Among men who had undergone radical prostatectomy, daily consumption of a beverage powder supplement containing soy protein isolate for 2 years did not reduce or delay development of biochemical recurrence of prostate cancer compared with men who received placebo, according to a new study.

“Prostate cancer is the most frequently diagnosed malignancy and the second most frequent cause of male cancer death in the United States and other Western countries but is far less frequent in Asian countries. Prostate cancer risk has been inversely associated with intake of soy and soy foods in observational studies, which may explain this geographic variation because soy consumption is low in the United States and high in Asian countries,” according to background information in the article, published in JAMA (2013; 310(2):170-178).

Though soy has been proposed to prevent prostate cancer development, and supplementing with soy products is common, evidence does not exist to determine if soy supplementation has any prostate cancer-related benefits. Maarten C. Bosland, DVSc, PhD, of the University of Illinois at Chicago, and colleagues examined whether daily consumption of a soy protein-based supplement would reduce the rate of recurrence or delayed recurrence of prostate cancer in men at high risk for recurrence after radical prostatectomy.

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The randomized trial was conducted from July 1997 to May 2010 at 7 US centers and included 177 men. Supplement intervention was started within 4 months after surgery and continued daily for up to 2 years, with prostate-specific antigen (PSA) measurements made at 2-month intervals in the first year and every 3 months thereafter. Participants were randomized to receive a daily serving of a beverage powder containing 20 g of protein in the form of either soy protein isolate (n=87) or as placebo, calcium caseinate (n=90).

The trial was stopped early for lack of treatment effects at a planned interim analysis with 81 evaluable participants in the intervention group and 78 in the placebo group. Overall, 28.3% of participants developed biochemical recurrence (defined as development of a PSA level of ≥0.07 ng/mL) within 2 years of entering the trial. Twenty-two (27.2%) of the participants in the intervention group developed confirmed biochemical recurrence, whereas 23 (29.5%) of the participants receiving placebo developed recurrence.” Among participants who developed recurrence, the median [midpoint] time to recurrence was somewhat shorter in the intervention group (31.5 weeks) than in the placebo group (44 weeks), but this difference was not statistically significant,” the authors wrote.

The research team wrote that their findings were at variance with the epidemiologic evidence on soy consumption and prostate cancer risk and with experiments with animal models of prostate carcinogenesis. They emphasized that clinical outcomes from purported preventive agents need to be confirmed in observational epidemiologic studies.

They suggested that the study participants were exposed to soy later in life. Soy may be protective if consumption starts early in life but not later or when prostate cancer is already present.