories (2, 3, 6, 7,
15). Houtkooper et al. (15) developed and then cross-validated an equation for predicting fat-free mass from
bioelectrical resistance in children aged lo-14 yr with a
standard error of the estimate of rt1.9 kg (15). However,
before this study, Deurenberg et al. (7) recognized that
the relationship between bioelectrical resistance and total body water is influenced by age, even within a narrow
age range in children. Deurenberg et al. therefore suggested using age-specific equations when the technique
of bioelectrical resistance was applied. More recently,
however, Kushner et al. (18) used data from neonates,
preschool children, prepubertal children, and adults to
derive one universal equation for estimating total body
water from height%esistance that was applicable across
a wide age range, although the authors remained uncertain of the accuracy of the equation in preschool children
(18). The purpose of this study was therefore to cross-validate the Kushner equation for predicting total body
water from height’/resistance and body weight in two
independent groups of 4- to 6-yr-old children studied in
Burlington, Vermont, and Phoenix, Arizona.
METHODS
Subjects. Thirty-one Caucasian children aged 4-6 yr
[ 15 females, 16 males; 5 t 0.8 (SD) yr] were studied at the
University of Vermont, and 30 Caucasian children aged
4-6 yr (14 females, 16 males; 5 t 0.2 yr) were studied at
the Clinical Diabetes and Nutrition Section of the National Institute of Diabetes and Digestive and Kidney
0 1993 the American
Physiological
Society
BIOELECTRICAL
IMPEDANCE
ANALYSIS
Diseases in Phoenix, Arizona. Informed consent was obtained from the parent or guardian for each child before
participation.
The studies were approved by the Committee on Human Research for the Medical Sciences at
the University
of Vermont and the Human Ethics Subpanel Committee of the National Institute
of Diabetes
and Digestive and Kidney Diseases.
Bioelectrical resistance and anthropometric
measurements. Total body resistance (Q) and reactance (capacitance) were measured in both laboratories with the use of
a tetrapolar bioelectrical impedance analyzer (RJL lOlA,
Detroit, MI) using electrode placement procedures as recommended by the manufacturer.
The analyzers were calibrated before each test by using the 500-Q resistor provided by the manufacturer.
Two measurements
of resistance and reactance were obtained from each child under
similar test conditions at either a 2-wk interval (for studies in Burlington)
or a l-wk interval (for studies in
Phoenix). The data reported were the average of duplicate measures. In both laboratories,
weight was measured when the child was in light clothing and without
shoes on a beam scale to the nearest 0.01 kg and was
followed by a measurement
of height to the nearest 0.5
cm via a fixed wall-mounted
metric ruler.
Measurement of total body water. Total body water was
measured from the “0 dilution space as previously described (12, 13). Baseline urine samples were taken before oral dosing with -0.15 and 0.21 g of H,180 per kilogram body mass in Vermont and Arizona, respectively. A
total of four urine samples were collected postdose. The
last urine sample was obtained 14 days after dosing in
Vermont and 7 days after dosing in Arizona. Samples
and prepared standard dilutions were analyzed in triplicate (Vermont) or duplicate (Phoenix) for H,180 by isotope ratio mass spectrometry at the Biomedical Mass
Spectrometry Facility at the Clinical Research Center at
the University of Vermont (12, 13) and at the National
Institutes of Health facility in Phoenix (lo), as previously described. Zero-time enrichments of Hz’80 were
calculated by back extrapolation of the semilogarithmic
plot of isotope enrichment in urine vs. time after dosing
to time 0, and the dilution space of 180 was calculated
according to the equation of Coward (4). The reproducibility of repeat measures of total body water with the use
of this protocol in adult subjects studied in Vermont had
a coefficient of variation of -3% and an intraclass corre-
n
Height,
cm
Weight,
kg
Resistance,
$2
Reactance,
C/V
Ht2/resistance,
Data
are means
15
112.7k8.25
(99.3-129)
20.98k4.72
(15.96-34)
745t64
(592-861)
73+7
(63-89)
17.4k6.7
(12.6-27.3)
cm2/$t
+ SD with
ranges
in parentheses;
YOUNG
1777
CHILDREN
lation coefficient of 0.98 (unpublished data). Total body
water was assumed to be equivalent to 180 dilution space
divided by 1.01 to correct for isotope exchange into nonaqueous compounds (24).
