International Journal of Obesity (1999) 23, 1276±1281 ß 1999 Stockton Press All rights reserved 0307±0565/99 $15.00 http://www.stockton-press.co.uk/ijo Body composition of healthy 7- and 8-year-old children and a comparison with the `reference child'

CHS Ruxton1*, JJ Reilly2 and TR Kirk3

1The Sugar Bureau, Dolphin Square, London, UK; 2University Department of Human Nutrition, Yorkhill Hospitals, Glasgow, UK; 3Centre for Food Research, Queen Margaret College, Edinburgh, UK

BACKGROUND: There are few longitudinal data on body composition in healthy children. This has prompted a reliance on notional standards such as the `reference child', to validate new methods of determining body composi- tion and comparing cross-sectional height, weight and fatness data. OBJECTIVES: These were twofold Ð to provide normative longitudinal data on changes in body composition in healthy pre-pubertal children, and to compare measures of growth and body composition with the appropriate age- speci®c reference child. DESIGN: A sample of healthy Scottish children aged 7 ± 8 y (n ˆ 257) was recruited during 1991=1992. Data on height, weight, skinfold thickness and resistance from bioelectrical impedance analysis were collected twice, 12 months apart. Percentage body fat was estimated from both skinfolds and bioelectrical impedance. RESULTS: Fat and fat-free mass, but not body mass index, differed between boys and girls. All measurements increased signi®cantly over the 12 month period except percentage body fat from skinfolds in boys. The reference child comparison revealed that our sample was taller, heavier and fatter and gained weight and fat mass at a greater rate than the Fomon standards. CONCLUSIONS: Data from the children in this study suggest that the reference child has a body composition which is now out of date. This may have important implications for body composition methodology. New references for height and weight may be required, but an upgrading of the body fat reference may con¯ict with public health aims to reduce obesity.

