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 beverages, 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 ISPϩ provided 42 mg. of genistein and 27 mg. of daidzein daily, whereas the other beverage, ISPϪ, 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: ISPϩ and to a lesser extent ISPϪ substantially increased the serum concentration and urinary output of the isoflavones and their metabolites. Serum cholesterol was significantly decreased by ISPϩ irrespective of the order in which the 2 soy beverages were administered and in apparent correlation with the total isoflavone concentration. There was no significant effect of the soy beverages on serum PSA and p105erbB-2 values.Conclusions: This study reveals that short-term exposure of elderly men with elevated serum PSA values to soy protein containing isoflavones decreases serum cholesterol but not the serum biomarkers PSA and p105erbB-2.KEY WORDS: prostate, prostate-specific antigen, soy proteins, biological markers, isoflavones Carcinoma of the prostate is a major health problem in American men. In 1998 there were more than 184,500 newly diagnosed cases and more than 39,000 deaths.1 Because there is no curative therapy for advanced prostate cancer, recent interest has focused on prostate cancer prevention. Several lines of evidence imply that dietary alterations in American men may decrease the risk for and incidence of prostate cancer. While it has been speculated that a diet with less than 30% fat may result in a decreased risk of prostate cancer, others believe that compliance with such a low fat diet would be poor. Other epidemiological studies indicate that the level of dietary soy consumption may be linked to a decreased risk of prostate cancer.2, 3 Soy contains large quantities of the isoflavone genistein (5,7,4í-trihydroxyisoflavone), a compound that is not present in other parts of the American diet ( fig. 1). 4 Men in southeast Asia, who consume 20 to 50-fold more soy daily than American men, have a 10-fold lower incidence of clinically significant prostate cancer and prostate cancer related death. 5,6 Thus, it has been hypothesized that American men may decrease the risk of prostate cancer by altering their diet to consume a higher level of soy proteins and, hence, genistein.
6Evidence that genistein is capable of inhibiting the human prostate growth wa...