Statistics. Differences in subject characteristics between laboratories (Vermont and Arizona) and between
gender were tested using a 2 X 2 analysis of variance . The
equation developed by Kushner et al. (18) for total body
water by using height2/resistance and body weight was
used to predict total body water measured in children in
the two independent laboratories. Predicted total body
water was compared with that measured by using an independent t test. Statistics were computed using Statplan
(Futures Group, Glastonbury, CT) and BMDP software
programs. The level of statistical significance was set at a
probability of P 5 0.05 for all tests. Data are cited as
means t SD unless otherwise stated.
RESULTS
Table 1 presents the subject characteristics, separated
by gender, for data collected in both Vermont and Arizona. There was no significant difference between laboratories or between genders for age, height, or weight. The
mean resistance, reactance, and measured total body
water were significantly different between males and females but were not statistically different between laboratories within either gender. There was no significant laboratory-by-gender interaction for any of the variables presented.
The relationship between measured total body water
and that predicted from the Kushner equation is shown
in Fig. 1 (? = 0.88; ? corrected for attenuation due to
unreliability of measuring total body water = 0.90; SE =
0.63 kg). There was no significant difference between the
means of the measured and predicted values of total body
water for boys or girls studied in Phoenix or Burlington
(Table 2).
The Kushner equation can be transformed into equations for estimating fat-free mass by using the published
age- and gender-specific constants for the hydration of
fat-free mass in children (9). The original data for these
constants were adjusted, since the original calculations
of Fomon et al. (9) were based on a deuterium exchange
of l.3%, whereas our recently published data suggest that
deuterium exchange is closer to 3.1% in girls and 5.1% in
16
112.3k6.92
(103-122)
20.3324.29
(14.67-29.68)
703+72
(532-845)
66-t8
(54-85)
18.3k3.9
(12.6-27.5)
n, no. of subjects.
IN
14
112.Ok6.13
(102.2-121)
18.59k2.7
(14.43-23.15)
770+69
(668-861)
71+8
(59-88)
16.5k2.4
(12.1-21.1)
Significant
differences
16
114.1k5.5
(105-128)
2 1.04k3.77
( 16.10-30.50)
716k61
(612-805)
6527
(52-77)
18.4-tl.7
(9.3-16.1)
were
determined
by 2-way
NS
NS
Gender
Gender
NS
analysis
of variance.
1778
RIOELECTRICAL
IMPEDANCE
ANALYSIS
IN
YOUNG
CHILDREN
2. Comparison of measured and predicted
total body water in children
TABLE
2
= 0.88;
r
18
SEE
=
0.63
regression
--.-....-
A
36
V
n
-
=14
2
:
-
$2
I-
-
line
of
kg
line
Vermont
identity
n
Predicted
water,
Measured
water,
W
rY
10
,’ :
I
I
a
10
I
12
TBW
I
PREdl;TED
I
I
16
18
20
(Kg)
FIG. 1. Relationship
between total body water (TBW)
measured
in
61 children
in Vermont
and Arizona
and that predicted
from Kushner
equation
(Ref. 18) using height2/resistance
and body weight.
boys (11, 13). The newly adjusted values for the hydration of fat-free mass were then expressed as a function of
age and sex by modeling
HFFM
Arizona
(%) = 76.9 - (0.25 X age) - (1.9 X gender)
(I)
where HFFM is the hydration of fat-free mass, age is
measured in years, and gender is 0 for females and 1 for
males.
