Soy Protein Supplementation Does Not Reduce Risk of Prostate Cancer Recurrence after Radical Prostatectomy
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 to men who received placebo, according to a study in the July 10 issue of JAMA, published online today.
Prostate cancer is the most frequently diagnosed cancer and second most frequent cause of male cancer death in Western countries, but it is far less frequent in Asian countries. Prostate cancer risk has been inversely associated with soy intake in observational studies, which could be a possible explanation for this geographic variation, but this hypothesis has not been tested in randomized clinical trials with cancer as the endpoint.
Study Details
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 of recurrence after radical prostatectomy. The randomized trial was conducted from July 1997 to May 2010 at seven U.S. 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 randomly assigned 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 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 a PSA level ≥ 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 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.
Adherence was greater than 90%. There were no differences in adverse events between the two groups.
Findings Highlight Importance of Clinical Trials
“The findings of this study provide another example that associations in observational epidemiologic studies between purported preventive agents and clinical outcomes need confirmation in randomized clinical trials. Not only were these findings at variance with the epidemiologic evidence on soy consumption and prostate cancer risk, they were also not consistent with results from experiments with animal models of prostate carcinogenesis, which also suggest reduced risk,” the researchers wrote.
The authors concluded, “One possible explanation for these discrepant results is that in both epidemiologic studies and animal experiments, soy exposure typically occurred for most or all of the life span of the study participants or animals; there are no reports of such studies in which soy exposure started later in life. Thus, it is conceivable that soy is protective against prostate cancer when consumption begins early in life but not later or when prostate cancer is already present. If this is the case, chemoprevention of prostate cancer with soy is unlikely to be effective if started later in life, given the high prevalence of undetected prostate cancer in middle-aged men.”
The study was supported in part by grants from the National Institute of Health, with minor support from the Prevent Cancer Foundation and the United Soybean Board. Solae LLC provided the intervention materials. For full disclosures of the study authors, visit www.jama.ama-assn.org.
The content in this post has not been reviewed by the American Society of Clinical Oncology, Inc. (ASCO®) and does not necessarily reflect the ideas and opinions of ASCO®.
