Abstract
We examined sex and ethnic differences in central fatness, as assessed by waist circumference measurements, in 13 590 Millennium Cohort Study 5-year-olds. Measurements were expressed as z-scores based on reference data from the British Standards Institute. The cohort, especially girls, had larger waist circumference measurements than the reference population. Black children had larger waists, and children from other minority ethnic groups had smaller waists than White children. Girls, and Black children, in the United Kingdom are at particular risk for central fatness. Further research is needed to clarify ethnic and other influences on fat distribution, and the health outcomes associated with central fatness.
Keywords: Children, ethnicity, waist circumference, central fatness
Introduction
Ethnic group differences in overweight and obesity, defined by body mass index (BMI) and body fat, have been established (1,2). Waist circumference is an accurate marker of central fatness (3), but research on ethnic differences in this anthropometric measure is more limited, despite growing evidence of its association with metabolic abnormalities (4). Data from the Millennium Cohort Study (MCS) provide a unique opportunity to examine differences in waist circumference in a contemporary, ethnically-diverse, cohort.
Subjects and methods
The MCS is a prospective study of the social, economic and health-related circumstances of children born in the United Kingdom (UK) at the start of the new century. It employed a stratified clustered sampling design to over-represent children living in disadvantaged areas and from ethnic minority groups. Additional information on the sampling framework has been reported previously (5). Ethics approval was obtained from the South West and London Multi-Centre Research Ethics Committees, UK.
We used waist circumference data (UK Data Archive, University of Essex, UK) from the third sweep of MCS, which took place at age 5 years. This was measured in duplicate to the nearest completed millimeter on bare skin or over light clothing, using a non-elasticated SECA tape (SECA, Hamburg, Germany) positioned horizontally midway between the costal margin and the iliac crest. The correlation between the first and second measurements was 0.98. A third measure was performed if the two measures differed by more than 2 cm. Measures taken over clothing were corrected by subtracting 2 cm (Appendix 1). The average of the two closest measures was used for analysis, expressed as a z-score adjusted for age and sex based on reference data from 8 868 children (aged 3–17). These reference data were collected by the British Standards Institute in 1977 (boys) and 1987 (girls), and represent a nationally representative sample of British children, broadly consistent with the sample used to develop the British 1990 reference data for height and weight (6,7).
In total, 14 403 singleton children were measured at age 5 years (mean [Standard deviation, SD] age: 5.2 (0.25]). Families in which the main survey respondent was not female (n=370) or the child's biological mother (n=41), or there were two cohort children from the same family (n=9) were excluded. Waist circumference data were missing in 314 children or implausible (z-scores > ±5) in 22; ethnicity was not recorded in 72. As some participants had more than one exclusion criterion, the final sample size was 13 590 children (6 658 girls).
Analyses were conducted in STATA/SE 10.0 (Stata Corporation, TX), using survey and non-response weights to account for the sampling design and attrition between contacts. Multivariable regression was used to test the influence of potential confounding factors (mother's highest academic qualification, maternal socio-economic status, or number of children in household) on sex and ethnic differences in waist circumference.
Results
The mean waist circumference of the cohort was 53.7 cm (SD: 4.3) and the mean z-score 0.5 (SD 1.1), indicating a greater waist circumference than the reference population. The waist circumference z-score for girls was marginally larger than for boys (Tables I and II).
Table I.
Waist circumference summary statistics according to sex and ethnic group
| Variables | N | Mean measure cm (SD*) | Mean z-score (SD) |
|---|---|---|---|
| Sex | |||
| Boys | 6 932 | 53.9 (4.3) | 0.50 (1.11) |
| Girls | 6 658 | 53.4 (4.4)† | 0.56 (1.10)† |
| Child's ethnicity | |||
| White | 11 576 | 53.7 (4.1) | 0.55 (1.02) |
| Indian | 328 | 52.6 (6.6)‡ | 0.15 (1.67)‡ |
| Pakistani | 548 | 52.5 (6.4)‡ | 0.12 (1.68)‡ |
| Bangladeshi | 207 | 53.3 (7.7) | 0.32 (1.91)‡ |
| Black | 412 | 54.9 (6.2)‡ | 0.78 (1.44)‡ |
| Mixed/Other | 519 | 53.2 (4.9) | 0.36 (1.24)‡ |
Standard Deviation.
Significantly different from boys (p<0.05).
Significantly different from White (p<0.05).
Table II.