An equation for estimating fat-free mass was then derived by dividing the Kushner equation by Eq. I
FFM(kg)
(ht2/resistance)0.59 + (wt X 0.065) + 0.04
= 0.769 - (0.0025 X age) - (0.019 X gender)
(2)
where FFM is fat-free mass; height is measured in centimeters, resistance in ohms, weight in kilograms, and age
in years; and gender is 0 for females and 1 for males.
The reliability of Eq. 2 was assessedin an additional
group of 26 children studied in Vermont in whom duplicate measures of height, weight, and resistance were obtained at the beginning and end of a Nday study. This
group of children was similar in age (5.0 t 0.8 yr, range
4-6 yr) and weight (20.2 t 3.0 kg, range 15.0-27.2 kg) to
the combined group in the main analysis. The coefficient
of variation for repeat measurements of fat-free mass
(kg), percent body fat (%body wt), and fat mass (kg) was
2.0, 2.8, and 2.9%, respectively (data are expressed as
%SD of repeat measurements divided by average of repeat measurements). The intraclass correlation coefficient for these repeat measurements was >0.98 for all
three variables.
DISCUSSION
The simplicity and low operating cost associated with
bioelectrical resistance makes it an appealing tool for estimating body composition under epidemiologic and field
conditions. The technique has been cross-calibrated
against several other body composition techniques
across a wide age range (8,15,19,20). The applicability of
hioelectrical resistance to children has been questioned
Data
Female
Male
Female
Male
15
16
14
16
11.6k2.4
12.2k2.6
ll.Ok1.6
12.322.0
11.5*2.0
12.Ok2.0
10.8k1.3
12.2t1.7
Data are means & SD; n, no. of subjects.
sured in children
studied
at 2 independent
dicted from ht2/resistance
and body weight
tion (Ref. 18).
-
8'
total body
kg
total body
kg
Data
Total body water was mealaboratories
or was prewith use of Kushner
equa-
by Deurenberg et al. (7), who suggested that age-specific
equations should be used. Furthermore, the applicability
of the Kushner equation from data collected from neonates, preschool children, prepubertal children, and
adults was questioned for preschool children. We therefore examined the accuracy of the Kushner equation in
predicting total body water in two independent groups of
4- to 6-yr-old children. The main finding was that the
equation accurately predicts total body water in younger
children and that the relationship between total body
water and height2/resistance in young children is robust
across independent laboratories. Moreover, reproducibility studies showed good reliability of body composition
estimates. The strength of the cross-validation of the relationship between height2/resistance and total body
water in young children suggests that the estimation of
total body water in young children with the use of bioelectrical resistance is not susceptible to interlaboratory variation, provided that the manufacturer’s recommended
electrode placement sites and calibration procedures are
followed.
There are marked differences in the literature explaining the relationship between total body water and
height2/resistance in children. The lack of concordance
among the various equations in children is explained, at
least in part, by Deurenberg et al. (7), who concluded that
the relationship between body composition and body resistance was altered in boys aged IO-15 yr and girls aged
IO-12 yr because of I) age-related differences in electrolyte concentration in the extracellular space relative to
the intracellular space and 2) age-related differences in
tissue composition with regard to electrolyte concentration. Deurenberg et al. therefore proposed the use of agespecific equations for estimating body composition from
bioelectrical resistance. Despite the concerns raised in
the study of Deurenberg et al., Kushner et al. (18) recently concluded that the relationship between total
body water and height2/resistance is not independently
influenced by age or body size, which implies that a single
equation can be used to predict total body water. However, the Kushner equation should not be extrapolated to
adolescent subjects given the absence of subjects in this
age range in the study of Kushner et al. (18). Therefore,
although the present study lends support to the notion of
one universal equation for estimating total body water
from bioelectrical resistance, the applicability of this
equation in adolescent children remains unclear.
