Change in BMI Accurately Reflects Change in Percent Fat and is a Stronger Predictor of Change in Fat Mass than Change in Lean Soft Tissue Mass

Abstract

Although there is a strong curvilinear relation between body mass index (BMI) and body fatness (%Fat), the associations between changes in BMI and changes in %Fat, fat mass (FM) and lean soft tissue (LST) are inadequately characterized in elderly women of different race. PURPOSE: To determine if two-year changes in BMI accurately reflect changes in %Fat, FM, and LST in elderly women and to assess the impact of race on these relationships. METHODS: Anthropometric measures and body composition via dual-energy X-ray absorptiometry were assessed prospectively in 197 community-dwelling white (W, 68.0±6.0yrs, n=144) and black (B, 66.4±5.7 yrs, n=53) women at baseline and after two years. RESULTS: The B and W women did not differ (p > 0.05) in age at baseline or in two-year changes in anthropometric or body composition variables except for bone mineral content (BMC, B; −11.1 ±55.8 vs. W; −36.8 ± 58.8 g, p = 0.007) and height (B; −0.3±0.6 vs. W; −0.5±0.6 cm, p = 0.036), the latter being weakly related to changes in LST (r = 0.15, p = 0.036). Change in BMI was linearly related to change in %Fat (R = 0.64, SEE = 1.54 %Fat, p < 0.001) and change in FM (R = 0.90, SEE = 1.19 kg FM, p < 0.001). After controlling for change in height, change in LST and change in BMI were curvilinearly related (R = 0.76, SEE = 1.10 kg LST, p < 0.001) and that relation was significantly lower than the relation between change in BMI and change in FM (p < 0.01). As expected, changes in BMI could not predict changes in BMC. Race, age, or hormone replacement therapy did not modulate any of the above relationships (all p > 0.05). CONCLUSIONS: Two-year change in BMI accurately reflects changes in body composition in relatively healthy community-dwelling black and white elderly women with being more sensitive to change in FM than LST. Race, age, and hormone replacement therapy did not impact the relations of changes in BMI to changes in %Fat, FM, and LST. Support: NIH RO1-AG020118, UIUC Research Board