Can waist circumference provide a new “third” dimension to BMI when predicting percentage body fat in children? Insights using allometric modelling

Abstract

Introduction: Body mass index (BMI) is often criticised for not being able to distinguish between lean and fat tissue. Waist circumference (WC), adjusted for stature, is proposed as an alternative weight-status index, as it is more sensitive to changes in central adiposity. Purpose: To combine the three dimensions of height, mass and WC to provide a simple, meaningful and more accurate index associated with percentage body fat (BF%). Methods: We employed a four independent sample design. Sample 1 consisted of 551 children (320 boys) (Mean ± S.D. of age = 7.2 ± 2.0 years), recruited from London, UK. Samples 2, 3 and 4 consisted of 5387 children (2649 boys) aged 7-17 years recruited from schools in Portugal. Allometric modelling was used to identify the most effective anthropometric index associated with BF%. The data from sample 2, 3 and 4 were used to confirm and cross validate the model derived in sample 1. Results: The allometric models from all four samples identified a positive mass exponent and a negative height exponent that was approximately twice that of the mass exponent and a waist circumference exponent that was approximately half the mass exponent. Consequently, the body-shape index most strongly associated with BF% was BMI√WC. The √WC component of the new index can simply be interpreted as a WC “weighting” of the traditional BMI. Conclusions: Compared to using BMI and WC in isolation, BMI√WC could provide a more effective and equally non-invasive proxy for BF% in children that can be used in public and community health settings.