Keywords: body composition; standards; children

Introduction and current methods for measuring body composition. While the concept of the reference child has been extremely useful in the development of age- and sex- Measurement of body composition in childhood is of speci®c constants in body composition, and in the considerable importance and has a variety of applica- interpretation of body composition measurements, tions,1,2 such as assessment of growth and nutritional there are two major concerns about the existing data status, public health (in de®ning the prevalence of used to derive the `reference child'. First, secular obesity) and interpreting energy expenditure data. Our trends in body fatness are occurring rapidly in the current understanding of the normal development of developed world in both children5,6 and adults.7 body composition during childhood is heavily depen- Second, the authors of the original reference data dent on the concept of Fomon's `reference child'.3 noted that their normative data should be considered This gives estimates of normal body composition preliminary and crude because of uncertainties about based on a combination of theoretical considerations the data and the large number of assumptions and empirical data on height and weight collected required. Empirical data on body composition of from children in the USA many years ago.4 The normal healthy children are scarce, and empirical reference child concept represents notional values data on actual longitudinal changes in body composi- for normative fat and fat-free mass (FFM) at particular tion during childhood are seldom available. ages in childhood. These in turn give rise to assump- The aims of this study were to: (a) obtain normative tions concerning hydration, density and potassium data on body composition and changes in body content of FFM which underpin many of the historical composition measured longitudinally in a sample of healthy Scottish 7- and 8-y-old children; and (b) compare these with data on the reference child. In addition to the estimates from skinfolds, data on fat *Correspondence: Dr C Ruxton, Research Manager, The Sugar mass, FFM and body fat from bioelectrical impedance Bureau, Duncan House, Dolphin Square, London SW1A 3PW, UK. analysis (BIA) are also presented since there are few E-mail: [email protected] Received 7 January 1999; revised 25 May 1999; accepted 22 June longitudinal data collected by this method in the 1999 literature. Body composition in children CHS Ruxton et al 1277 Methods nearest 1 mm. Fat mass, FFM and percentage body fat were calculated using the equation of Slaughter et al,10 which has been shown to produce unbiased Subjects estimates of fatness validated against hydrodensito- Following ethical approval from Queen Margaret metry in Scottish pre-pubertal children.11 All skinfold College and Lothian Education Authority, healthy measurements were made by a single trained observer children aged 7 ± 8 y attending ®ve primary schools (CHSR). in Lothian Region, Scotland, were recruited for a study of dietary intake and anthropometry. Data collection for the study ran from February 1991 to March 1992, excluding school holidays. The children Bioelectrical impedance analysis (BIA) were a representative group for Edinburgh children of The equipment used was a BIA101 plethysmograph this age in terms of social class and gender, and have (RJL Systems, USA), measuring resistance to 1000 O been described in detail elsewhere.8 Height, weight in 1 O increments. BIA data were taken from 135 and skinfold thickness were measured during school (52%) children at the ®rst measurement and 238 hours in 98% of the target group (n ˆ 257). Eleven (93%) children at follow-up under unfasted condi- children were not measured due to absenteeism on the tions. The lower rate of participation at the ®rst measurement day, and one child refused to be mea- measurement was due to a delay in ethical approval sured. After approximately 12 months, the measure- for this part of the study and affected data collection ments were repeated, this time obtaining 90% of the in two schools. Measurements of impedance were original sample (n ˆ 240). The 29 children not mea- taken with the child placed in the supine position sured at the follow-up included 13 absentees, one with legs slightly apart and arms not touching the child who refused (same child as on the ®rst occa- body. A constant current of 80 mA at a signal fre- sion), and 15 children who had moved to other quency of 50 kHz was then applied. The equation of schools. Houtkooper et al 12 was used to calculate FFM since it has been successfully cross-validated against hydro- Height densitometry in pre-pubertal Scottish children.13 This A portable stadiometer (Minimetre, Child Growth equation is as follows: Foundation, UK) measuring to 180 cm in 0.1 cm increments was used to measure height after being FFM kg†ˆ0:61 RI†‡0:25 weight; kg†‡1:31 checked for accuracy in its position on the wall. Children were positioned and height was measured where RI equals height2 (cm)=resistance (ohms). to the nearest 0.1 cm according to the method of Fat mass (kg) was then calculated by subtracting Tanner et al.9 FFM from weight and both were expressed as a proportion of weight. Percentage body fat values which fell below a selected cut-off point of 3.99% Weight were deemed to be unphysiological1 and were A set of ¯oor scales (Soenle, Germany) weighing to excluded from subsequent analyses. This affected 127 kg in 0.5 kg increments was used to measure ®ve cases at baseline and two at follow-up and may weight after being checked for accuracy using a set have been due to the low body weight and height of of standard weights, weighing in 0.5 kg increments to these children producing false results when resistance 55 kg. Weight to the nearest 0.5 kg was measured after was measured. asking the child (wearing light indoor clothing and with shoes removed) to stand motionless on the scales with feet together. Body mass index (BMI) was computed as weight (kg)=height2(m). Statistical analysis Data were entered into SPSS 7.5 for Windows 95 and Skinfold thickness comparisons were made between baseline and follow- A set of metal Harpenden skinfold callipers was used up values using paired t-tests. In the case of estimates (British Indicators Ltd, UK), which exert a constant of fat and FFM from BIA, this resulted in a drop in pressure of 10 g=mm, measuring to 40 mm in 0.1 mm sample size due to the smaller group participating at increments. The callipers were calibrated by the baseline. Differences between boys and girls were manufacturers prior to being used in the study, after examined using independent t-tests. Signi®cance was which they were not used by any person other than the de®ned as P < 0.05. A comparison was made with the observer. Skinfold thicknesses at the tricep and sub- reference child by calculating con®dence intervals for scapula regions were located and measured according each anthropometric variable. Our sample were to the technique described by Tanner et al.8 This was deemed to be statistically different if the reference repeated three times on the left-hand side of the body child value lay outside the appropriate con®dence and the mean of the measurements recorded to the interval. Body composition in children CHS Ruxton et al 1278 Results In comparison with the reference child, boys and girls were signi®cantly taller, heavier, fatter and had a greater FFM at both ages. However, when percentage FFM was considered, boys and girls had lower levels The mean age of the boys was 7.49 (0.37) y at baseline than the reference child. The con®dence intervals for and 8.51 (0.39) y at follow-up, while the correspond- each of the anthropometric and body composition ing mean ages for girls were 7.52 (0.36) and 8.53 variables are given in Table 1. (0.39) y. Measures of fatness calculated from BIA are given Mean (s.d.) values for height, weight, BMI and in Table 2. FFM and percentage FFM were greater in measures of fatness calculated from skinfold thickness boys than girls at age 7 (P < 0.05) and at age 8 are given in Table 1 alongside values for the reference (P < 0.005). Fat mass and percentage body fat were child. There were no signi®cant gender differences for lower in boys compared with girls at age 7 (P < 0.01) height, weight and BMI at either age. However, fat and at age 8 (P < 0.001). mass in girls was greater at age 8 (P < 0.05), while Table 3 shows how height, weight and measures of FFM was greater in boys at ages 7 and 8 (P < 0.001). fatness changed over the 12 months in boys and girls.

