Soy Nutrition Institute
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Jun 30, 2013

Effects of isoflavones on PSA levels in prostate cancer patients

Studies addressing the impact of isoflavone exposure on PSA levels in prostate cancer patients have produced intriguing results. Two studies published in July examined this relationship, one was a two-year study focused on tumor recurrence in 177 men treated for their cancer (1) and the other, a 6-week study involving only 44 men that intervened prior to radical prostatectomy (2). Neither study found that PSA levels were affected. However, the lack of effect is not especially surprising given the low dose of isoflavones used in these studies. In the two-year study, men were randomly assigned to the placebo or 41 mg/day isoflavones (expressed in aglycone equivalents), 24 mg of which was genistein. In the shorter and smaller study, men in the isoflavone group consumed 51 mg, but because the supplement was derived from soygerm, the daily intake of genistein, the most biologically-active isoflavone in soybeans, was only about 7 mg per day. For perspective on these exposure levels, there are about 12 mg genistein in one cup of soymilk made from whole soybeans. Hoping to see robust effects on cancer-related endpoints in cancer patients over a relatively short period of time in response to such low isoflavone intakes may be unrealistic. In contrast to these studies, research published in 2009 found that in prostate cancer patients, genistein down-regulated by 76% the expression of an enzyme involved in metastasis (3). This study intervened with 150 mg/day. Future clinical studies evaluating effects of isoflavones in prostate cancer patients should involve larger exposures than were used in the two most recently published studies. 1. Bosland MC, Kato I, Zeleniuch-Jacquotte A, et al. Effect of soy protein isolate supplementation on biochemical recurrence of prostate cancer after radical prostatectomy: a randomized trial. JAMA 2013;310:170-8. 2. Hamilton-Reeves JM, Banerjee S, Banerjee SK, et al. Short-term soy isoflavone intervention in patients with localized prostate cancer: a randomized, double-blind, placebo-controlled trial. PLoS One 2013;8:e68331. 3. Xu L, Ding Y, Catalona WJ, et al. MEK4 function, genistein treatment, and invasion of human prostate cancer cells. J Natl Cancer Inst 2009;101:1141-55.