Influence of sex and ethnicity on waist circumference z-scores
| Adjusted regression coefficients (95% Confidence Intervals) | |||
|---|---|---|---|
| Variables | All children | Boys | Girls |
| Sex | |||
| Boys | 0 | ||
| Girls | 0.07 (0.02, 0.11) | ||
| Child's ethnicity | |||
| White | 0 | 0 | 0 |
| Indian | −0.39 (−0.59, −0.19) | −0.45 (−0.81, −0.09) | −0.33 (−0.57, −0.10) |
| Pakistani | −0.38 (−0.53, −0.24) | −0.52 (−0.72, −0.33) | −0.25 (−0.41, −0.09) |
| Bangladeshi | −0.20 (−0.42, 0.02) | −0.29 (−0.54, −0.04) | −0.10 (−0.42, 0.22) |
| Black | 0.24 (0.06, 0.41) | 0.16 (−0.05, 0.37) | 0.33 (0.15, 0.51) |
| Mixed/Other | −0.20 (−0.33, −0.06) | −0.24 (−0.44, −0.03) | −0.16 (−0.34, 0.02) |
* Adjusted for mother's highest academic qualification (collected at 9 months in the MCS), maternal socio-economic status (collected at 9 months), and the number of children in household (collected at age 5).
Black children had larger waist circumference than white children, while those of all other ethnic groups had smaller waists (Table I). Its variance was larger in all ethnic minority groups relative to White children. Following adjustment for confounding factors, ethnic differences remained (Table II). Analyses stratified by sex revealed that Indian, Pakistani and Bangladeshi boys, and those from Mixed/Other ethnic groups, had smaller waists than White boys. Indian and Pakistani girls had smaller waists than White girls, while Black girls had larger waists. Further adjustment for body mass index removed these ethnic differences (data not shown), reflecting ethnic differences in overall adiposity.
Discussion and conclusions
Children in this cohort had larger waist measurements than the reference population dating from the 1970s and 1980s, suggesting an increase in central fatness and not just general fatness assessed with BMI (8). Nevertheless, the reference population were predominantly Caucasian (6), perhaps explaining some of this difference. However, our findings do suggest that waist circumference has risen more steeply in girls than boys, indicating that they are at greater risk of this central fatness, even at a young age. This study found clear evidence of high levels of central fatness in one particular ethnic group, namely, Black children. Although waist circumference was not greater in South Asian children, body fat percentage measured by dual-energy x-ray absorptiometry has been reported to be highest in this group (2).
While there is increasing evidence of ethnic differences in obesity and other measures of body composition, this remains limited and inconclusive. We have previously reported that Black UK children have the greatest infant weight gain (9) and are among those most likely to be obese or overweight, as determined by BMI during the preschool years (1,2,10). Our findings are comparable with evidence from the United States reporting more central adiposity in Black, than White, adolescent females (11); they do, however, differ to those from Sisson et al. (12) who reported significantly higher waist circumference measurements in White, than Black, boys and no ethnic differences in girls, which may reflect sample differences and factors controlled for in our respective analyses.
This is, to our knowledge, the first population-based study of ethnic differences in waist circumference in the UK, using trained interviewers and standardised protocols. The high correlation (0.98) between repeat measurements supports this approach. In this study we have also incorporated and documented a suitable approach to the size adjustments needed when measures are taken over clothing, which could be used in future epidemiological research (Appendix 1).
Further research is needed to examine ethnic differences in body composition. While BMI is a reasonable proxy for body fatness at a population level, it is less reliable when examining ethnic group differences (13), suggesting that a range of anthropometric measurements, including waist circumference, should be used. Future studies should focus on other genetic and environmental risk factors for central fatness in early childhood, as well as its association with adverse health outcomes in the short and longer term.
Acknowledgments
We would like to thank all the Millennium Cohort families for their participation, the director of the Millennium Cohort Study and colleagues in the management team at the Centre for Longitudinal Studies, Institute of Education, University of London. We would also like to thank other members of the MCS Child Health Group at the UCL Institute of Child Health, London, UK (Helen Bedford, Carly Rich, Jane Ahn, Irina Chis Ster, Leah Li, Anna Pearce, Catherine Law, Catherine Peckham, Jugnoo Rahi, Phillippa Cumberland and Richard Jenkins).
Funding
Lucy Griffiths is supported by the Wellcome Trust (grant 084686/Z/08/A), Carol Dezateux by HEFCE, and Tim Cole by the MRC (G0700961). The Centre for Paediatric Epidemiology and Biostatistics is supported in part by the Medical Research Council in its capacity as the MRC Centre of Epidemiology for Child Health (G0400546). Research at the UCL Institute of Child Health and Great Ormond Street Hospital for Children receives a proportion of the funding from the Department of Health's National Institute for Health Research Biomedical Research Centres funding scheme. The Millennium Cohort Study is funded by grants to Professor Health Joshi, Director of the study, from the Economic and Social Research Council and a consortium of government funders. The study sponsors played no part in the design, data analysis and interpretation of this study, the writing of the manuscript, or the decision to submit the paper for publication and the authors’ work was independent of their funders.
Declaration of interest: The authors report no conflicts of interest. The authors alone are responsible for the content and writing of the paper.
Supplementary material available online
Appendix 1.
References
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