THE EFFECT OF ISOLATED SOY PROTEIN ON PLASMA BIOMARKERS IN ELDERLY MEN WITH ELEVATED SERUM PROSTATE SPECIFIC ANTIGEN

Purpose: We performed a randomized double-blind crossover pilot study in elderly men with elevated prostate specific antigen (PSA) on the effects of the daily consumption of 2 soy bever-ages, each containing 20 gm. of isolated soy protein, on the isoflavone concentration in blood and urine, and on the 3 serum biomarkers cholesterol, PSA and the soluble p105 component of the p185erbB-2 proto-oncogene. Materials and Methods: A total of 34 men supplemented their diet by consuming 1 of 2 soy protein beverages assigned randomly twice daily for a 6-week period. In a second 6-week period they consumed the other soy protein beverage. The beverage ISP1 provided 42 mg. of genistein and 27 mg. of daidzein daily, whereas the other beverage, ISP2, provided only 2.1 and 1.3 mg. of these isoflavones daily, respectively. Blood and 24-hour urine samples were obtained before the study, at 2-week intervals during the study and 2 weeks after study completion. Results: ISP1 and to a lesser extent ISP2 substantially increased the serum concentration and urinary output of the isoflavones and their metabolites. Serum cholesterol was significantly decreased by ISP1 irrespective of the order in which the 2 soy beverages were administered an