Reformulation of BMI and percent body fat to remove the height bias in 8-year-olds

BMI and percent body fat (%BF) are both related to height (Ht) in prepubertal children, so may misrepresent childhood adiposity, especially in tall or short children. We sought to construct replacement functions for BMI and %BF that are independent of Ht. Fat mass (FM) was measured using dual-energy X-ray absorptiometry, together with Ht and body mass (BM) in 746 healthy boys and girls aged 8 years (0.34 s.d.). Relationships between BM, FM, and Ht were measured and values of p and q derived such that the functions BM.Ht(-p) and FMA M-q were unrelated to Ht. BM was not directly proportional to Ht(2), BMI being significantly related to Ht in both boys and girls (P < 0.001). BM was proportional to Ht(3), BM.Ht(-3) being independent of Ht. Similarly, FM was not directly proportional to BM and %BF was significantly related to Ht (P < 0.001). While FM was proportional to BM2, FM.BM-1.5 was the function found to be independent of Ht. Using the 85th and 95th percentiles as the cutoffs for overweight and obesity respectively, 6.4% of the boys and 6.8% of the girls were classified differently by BMI and the Ht independent measure BM.Ht(-3). Similarly, 10.1 % boys and 13.7% girls were classified differently by %BF and the Ht independent measure FM.BM-1.5. We propose that improved diagnostic accuracy of body composition in 8-year-olds is provided by the BM function (BMF, BM.Ht(-3)) and FM function (FMF, FM.BM-1.5) replacing BMI and %BF, which both overestimate the adiposity of taller children and underestimate it in shorter children.