Introduction

The rapid increase in childhood obesity, which is currently affecting many countries is of major public health concern.^{1,2,3,4,5,6,7} This excessive adiposity is evident in preschool children^8,9,10 making it important to know exactly what levels of body fat are present today in youngsters of this age.

Excessive fatness has adverse health effects, even in childhood.¹¹ Obesity in youngsters is associated with orthopaedic problems, higher blood pressure, raised lipids, altered insulin sensitivity, arterial dysfunction, early signs of cardiovascular disease, psychosocial problems and advanced pubertal maturation.^{12,13,14,15,16,17,18,19,20} Furthermore, serial measurements of body mass index (BMI) indicate that childhood obesity tracks strongly into adult life^21,22 where obesity has serious consequences.^11,23 Thus, curbing early obesity in children has become a health priority.^3,24

Until recently, excessive fatness has generally been documented in young children from simple measurements of weight, weight for height or BMI (weight/height²). These methods do not distinguish levels of body fat content separately from lean tissue mass and bone mass. Although underwater weighing does estimate fat stores, this technique is unsuitable for use in young children²⁵ while bioimpedence²⁶ is unduly influenced by hydration. Skinfold measurements have been widely used to assess adiposity in the past, but are considered to be imprecise.²⁷ Fortunately, the advent of dual-energy X-ray absorptiometry (DXA) enables fat tissue to be quantitated separately from lean tissue with precision and safety in children.^28,29

To date, few DXA studies of body composition evaluating large samples of young children of the same gender and ethnicity within a narrow age band have been undertaken. Information concerning longitudinal changes in the body fat and lean mass are even rarer, but are urgently needed.³⁰ The aim of our present study was to measure the body composition of a large group of contemporary New Zealand girls of similar chronological age in order to ascertain the size and variability of their fat stores round the time of entry to school and to quantify prospective changes in body composition after 4 years.

Methods

We report the body composition of 89 New Zealand Caucasian girls aged 4–5 y who were enrolled for studies of bone health and body composition conducted during the last 5 y in our laboratory. Of these, 66 girls participated in cross-sectional studies only. The remaining 23 girls were studied longitudinally exactly 4 y after baseline measurement, when they were aged 8–9 y. Girls were recruited by advertisement. The research was approved by our local hospital Ethics Committee and signed informed parental consent was obtained for every child participant.

The girls were weighed (electronic balance, Model 169N Tanita Corp., Tokyo, Japan) and measured (Harpenden stadiometer, Holtain Ltd, Croswell, Crymych, UK) without shoes in light clothing. BMI was determined as weight in kilograms divided by height in metres squared. Subjects were also classified as having high adiposity if their percentage fat values were greater than those estimated to correspond with high BMI values,³¹ as recently proposed by the International Obesity Taskforce.³² Tanner staging of pubertal development was self-assessed³³ in older girls using standard pictures and descriptions of breast development as described previously.³⁴ Metal objects (watches, jewellery, buckles) worn by the children were removed and body composition was measured by one operator using the same Lunar DPX-L scanner (Lunar Corporation, WI, USA) by procedures described elsewhere.³⁵ Total body scans were analysed with software package 1.35. The long-term stability of our DXA scanner was monitored by measuring phantom tissue blocks daily and a Hologic spine phantom 3 times per week. Measurements were stable throughout the study period. Our in vivo DXA precision values (coefficients of variation in adults) are (%): fat percentage(2.4%), fat mass (2.5%), lean mass (1.1%) and bone mineral content (BMC, 1.5%), respectively. Similar repeatability is obtained from in vivo repeat scan measurements performed on children.²⁹

Statistical analysis was performed with STATA (StataCorp Stata Statistical Software Release 7, Stata Corporation 2001, College Station, TX, USA) data being log-transformed where appropriate. Raw data are presented in Table 1 and Table 4 as means and s.d.'s. The tertile cutpoints for relative adiposity (percentage body fat) at baseline for the whole study population were determined and the girls were then subdivided from these into three groups having Group 1=low (<15.4%), Group 2=medium (15.4%<19.1%) or Group 3=high (19.2%) initial percentage fat values. Data were analysed by ANOVA and when P<0.05 Tukey's test was applied to ascertain levels of statistical difference between different groups.

Table 1 - Characteristics of the 89 girls grouped according to their baseline total body fat percentage.

