Tracking Body Composition Changes Over Time: Comparison of Methods

Abstract

Tracking changes in body composition (BC) is critical for observational and intervention studies. Little is known about the best approaches to tracking BC changes. PURPOSE: To compare air plethysmography (AP) and bioelectrical impedance (BI) estimates of BC in a group of college students, some who lose and some who gain body mass (BM). METHODS: 66 college students (26 males, 40 females, 18.1 y) were measured at the beginning and end of their first year at college. AP and BI were used to estimate fat mass (FM), fat-free mass (FFM), and body fat percentage (%BF). BI also assessed kg of trunk fat. BM, body mass index (BMI), and waist circumference (WC) were measured using standard procedures. Repeated measures ANOVA determined whether AP and BI yielded similar results. Bivariate correlation and multiple regression determined which variables were related to changes in BM, BMI and WC. RESULTS: There was wide variation in BC change, but the majority of individuals gained in BM, WC, and FM. On average, subjects gained 1.6±2.9 kg in BM, 1.24±3.02 cm in WC, and 0.35±1.03 kg/m2 in BMI. %BF increased by 0.64±3.42 and 1.37±2.76% according to AP and BI, respectively. AP yielded higher estimates of %BF than BI (24.0 ± 10.5% vs. 22.8 ± 8.6%, p=0.029). The change in FM estimated from AP was highly and significantly (p<0.01) related to change in BM (r=0.81), BMI (r=0.77), and WC (r=0.61). Change in FM estimated from AP was a stronger predictor of change in WC, BMI, and BM than the AP-derived change in FFM or %BF or any of the variables estimated by BI. CONCLUSION: AP and BI both reflect an increase in FM over time in college students. However, differences in estimates from the 2 methods do not allow them to be used interchangeably. The changes in FM according to AP are most reflective of the changes in BM, BMI, and WC.