Table 1 Comparison of absolute values with Fomon reference data

Age 7 Age 8

Variables Reference Observed (s.d.) CI Reference Observed (s.d.) CI

Boys n ˆ 133 n ˆ124 Height (cm) 121.7 125.0 (5.4) 127.0 130.2 (5.7) 124.1 ± 125.9 129.2 ± 131.3 Weight (kg) 22.9 25.1 (4.1) 25.3 28.2 (4.6) 24.4 ± 25.8 27.3 ± 29.0 BMI 15.5 16.0 (1.8) 15.7 16.5 (2.0) 15.7 ± 16.4 16.2 ± 16.9 Fat mass (kg) 2.9 4.4 (2.6) 3.3 4.7 (2.8) 3.9 ± 4.8 4.4 ± 5.4 Percentage body fat 12.8 16.5 (6.1) 13.0 16.9 (6.4) 15.5 ± 17.6 15.8 ± 18.0 FFM (kg) 19.9 20.7 (2.5) 22.0 23.5 (2.8) 20.3 ± 21.2 22.7 ± 23.7 Percentage FFM 87.2 83.4 (6.0) 87.0 83.1 (6.4) 82.4 ± 84.5 81.9 ± 84.2 Girls n ˆ 124 n ˆ 116 Height (cm) 120.6 124.0 (5.2) 126.4 130.0 (5.5) 123.1 ± 124.9 128.5 ± 130.6 Weight (kg) 21.8 24.2 (4.2) 24.8 27.8 (4.9) 23.6 ± 25.1 26.9 ± 28.7 BMI 15.0 15.8 (2.0) 15.5 16.5 (2.2) 15.4 ± 16.1 16.1 ± 16.9 Fat mass (kg) 3.7 4.6 (2.5) 4.3 5.8 (2.9) 4.4 ± 5.3 5.3 ± 6.4 Percentage body fat 16.8 19.0 (6.4) 17.4 20.0 (6.3) 17.8 ± 20.1 18.9 ± 21.2 FFM (kg) 18.1 19.6 (2.5) 20.5 22.0 (2.5) 19.1 ± 20.0 21.5 ± 22.4 Percentage FFM 83.2 81.0 (6.4) 82.6 79.9 (6.2) 79.9 ± 82.1 78.8 ± 81.1

Fat mass, FFM and percentage body fat estimated from skinfolds.10 CI ˆ con®dence interval.

Table 2 Fat mass, FFM and percentage body fat from BIA at ages 7 and 8 y

Age 7 Age 8

Variable Mean s.d. CI Mean s.d. CI

Boys n ˆ 65 n ˆ 122 Fat mass (kg) 3.4 2.0 2.7 ± 3.8 4.3 2.2 3.6 ± 4.8 Percentage body fat 13.3 5.4 11.5 ± 14.4 14.8 5.1 13.5 ± 16.0 FFM (kg) 21.0 2.7 20.2 ± 21.6 24.0 3.2 22.5 ± 24.0 Percentage FFM 86.7 5.4 85.4 ± 88.0 85.2 5.1 84.3 ± 86.2 Girls n ˆ 75 n ˆ 116 Fat mass (kg) 4.0 2.1 3.4 ± 4.4 5.2 2.4 4.7 ± 5.9 Percentage body fat 16.0 6.1 14.3 ± 17.2 18.2 5.6 17.2 ± 20.0 FFM (kg) 20.0 2.8 19.3 ± 20.6 22.6 3.2 21.6 ± 23.1 Percentage FFM 84.0 6.1 82.6 ± 85.4 81.8 5.6 80.8 ± 83.0

CI ˆ con®dence interval. Body composition in children CHS Ruxton et al 1279 Table 3 Changes in height, weight, BMI, FFM and percentage body fat over 12 months

Mean change Significance bof Correlation between Variable n (s.d.) a change; P < baseline and follow up; r ˆ