Full table

Table 4 - Changes (4 y) in anthropometry and final body composition, and pubertal status of 23 girls studied longitudinally grouped by their baseline fat percentage status.

Full table

Results

Table 1 shows the characteristics of the study population at baseline. Total fat mass values varied widely. Although the girls were only 4 or 5 y of age, we found that there was a 10-fold difference between the lowest and highest values for fat mass. This compared with a four-fold difference for percentage fat values, and only an approximately two-fold difference for bone mineral content and lean mass between the minimum and maximum values for these variables.

Although the group of 23 girls who were studied prospectively were 3 months older (P<0.04) at baseline than the 66 girls studied only once at baseline, these groups did not differ in weight or BMI or percentage fat after adjusting for age (P0.05). Thus, we feel that these 23 girls studied again after 4 y were satisfactorily representative of the greater group. As expected, all 23 girls grew substantially and their adiposity increased with age. Although only six of these girls (all from group 3) had percentage fat values indicating overweight or obesity at baseline, nine subjects met these criteria four years later (Table 2). Moreover, we found that fat mass expanded considerably more in 4 y than other tissues, although all times 1 and 2 measurements were highly correlated (Table 3).

Table 2 - Distribution of high percentage fat at time 2 in the 23 girls who were studied longitudinally.

Full table

Table 3 - Relative gain in body composition variables as a percentage of their baseline values in the 23 girls studied longitudinally and strength of the associations between measurements at times l and 2.

Full table

The increases in fat mass and percentage fat tended to be most pronounced in girls with high initial adiposity, although this was not true for every individual (Figure 1 and Figure 2). Nevertheless, the average fat mass gain was three times higher in group 3 girls than in those from group 1 (Table 4). We found, by logistic regression, in a model adjusting for age and height at time 2 that 63.5% of the variance in percentage body fat at time 2 was explained by fat mass at time one. By 8–9 y of age, girls drawn from the highest group of initial adiposity weighed more, had higher BMI values, more body fat and a higher fat percentage than girls from group 1 (low initial adiposity). However, despite a 12 kg total average weight difference, the lean mass and BMC values of these older groups did not differ.

Figure 1.

Fat mass values at baseline (time 1) and 4 years later (time 2) in the 23 girls studied prospectively. Numbers indicate baseline fat percent group. Lines show mean values and 95% confidence intervals of the regression line.

Full figure and legend (20K)

Figure 2.

Fat percentage values at baseline (time 1) and 4 years later (time 2) in the 23 girls studied prospectively. Numbers indicate baseline fat percentage group. Lines show mean values and 95% confidence intervals of the regression line.

Full figure and legend (21K)

At baseline, all girls were prepubertal (Tanner breast stage l), whereas by the end of the study some had commenced pubertal development. As anticipated, pubertal status tended to be more advanced in girls from the top initial adiposity group than in those from other groups, although the numbers were too small for this trend to attain statistical significance.

Discussion

Our observational study shows that, in contemporary New Zealand girls, a wide variation in body fat mass is already present by the age of 5 y. Importantly, our results establish that higher adiposity subsequently tracks in these young girls, so that youngsters with high intial fat percentage have a greater likelihood of showing greater adiposity 4 y later than those who were initially leaner. Fat mass at five appears to be a strong determinant of weight and fatness at 9 y of age. This is a concern, given the rapid rises in adiposity currently being documented among preschool children in much of the developed world.^8,9,10 If we are to curb the epidemic of obesity affecting New Zealanders, there is an urgent need to put in place strategies to limit excessive early gain of excessive fat in young girls.

It is not perhaps generally appreciated that even at a young age, differences in the total body fat content make a larger contribution to the differences in body weight observed in girls of the same height and age than do differences in their lean mass and bone mass. This is evident from our results. In 4- and 5-y-old girls, the variation in fat mass at baseline was greater than that observed for either lean mass or bone mass. Thus, at baseline, girls in the high adiposity group had twice the body fat mass of those in the low group (5.3 vs 2.3 kg), although the mean values for lean mass in these groups differed by approximately only 1 kg and their average bone mineral content by only 93 g. Accordingly, the wide difference in the average fat percentage values in these groups (23.8 vs 13.2%) was mostly because of variations in fat mass. A similar picture was present in the older girls, where variation in absolute fat mass explained most of the 12 kg weight difference of girls in the high and low adiposity groupings.

