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Soy protein supplement failed to prevent prostate cancer recurrence
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Daily consumption of a soy protein powder beverage for 2 years failed to delay or prevent disease recurrence in men who had undergone radical prostatectomy for prostate cancer, according to results of a randomized, double blind trial.
Soy consumption has been suggested to reduce the risk for prostate cancer recurrence, but the hypothesis has not been tested as part of a randomized trial in which prostate cancer was a clinical endpoint.
An estimated 48% to 55% of men diagnosed with prostate cancer use dietary supplements including soy products, despite a lack of definitive evidence that shows soy supplementation offers cancer-related benefits in this population, according to background information provided by researchers.
The current study included 177 men at high-risk for cancer recurrence who were treated across seven centers between July 1997 and May 2010.
All patients received 20 grams of protein daily in the form of a beverage powder that contained either soy protein isolate (n=87) or placebo, calcium caseinate (n=90). Supplemental intervention began within 4 months after surgery and continued up to 2 years. Researchers took PSA measurements at 2-month intervals during the first year, and every 3 years thereafter.
The trial was halted early due to a lack of treatment effects. The published results include 81 evaluable participants in the intervention group and 78 in the placebo group.
Overall, biochemical recurrence occurred in 28.3% of patients within 2 years. Confirmed biochemical recurrence occurred in 29.5% of patients assigned placebo and 27.2% of patients in the intervention arm. The median time to recurrence among those who developed recurrence was 31.5 weeks in patients in the intervention arm vs. 44 weeks among those assigned placebo (P=.62), a difference that was not statistically significant.
Maarten C. Bosland
“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,” Maarten C. Bosland, DVSc, PhD, professor and director of graduate studies at the University of Illinois at Chicago, and colleagues wrote. “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.”
Researchers observed no significant differences in adverse events between the groups. Adherence was similar in the two arms.
“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,” the researchers wrote. “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.”
Disclosure: The research was supported by NIH grants, the Prevent Cancer Foundation and the United Soybean Board. The researchers report consulting roles with Eigen, GTX and Bayer, a speaking role with Janssen and a clinical trial investigator role with Steba Biotech.
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