Soy may aid men at high risk of prostate cancer
worldwide, may benefit from increased intake of soy isoflavones,
suggests a new study from the University of Minnesota.
The research, published in the Journal of Nutrition, adds to earlier studies that claimed to be the first prospective study to report an inverse association between isoflavone and prostate cancer in Japanese men (J. Nutrition, Vol. 137, pp. 1974-1979; Cancer Epidemiol Biomarkers Prev, Vol. 16, pp. 538-545). Over half a million news cases of prostate cancer are diagnosed every year world wide, and the cancer is the direct cause of over 200,000 deaths. More worryingly, the incidence of the disease is increasing with a rise of 1.7 per cent over 15 years. The new randomised controlled trial investigated the potential of soy isoflavones to increase the excretion of urinary estradiol (E2) and lower ratio of urinary 2-hydroxy estrogens to 16-hydroxyestrone (2:16 OH-E1), two oestrogen metabolites suggested to initiate hormone-related cancers. Indeed, according to background information in the new study, soy supplementation has been shown to increase the ratio 2:16 OH-E1 in women. However, no studies to date have looked at such effects in men. Lead researchers Jill Hamilton-Reeves recruited 58 men at high risk for developing advanced prostate cancer and supplemented their normal diets with one of three protein isolates: isoflavone-rich soy protein isolate, providing 107 mg isoflavones per day; isoflavone-poor soy protein isolate, providing less than six mg isoflavones per day; or milk protein isolate. All supplements provided 40 grams of protein per day. After three and six months of supplementation, the researchers report that both soy groups had higher E2 urinary excretion than subjects receiving the milk protein. Moreover, after six months of supplementation, a significantly higher urinary 2:16 OH-E1 ratio was observed amongst individuals receiving the isoflavone-rich soy protein isolate than the milk protein. "Increased urinary E2 excretion and 2:16 OH-E1 ratio in men consuming soy protein isolate are consistent with studies in postmenopausal women and suggest that soy consumption may be beneficial in men at high risk of progressing to advanced prostate cancer as a result of effects on endogenous oestrogen metabolism," wrote the researchers. The earlier study (Cancer Epidemiol Biomarkers Prev) linking isoflavones to potential protection from prostate cancer stated that the benefits could be due to the weak oestrogenic activity of soy isoflavones, which may act to reduce testosterone levels and inhibit 5 alpha-reductase - an enzyme involved in the metabolism of testosterone. Moreover, an animal study published in Biology of Reproduction (2004, Vol. 70, pp. 1188-1195) claimed that the metabolite of the soy isoflavone daidzein stopped the effect of the male hormone dihydrotestosterone (DHT), which has been linked to prostate growth and male baldness. Source: Journal of Nutrition October 2007, Volume 137, Pages 2258-2263 "Soy Protein Isolate Increases Urinary Estrogens and the Ratio of 2:16-Hydroxyestrone in Men at High Risk of Prostate Cancer" Authors: J.M. Hamilton-Reeves, S.A. Rebello, W. Thomas, J.W. Slaton and M.S. Kurzer