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 el derly men with elevated prostate specific antigen (PSA) on the effects of t he daily consumption of 2 soy beverages, each containing 20 gm. of isolated soy protein, on the isoflavone concentration in blood and urine, and on th e 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 consumi ng 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 bever age. The beverage ISP+ provided 42 mg. of genistein and 27 mg. of daidzein daily, whereas the other beverage, ISP-, provided only 2.1 and 1.3 mg. of t hese 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: ISP+ and to a lesser extent ISP- substantially increased the serum concentration and urinary output of the isoflavones and their metabolites. Serum cholesterol was significantly decreased by ISP+ irrespective of the order in which the 2 soy beverages were administered and in apparent correl ation with the total isoflavone concentration. There was no significant eff ect of the soy beverages on serum PSA and p105erbB-2 values. Conclusions: This study reveals that short-term exposure of elderly men wit h elevated serum PSA values to soy protein containing isoflavones decreases serum cholesterol but not the serum biomarkers PSA and p105erbB-2.