Body Mass Index as a Phenotypic Expression of Adiposity: Quantitative Contribution of Muscularity in a Population-Based Sample

OBJECTIVE: Although widely applied as a phenotypic expression of adiposity in population and gene-search studies, body mass index (BMI) is also acknowledged to reflect muscularity even though relevant studies directly measuring skeletal muscle (SM) mass are lacking. The current study aimed to fill this important gap by applying advanced imaging methods to test the hypothesis that, after controlling first for adiposity, SM mass is also a significant determinant of BMI in a population-based sample. DESIGN: Whole-body magnetic resonance imaging scans were completed in CARDIA Study subjects aged 33-45 years. Physical activity (PA) levels, alcohol intake, and adequacy of food intake were assessed by standardized questionnaires. SUBJECTS: 58 African-American (AA) and 78 Caucasian (C) men; 63 AA and 64 C women. MEASUREMENTS: Whole-body AT and SM volumes. RESULTS: AT was significantly predicted by not only BMI, but PA and alcohol intake with total model R(2)s of 0.68 (p<0.0001) for men and 0.89 (p<0.0001) for women. Men had more SM than AT at all levels of BMI while SM predominated in women at lower BMIs (C <26 kg/m(2); AA <28 kg/m(2)). Both AT and SM contributed a similar proportion of between-subject variation in BMI in men. In contrast, AT contributed ~30% more than SM to the variation in BMI in women. Developed allometric models indicated SM associations with AT, PA, and race after adjusting for height. There was little association of age, lifestyle factors, or race with BMI after controlling for both AT and SM. CONCLUSION: Variation in muscularity provides a mechanistic basis for the previously observed non-specificity of BMI as a phenotypic expression of adiposity. These quantitative observations have important implications when choosing adiposity measures in population and gene-search studies.