Boys Height (cm=y) 120 5.4 (1.6) 0.001 0.86 Weight (kg=y) 120 3.2 (1.7) 0.001 0.82 BMI 120 0.54 (1.4) 0.001 0.95 Percentage body fat (skinfold) 120 0.6 (3.3) ± 0.75 Percentage FFM (skinfold) 120 7 0.6 (3.3) 0.05 0.86 Fat mass (kg) BIA 56 1.0 (0.9) 0.001 0.90 Percentage body fat (BIA) 56 1.9 (3.1) 0.001 0.96 FFM (kg) BIA 56 0.1 (2.3) 0.001 0.94 Percentage FFM (BIA) 56 7 1.8 (3.1) 0.001 0.82 Girls Height (cm=y) 116 5.6 (1.2) 0.001 0.84 Weight (kg=y) 116 3.4 (1.8) 0.001 0.85 BMI 116 0.7 (1.1) 0.001 0.83 Percentage body fat (skinfold) 116 1.2 (3.5) 0.001 0.98 Percentage FFM (skinfold) 116 7 1.2 (3.5) 0.001 0.84 Fat mass (kg) BIA 68 1.4 (1.2) 0.001 0.85 Percentage body fat BIA 68 2.8 (3.3) 0.001 0.87 FFM (kg) BIA 68 2.3 (1.0) 0.001 0.96 Percentage FFM (BIA) 68 7 2.8 (3.2) 0.001 0.85

aFor height and weight, decimal time between the baseline and follow-up measurements was taken into account. bSigni®cance of change analysed using a paired t-test.

