Identifying adolescents with high percentage body fat: a comparison of BMI cutoffs using age and stage of pubertal development compared with BMI cutoffs using age alone

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Abstract
Objective: To ascertain whether body mass index (BMI) cutoffs calculated according to age and pubertal stage are superior to BMI cutoffs calculated on the basis of age alone at correctly identifying children and adolescents with high levels of adiposity. Design: Cross-sectional study. Setting: Dunedin, a university city in New Zealand. Subjects and measurements: In total, 368 healthy Caucasian children and adolescents (179 males, 189 females) aged 8.5–15.5 y had their percentage body fat measured by dual-energy X-ray absorptiometry (DXA). High adiposity was defined as 25% body fat in males and 35% body fat in females. The sensitivity, specificity and areas under the receiver operating characteristic curves of two Z-score distributions of BMI were calculated for each gender. Results: Forty-nine males (27.3%) and 43 females (22.8%) had high DXA-measured percentage body fat. In all, 86% of males and females with high adiposity were correctly identified by BMI cutoffs calculated according to age alone, with 89% of males and 97% of females with low adiposity being correctly classified. Similar results were observed when BMI cutoffs utilising both age and stage of pubertal development were used; 90% of males and 88% of females with high adiposity and 92% of both males and females with low adiposity were correctly classified according to BMI. Conclusions: Categorisation of BMI according to both age and pubertal stage of development does not produce cutoffs that are superior to BMI cutoffs calculated on the basis of age alone at identifying children with high DXA-measured adiposity. Sponsorship: University of Otago and the Health Research Council, New Zealand.