Objective: One of the multiple health benefits of soy protein or its isoflavones may be their purported favorable effect on body composition. We examined the effect of isoflavones extracted from soy protein on overall and regional body composition, taking into account appetitive hormones as potential mediators, as well as the direct effect on appetitive hormones.
Methods: This randomized, double-blind, placebo-controlled multicenter trial included 229 healthy postmenopausal women (age, 45.8-65 y; body mass index, 24.9 +/- 3.0 kg/m) who consumed placebo or soy isoflavone (80 or 120 mg/d) tablets for 12 months. We used intent-to-treat analysis to examine changes in body composition (whole-body lean mass, whole-body fat mass, androidal fat mass, and androidal-to-gynoidal fat mass ratio) and appetitive hormones (insulin, leptin, ghrelin, and adiponectin) in response to treatment.
Results: Repeated-measures analysis of variance indicated that soy isoflavone treatment did not exert a significant effect on body composition measures (P value from 0.36 to 0.79) or appetitive hormone concentrations; the inclusion of covariates in statistical models did not alter these results. Independently of treatment, leptin and ghrelin related inversely to each body composition measure (P values from 0.044 to < or = 0.0001). Adiponectin related inversely to all fat measures (P values from 0.0004 to <0.0001). Time since last menstrual period related directly to all fat measures (P values from 0.06 to 0.0055). Dietary fat contributed to whole-body (P = 0.028) and androidal (P = 0.017) fat mass.
Conclusions: Our findings do not support a favorable effect of soy isoflavone tablets on body composition in healthy postmenopausal women.
Trial registration: ClinicalTrials.gov NCT00043745.