Paired longitudinal data were available for 120 boys signi®cantly greater rate than the reference child. and 116 girls for measures which did not relate to The increase in absolute FFM was greater in boys BIA. All mean values were highly correlated and compared with the reference child but not in girls. increased signi®cantly over the 12 months, except Gains in height were similar to the reference child in percentage FFM in both sexes, which signi®cantly both sexes. decreased, and percentage body fat from skinfolds in boys, which remained constant. Height and weight velocity were similar between the sexes. Girls appeared to gain body fat and lose FFM at a greater Discussion rate than boys but any apparent differences were not statistically signi®cant. Table 4 compares the changes in height, weight, Despite similarities in height and weight between the FFM and fat mass (both from skinfolds) with expected sexes at ages 7 and 8, this pre-pubertal group of changes in the reference child. Con®dence intervals children were already showing body composition for the variables are also given. When compared with differences commensurate with their gender. While the `expected' changes from Fomon's3 data, our boys BMI was similar between the sexes, values for per- and girls gained both weight and fat mass at a centage body fat (calculated from both skinfolds and BIA) suggested that the girls were fatter than the boys. Similar observations about pre-pubertal differences in Table 4 Comparison with Fomon reference data Ð changes in 14,15 height (cm=y) and body composition (kg=y) for ages 7 ± 8 y fat and FFM have been made by others. Pietrobelli et al 15 detected differences in fatness between boys Variable Reference Observed (s.d.) CI and girls (P < 0.05) using BMI as a proxy for body Boys nˆ 120) fatness. We found no such difference using this Change in height (cm) ‡ 5.1 ‡ 5.4 (0.14) method in our sample. 5.1 ± 5.6 Change in weight (kg) ‡ 2.4 ‡ 3.2 (1.6)a A possible explanation for the apparent lack of 2.9 ± 3.5 agreement between percentage body fat and BMI in Change in fat mass (kg) ‡ 0.4 ‡ 0.7 (1.2)a our study could be that boys had more FFM but less 0.5 ± 1.0 Change in FFM (kg) ‡ 2.0 ‡ 2.5 (1.3)a body fat than girls for a similar weight. Thus, differ- 2.3 ± 2.7 ences in the proportion of these components might be Girls (n ˆ 116) masked by BMI, which is dependent on weight and Change in height (cm) ‡ 5.8 ‡ 5.6 (0.11) 16 5.4 ± 5.8 height. Bandini et al have also reported a low Change in weight (kg) ‡ 2.9 ‡ 3.4 (1.9)a accuracy of BMI compared with skinfolds when 3.1 ± 3.8 estimating fatness in pre-pubertal children. Change in fat mass (kg) ‡ 0.6 ‡ 1.0 (1.3)a 0.8 ± 1.3 When the changes in anthropometry and body Change in FFM (kg) ‡ 2.3 ‡ 2.4 (1.2) composition occurring between baseline and follow- 2.1 ± 2.6 up were examined, all measures increased except Fat mass and FFM calculated from skinfolds.10 percentage FFM in both sexes and percentage body aSigni®cantly different from Fomon standard. fat from skinfolds in boys. This may have been Body composition in children CHS Ruxton et al 1280 because boys were accumulating truncal rather than reference child. When growth rates were examined, peripheral fat mass, since the estimate of fat mass our sample was gaining height at a similar rate to the from BIA did increase signi®cantly over the 12 reference child, but putting on proportionally more months. However, it is acknowledged that the BIA weight and body fat and failing to gain suf®cient longitudinal sample was small and this may have FFM. This lends support to a worrying trend that contributed bias. The only way of properly assessing our rapidly growing modern children are getting differences in fat compartments over time is by using rapidly fatter as well. a multi-compartmental model, which would have been Two implications arise from this ®nding. If the inappropriate for a ®eld study design such as ours. reference child is no longer an appropriate representa- However, there are bene®ts of our two-compartment tion of normal body composition, current assumptions ®eld study over that of a technically superior multi- about the nature of FFM in children may need to be compartmental study. Sample sizes can be larger, revised since many of these are based on Fomon's ethical approval is more likely to be given and a data. Since age-speci®c assumptions about the com- more representative group of children can be studied. position of FFM underpin many body composition The change in percentage FFM in both boys and methods, an investigation into the validity of the girls over time was intriguing because the value went constants used in these methods is warranted. Such down instead of up. It seemed that FFM as a propor- an approach would require empirical data based on tion of body weight was being lost in favour of a gain multi-component models Ð an option that was not in fat mass. This is a worrying trend and could available at the time of the construction of Fomon's represent insuf®cient exercise in this age group. reference child. Girls appeared to be gaining percentage body fat A second implication of the present study's ®ndings and losing percentage FFM at a greater rate compared relates to the use of body composition standards in with boys, but the difference failed to reach signi®- clinical and research situations to estimate the pre- cance. This may have been because variation in valence of childhood obesity. While it is possibly percentage body fat was too broad within our acceptable that a new reference would take into sample size, and it would be interesting to repeat the account secular changes in height and weight, any gender comparison in a larger group. upward revision of the level of `normal' fatness might The primary purpose of this study was to provide not be appropriate in a public health context where a normative data on body composition, and longitudinal key aim is to identify and treat children whose body body composition changes in a sample representative of fat levels are unacceptably high. A full debate is Edinburgh 7 ± 8 y-olds. The choice of the skinfold needed on how this issue should be resolved. thickness method to estimate body composition might To conclude, this study demonstrates that modern be considered unusual and needs to be justi®ed. The children have outgrown the Fomon standard for the advantages of skinfold thicknesses for our study were `reference child' and are now taller, heavier and deemed to be twofold. First, in ®eld-based community fatter than expected. This may have implications for studies of this type, `®eld' methods of estimating body paediatric body composition methodology and the use composition (for example skinfolds, BIA) are the only of such standards for public health purposes. practical options.2 Second, we have previously shown that the Slaughter prediction equation,10 derived origin- ally from a multi-component model, provided unbiased Acknowledgements estimates of body fatness relative to hydrodensitometry The project was ®nancially supported by a Queen in Scottish pre-pubertal children.11 The use of skinfolds Margaret College studentship (CHSR) and contribu- is known to be subject to technical error, but this was tions from the Kellogg Company of Great Britain and minimized by the use of a single trained observer, the Scottish Dairy Council. The assistance of Dr although it is worth considering that such errors are Angelo Pietrobelli in reviewing this manuscript is inherent even in reference methods such as hydrodensi- gratefully acknowledged. tometry.1 Errors in prediction equations to estimate References body composition from skinfolds have been subjected 1 Lohman TG. Advances in body composition assessment. 1 to detailed considerations of acceptability and the Monograph no. 3. Human Kinetics Publishers: Champaign, equations used here produced errors which were IL, 1993. within the limits of acceptability.2,11 2 Reilly JJ. Assessment of body composition in infants and The comparison with the reference child,3 based on children. Nutrition 1998; 14: 821 ± 825. 3 Fomon SJ, Haschke F, Ziegler EE, Nelson SE. Body composi- theoretical considerations and data collected in the tion of reference children from birth to age 10 years. 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