Supplementation with Soy-Protein−Rich Foods Does Not Enhance Weight Loss

The objective of this study was to determine whether a weight-loss diet high in soy-protein−rich foods leads to greater weight loss and reductions in waist circumference, fat mass, and cardiovascular disease risk factors than a control diet. Groups of overweight women (body mass index [calculated as kg/m2] of 28 to 33, aged 25 to 49 years) were counseled to decrease their caloric intake by 500 kcal/day for a period of 12 weeks; in addition, the soy-protein−rich group was counseled to consume 15 g soy protein/1,000 kcal daily. Soy-protein−rich foods were provided to subjects. Body weight, waist circumference, percent body fat, fat-free mass, lipids, glucose, and insulin were measured at repeated intervals. Forty-seven women completed the study. Both groups lost a similar amount of weight both when a completers only (−3.18%±0.63% vs −4.04%±0.95% for soy-protein−rich and control diets, respectively) and a last-observation-carried-forward analysis (−1.93%±0.50% vs −2.50%±0.67%, for soy-protein−rich and control diets, respectively) were performed. There was no difference between groups in change in percent fat mass (−5.31%±1.50% for soy-protein−rich diet vs −3.94%±1.68% for control diet), percent fat-free mass, and waist circumference. There was no dietary assignment-by-group interaction on lipid, glucose, and insulin concentrations when analyses were done on completers only or on all subjects using a last-observation-carried-forward approach. Percent change in any of the biochemical parameters studied over the 12-week period was not significantly different between groups. Our results do not lend support to the emerging notion that soy-protein−rich foods could be considered potential functional foods for weight management, in the quantities consumed in this study.