Relative adiposity is expected to increase during growth.^36,37 However, our longitudinal measurements show that girls who have acquired a high fat percentage by the age of five generally continue to gain large amounts of body fat mass over 4 y. These results provide strong support for the view that body fatness tends to track strongly from a young age in girls.

Although current measurements of adiposity or BMI from a representative sample of children are not available for the whole country, recent research indicates that high levels of overweight and obesity are present in New Zealand youngsters today.⁵ At baseline, our 89 girls had a mean BMI of 16.1: this was significantly higher (P<0.03) than the mean BMI value of 15.8 in 435 girls aged 5 y, from a cohort of children born in 1972–73 from the same city.³⁸ The 23 girls we studied longitudinally increased their BMI by 18% in 4 y, which was substantially greater than the BMI rise of only 3.8% (95% CI 2.9–4.6) recorded between 5 and 9 y of age in the earlier Dunedin birth cohort study.³⁸ These comparisons provide striking evidence for the view that many prepubertal New Zealand children are fatter now, and are gaining fat faster, than those born 30 y ago.

Previous evidence for tracking of adiposity in childhood has been based on methods which do not distinguish gains in fat mass from gains in lean mass and bone mass.³⁹ Our study is one of the first with the ability to quantitate fat mass separately from lean mass to have yet been undertaken in young prepubertal children. Strengths of our study design include the use of DXA to quantitate body fat and examination of a large sample of children of the same gender and the same ethnicity. Chronological age was limited to a very narrow range, but we placed no restrictions for study entry on weight or height; all healthy children being eligible to participate. In addition, our longitudinal measurements, although limited to only 23 girls, were made at a precise time interval and were measured directly: they were not estimated by extrapolation. However, we acknowledge that because the present study sample was recruited from volunteers, it is not a true representative random sample of the population. We did not attempt to measure energy intake or energy expenditure in the study children. Nor did we collect any information concerning parental obesity status, fetal health or birth weight, which influence child adiposity.^40,41 We are merely reporting the observed changes in body fat and are not attempting to identify mechanisms responsible for child to child differences in fat gain.

At five, small girls with high adiposity frequently do not look overfat. Accordingly, parents often fail to notice the problem. However, by 9 y of age, excessive adiposity in the same children is generally all too evident. Our results establish that 4- and 5-y-old girls with high initial relative adiposity tend to maintain a considerably higher trajectory of fat gain, than girls who were leaner at baseline. Nevertheless, it is reassuring to note that not every child with high initial adiposity gained large amounts of fat. Thus, although worsening adiposity is more likely as childhood advances, it is not an inevitable consequence of having a high fat percentage at 5 y of age. Whether or not excessive adiposity becomes more severe over time will depend on the balance each child achieves between their energy intake and their energy expenditure. Our longitudinal measurements indicated that girls from the low percentage fat group were gaining on average 2 g fat per day, whereas those from the high percentage fat group were accumulating about 6 g fat daily.

Parents need to know throughout growth whether or not their child is too heavy. We believe that this information would encourage the adoption of strategies to limit excessive fat gain. An opportune time to provide this information would be when the child starts school. Although measurements of percentage fat yield the best information regarding excess fatness, these are not as accessible as measurements of weight and height. Thus, it seems reasonable to advocate that children should be weighed and measured carefully then, so that BMI status can be determined and the parents told where their child ranks on the BMI growth charts. This would facilitate early identification of children having elevated BMI values and may encourage families to adopt lifestyle changes to limit excessive fat gain. If the current obesity epidemic is to be curbed, it is desirable that sensible eating patterns and regular physical activity patterns should be put in place for all children during their early years when the seeds of adult obesity are being sown.

