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 FM.BM 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 BM 2, 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.