How well do waist circumference and body mass index reflect body composition in pre-pubertal children?

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Abstract
Objective: To investigate the quantitative relationship between waist circumference (WC) and height (Ht), and subsequently the association between waist circumference index (WCI), body mass index (BMI) and body composition in pre-pubertal children. Design: Cross-sectional sample (n=227; boys=127) of pre-pubertal black children (age range 8.8–11.0 years) from the Bone Health sub-study of the Bt20 birth cohort study set in Soweto-Johannesburg, South Africa. Measures of height, weight and WC by anthropometry, total and truncal fat and lean mass by dual-energy X-ray absorptiometry were used in the analysis. Pearson's correlation coefficients were used to examine the associations between BMI, WC and body composition outcomes. Results: WC was independent of height when height was raised to a power of ∼0.8. BMI and WCI (WC/Ht) were significantly associated with total and truncal fat and lean mass in both sexes (all P<0.001). BMI showed consistently and significantly higher correlations with body composition than WCI and this association was significantly greater for fat mass than lean mass. Conclusion: BMI, rather than WCI, would be a better screening tool for total and truncal fat mass in both sexes before puberty.