Soy Protein Supplementation In Men Following Radical Prostatectomy: A 2-Year Randomized, Placebo-Controlled Clinical Trial

Background: Many studies have addressed effects of dietary supplementation with soy protein, but most have been inconsistent and few have been long-term studies in men.Objectives: This study was a secondary analysis of body weight, blood pressure, thyroid hormones, iron status, and clinical chemistry in a 2-y trial of soy protein supplementation in middle-aged to older men.Methods: Data were analyzed as secondary outcomes of a randomized controlled trial of dietary supplementation with 20 g/d soy protein isolate, providing 41 mg/d total isoflavones and 23 mg/d genistein, in 44- to 75-y-old men who were at risk of cancer recurrence following prostatectomy randomized to soy (n = 50) or a casein-based placebo (n = 43). Weight, blood pressure, and blood samples were collected at baseline, every 2 mo in year 1, and every 3 mo in year 2.Results: Compared with casein, soy supplementation did not affect body weight, blood pressure, serum total cholesterol, calcium, phosphorus, and thyroid hormones. Serum ferritin concentrations doubled over 2 y in both groups (117-129%), whereas hemoglobin and hematocrit increased slightly. In an exploratory subgroup analysis of soy group data, weight increased in subjects producing equol but not in nonproducers. Blood pressure was reduced in nonequol producers but not in producers. Other endpoints were not affected by equol production status.Conclusions: Soy protein supplementation for 2 y compared with a casein-based placebo did not affect body weight, blood pressure, serum total cholesterol, iron status parameters, calcium, phosphorus, and thyroid hormones. Exploratory analysis suggests that equol production status of subjects on soy may modify effects of soy on body weight and possibly blood pressure.