References

1. Troiano RP, Flegal KM, Kuczmarski RJ, Campbell SM, Johnson CL. Overweight prevalence and trends for children and adolescents. Arch Pediatr Adolesc Med 1995; 149: 1085–1091. | PubMed | ISI | ChemPort |
2. Prentice AM, Jebb SA. Obesity in Britain: gluttony or sloth? BMJ 1995; 311: 437–439. | PubMed | ChemPort |
3. Dietz WH, Gortmaker SL. Preventing obesity in children and adolescents. Ann Rev Pub Health 2001; 22: 337–353.
4. Magarey AM, Daniels LA, Boulton TJC. Prevalence of overweight and obesity in Australian children and adolescents: reassessment of 1985 and 1995 data against new standard international definitions. Med J Aust 2001; 174: 561–564. | PubMed | ISI | ChemPort |
5. Tyrrell VJ, Richards GE, Hofman P, Gillies GF, Robinson E, Cutfield WS. Obesity in Auckland school children: a comparison of the body mass index and percentage body fat as the diagnostic criterion. Int J Obesity 2001; 25: 164–169.
6. James PT, Leach R, Kalamara E, Shayeghi M. The worldwide obesity epidemic. Obes Res 2001; 9(Suppl 4): 228S–233S. | PubMed | ISI |
7. Prentice AM. Overeating: the health risks. Obes Res 2001; 9(Suppl 4): 234S–238S. | PubMed |
8. de Onis M, Blossner M. Prevalence and trends of overweight among preschool children in developing countries. Am J Clin Nutr 2000; 72: 1032–1039. | PubMed | ISI | ChemPort |
9. Bundred P, Kitchiner D, Buchan I. Prevalence of overweight and obese children between 1989 and 1998: population based series of cross sectional studies. BMJ 2001; 322: 326–328. | Article | PubMed | ChemPort |
10. Deckelbaum RJ, Williams CL. Childhood obesity: the health issue. Obes Res 2001; 9(Suppl 4): 239S–243S. | PubMed |
11. Dietz WH. Health consequences of obesity in youth: childhood predictors of adult disease. Pediatrics 1998; 101(Suppl S): 518–525. | PubMed | ISI | ChemPort |
12. Goulding A, Taylor RW, Jones IE, McAuley KA, Manning PJ, Williams SM. Overweight and obese children have low bone mass and area for their weight. Int J Obesity 2000; 24: 627–632.
13. Goulding A, Jones IE, Taylor RW, Manning PJ, Williams SM. More broken bones: A 4-year double cohort study of young girls with and without distal forearm fractures. J Bone Min Res 2000; 15: 2011–2018.
14. Berenson GS, Srinivason SR, Nicklas TA. Atherosclerosis: a nutritional disease of childhood. Am J Cardiol 1998; 82: 22T–29T. | Article | PubMed | ChemPort |
15. Lindsay RS, Hanson RL, Roumain J, Ravussin E, Knowler WC, Tataranni PA. Body mass index as a measure of adiposity in children and adolescents: relationship to adiposity by dual energy x-ray absorptiometry and to cardiovascular risk factors. J Clin Endocrinol Metab 2001; 86: 4061–4067. | Article | PubMed | ISI | ChemPort |
16. Freedman DS, Khan LK, Dietz WH, Srinivasan SR, Berenson GS. Relationship of childhood obesity to coronary heart disease risk factors in adulthood: The Bogalusa Study. Pediatrics 2001; 108: 712–718. | Article | PubMed | ISI | ChemPort |
17. Tounian P, Aggoun Y, Dubern B, Varille V, Guy-Grand B, Sidi D, Girardet J-P, Bonnet D. Presence of increased stiffness of the common carotid artery and endothelial dysfunction in severely obese children: a prospective study. Lancet 2001; 1400–1404. | Article | PubMed | ChemPort |
18. Gower BA, Nagy TR, Goran MI. Visceral fat, insulin sensitivity and lipids in prepubertal children. Diabetes 1999; 48: 1515–1521. | Article | PubMed | ISI | ChemPort |
19. Garnett SP, Cowell CT, Baur LA, Fay RA, Lee J, Coakley J, Peat JK, Boulton TJ. Abdominal fat and birth size in healthy prepubertal children. Int J Obesity 2001; 25: 1667–1673.
20. Herman-Giddens ME, Slora EJ, Wasserman RC, Bourdony CJ, Bhapkar MV, Koch GG, Hasemeier CM. Secondary sexual characteristics and menses in young girls seen in office practice: a study from the pediatric research in office settings network. Pediatrics 1997; 99: 505–512. | Article | PubMed | ChemPort |
21. Whitaker RC, Wright JA, Pepe MS, Seidel KD, Dietz WH. Predicting obesity in young adulthood from childhood and parental obesity. N Engl J Med 1997; 337: 869–873. | Article | PubMed | ISI | ChemPort |
