Effect of soy supplementation on regional lean and fat mass and resting metabolic rate in caucasian and African American postmenopausal women

OBJECTIVE: To investigate the effect of soy supplementation on regional lean and fat mass and on resting metabolic rate (RMR) in postmenopausal women. DESIGN: Randomized, double-blinded, placebo-controlled trial. MATERIALS AND METHODS: Thirty-nine postmenopausal women were randomized for 3 months to a daily shake supplement containing either soy protein or a casein placebo. At baseline and at follow up, dual x-ray absorptiometry (DXA) scans were performed to measure total, trunk, arm, and leg lean and fat mass, and regional bone mineral density; RMR was determined by indirect calorimetry. Medians ± interquartile ranges were determined, and the change over time was calculated. We tested the significance of change over time using the Wilcoxon rank-sum test, with statistical significance set at p<0.05. RESULTS: Groups were similar at baseline. Soy supplementation did not affect total or regional lean or fat mass, bone mineral density, or RMR in the group as a whole. However, in the soy group, Caucasians gained 1.9 kg of weight compared to a loss of 1.45 kg in African Americans (p=0.0161). Caucasians tended to lose fat in the trunk compared to a loss of fat in the extremities for African Americans with the supplement. Caucasians gained lean mass in the trunk compared to the extremities with the supplement (p=0.0209). Bone mineral density decreased in the trunk compared to the extremities in Caucasians (p=0.0382), with no change in African Americans. In Caucasians but not African Americans, the supplement increased average RMR compared to placebo (p= 0.0161). CONCLUSION: Daily soy supplementation produces differential effects by race on regional lean and fat mass. In Caucasians, average RMR is increased, possibly related to an increased lean trunk mass with the supplement. In African Americans, soy supplementation reduces body weight by decreasing fat mass in the extremities, and lean mass in the trunk and extremities.