BIOELECTRICAL
IMPEDANCE
It is important to consider methodological
differences
other than differences in age and puberty status of subjects in attempts to clarify the lack of consistency among
studies. For example, application
of the equation of
Houtkooper
et al. (15) developed from data on adolescent children to our data set leads to an overestimate
in
fat-free mass of ~3 kg. It is unclear how much of this
discrepancy
might be due to differences in the relationship between body composition
and bioelectrical
resistance between young children and adolescents and how
much might be due to methodological differences. In the
study of Houtkooper
et al., deuterium was used to measure total body water without correction for isotope exchange. This is an important correction to apply, since
deuterium is known to overestimate total body water by
4-6% in humans (13) because of the exchange of isotope
into nonaqueous compounds (5). In addition, total body
water was sampled in breath in the study of Houtkooper
et al. without fractionation
corrections
being applied to
take into account differences in evaporative water rates
in water containing deuterium (25). Both of these errors,
if unaccounted for, would lead to a systematic overestimation of total body water and therefore fat-free mass.
These two factors may possibly explain a portion of the
discrepancy between our new equation and that of Houtkooper et al. Thus, the lack of consensus among body
composition
investigators
concerning
various methodological issues complicates the comparison of equations.
There are several limitations that should be borne in
mind when using bioelectrical
resistance
to estimate
body composition.
First, bioelectrical resistance is limited to its ability to predict total body water. We overcame this limitation by using published constants for the
hydration of fat-free mass (9) to transform
the equation
for predicting total body water to one capable of predicting fat-free mass. We realize the limitation of this approach with regard to the possibility of physiological factors other than age and gender affecting the hydration of
fat-free mass. However, in the absence of this information we suggest that for normally hydrated children the
present equations can be used to satisfactorily
estimate
fat-free mass. These equations may need to be modified
in the future as more information
on the hydration status
of fat-free mass in children becomes available. The second limitation is that bioelectrical
resistance
has not
been assessed for its use in measuring change in body
composition
in children in response to environmental
stimuli such as diet or exercise. Bioelectrical resistance is
therefore currently
limited to cross-sectional
measurements of body composition in young children and should
not be applied to determine change in response to intervention treatment.
In summary, the Kushner equation describing the relationship between total body water and height”/resistance accurately predicts total body water in young children and is robust across data sets collected from independent laboratories. The equation for total body water
can be transformed
for estimating fat-free mass by using
published constants
for the hydration of fat-free mass,
thus allowing for accurate and noninvasive measurement
of bodv comnosition in voung ” children.
d
I
J
ANALYSIS
IN
YOUNG
1.779
CHILDREN
We thank
David
Ebenstein
(Burlington)
and Ingeborg
Harper
(Phoenix)
for expert technical
assistance
with isotope ratio mass spectrometry
and the nursing staff at both research
centers for assistance
in performing
these studies. Most importantly,
we extend our appreciation to the families
and young children
at both research
centers who
volunteered
their time to take part in these research
studies.
These studies were supported
by a Biomedical
Research
Support
Grant
from the University
of Vermont,
a grant from the American
Diabetes
Association,
the United
States Department
of Agriculture
Grant 92-01048,
and the National
Institute
of Child Health and Human Development
Grant RX-HD-28720
to M. I. Goran; the National
Institute
of Aging Grants
R29-AG-07857
and K04-AG-00564
and a
grant from the American
Association
of Retired Persons Andrus Foundation to E. T. Poehlman;
a grant from the Benjamin
Delessert
Foundation, Paris, France,
to A.-M.
Fontvieille;
and in part by the Sims
Obesity/Nutrition
Research
Center
and General
Clinical
Research
Center Division
of Research
Resources
Grant RR-109.
Address
for reprint
requests:
M. I. Goran,
Div. of Endocrinology,
Metabolism
and Nutrition,
Dept. of Medicine,
College of Medicine,
Univ. of Vermont,
Burlington,
VT 05405.
Received
17 December
1992; accepted
in final
form
8 June
1993.
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