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/m(2)] 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.