Associations between body mass index and body composition measures in a birth cohort

BACKGROUND: Associations among body composition measures have been limited to cross-sectional analyses of different subjects. We identified cross-sectional relationships between body mass index (BMI) and other body composition measures and predicted body composition measures from BMI throughout childhood and adolescence. METHODS: BMI was calculated and % body fat (%BF), fat mass index (FMI), and fat-free mass index (FFMI) were measured using dual energy x-ray absorptiometry at ages 5, 9, 11, 13, 15, and 17 years in a birth cohort (n=629). Sex-specific body composition measures were calculated for BMI-for-age percentiles, associations between BMI and body composition measures were characterized; and body composition measures were predicted from BMI. RESULTS: %BF, FMI, and FFMI generally increased with BMI-for-age percentiles at each age. Correlations between BMI and %BF or FMI were generally higher at BMI-for-age percentiles ≥ 95% than for lower BMI-for-age percentiles. Correlations between BMI and FFMI were generally higher for participants at very low and very high BMI-for-age percentiles than at moderate BMI-for-age percentiles. Age- and sex-specific predictions from BMI are provided for %BF, FM, and FFMI. CONCLUSIONS: Sex-specific body composition measures throughout childhood and adolescence are presented. BMI is a better indicator of adiposity at higher than at lower BMI values.