22. Williams SM. Overweight at age 21: the association with body mass index in childhood and adolescence and parents' body mass index. A cohort study of New Zealanders born in 1972–1973. Int J Obesity 2001; 25: 158–163.
23. Power C, Lake JK, Cole TJ. Measurement and long-term health risks of child and adolescent fatness. Int J Obesity 1997; 21: 507–526.
24. Poskitt EME. Defining childhood obesity: fiddling whilst Rome burns? Acta Paediatr 2001; 90: 1361–1367.
25. Maynard LM, Wisemandle WA, Roche AF, Chumlea WC, Guo SS, Siervogel RM. Childhood body composition in relation to body mass index. Pediatrics 2001; 107: 344–350. | Article | PubMed | ISI | ChemPort |
26. Bioelectrical impedance analysis in body composition measurement: National Institutes of Health Technology Asessment Conference Statement. Am J Clin Nutr 1996; 64: 524S–532S.
27. Wells JCK, Fuller NJ, Dewit O, Fewtrell MS, Elia M, Cole TJ. Four-component model of body composition in children: density and hydration of fat-free mass and comparison with simpler models. Am J Clin Nutr 1999; 69: 904–912. | PubMed | ISI | ChemPort |
28. Taylor RW, Gold E, Manning P, Goulding A. Gender differences in body fat content are present well before puberty. Int J Obesity 1997; 21: 1082–1084.
29. Figueroa-Colon R, Mayo MS, Treuth MS, Aldridge RA, Weinsier RL. Reproducibility of dual energy X-ray absorptiometry measurements in prepubertal girls. Obes Res 1998; 6: 262–267. | PubMed | ISI | ChemPort |
30. Goran MI. Metabolic precursors and effects of obesity in children: a decade of progress, 1990–1999. Am J Clin Nutr 2001; 73: 158–171. | PubMed | ChemPort |
31. Taylor RW, Jones IE, Williams SM, Goulding A. DXA body fat percentages corresponding to recently recommended body mass index cutoffs denoting overweight and obesity in children and adolescents aged 3 to 18 years. Am J Clin Nutr 2002; 76: 1416–1421. | PubMed | ISI | ChemPort |
32. Cole TJ, Bellizzi MC, Flegal KM, Dietz WH. Establishing a standard definition for child overweight and obesity worldwide: international survey. BMJ 2000; 320: 1240–1243. | Article | PubMed | ChemPort |
33. Duke PM, Litt IF, Gross RT. Adolescents' self-assessment of sexual maturation. Pediatrics 1980; 66: 918–920. | PubMed | ISI | ChemPort |
34. Goulding A, Cannan R, Williams SM, Gold EJ, Taylor RW, Lewis-Barned NJ. Bone mineral density in girls with forearm fractures. J Bone Min Res 1998; 13: 143–148.
35. Goulding A, Gold E, Cannan R, Taylor RW, Williams S, Lewis-Barned NJ. DEXA supports the use of BMI as a measure of fatness in young girls. Int J Obesity 1996; 20: 1014–1021.
36. Siervogel RM, Maynard LM, Wisemandle WA, Roche AF, Guo SS, Chumlea WC, Towne B. Annual changes in total body fat and fat-free mass in children from 8 to 18 years in relation to changes in body mass index—The FELS Longitudinal Study. Ann NY Acad Sci 2000; 904: 420–423. | PubMed | ChemPort |
37. Bray GA, DeLany JP, Harsha DW, Volaufova J, Champagne CM. Body composition of African American and white children: a 2-year follow-up of the BAROC study. Obes Res 2001; 9: 605–621. | PubMed | ChemPort |
38. Williams S. Body mass index reference curves derived from a New Zealand birth cohort. NZ Med J 2000; 113: 308–311.
39. Katzmarzyk PT, Perusse L, Malina RM, Bouchard C. Seven-year stability of indicators of obesity and adipose tissue distribution in the Canadian population. Am J Clin Nutr 1999; 69: 1123–1129. | PubMed | ISI | ChemPort |
40. Whitaker RC, Deeks C, Baughcum AE, Specker BL. The relationship of childhood adiposity to parent body mass index and eating behaviour. Obes Res 2000; 8: 234–240. | PubMed | ChemPort |
41. Davison KK, Birch LL. Child and parent characteristics as predictors of change in girls' body mass index. Int J Obesity 2001; 25: 1834–1842.

Acknowledgements

We thank all the participants and their families for their willing cooperation and interest in our study. This work was supported by the Health Research Council of New Zealand, and the Otago Medical Research Foundation.