Prostate cancer is the most common cancer among men — and the second most frequent cause of male cancer death in the United States and Western countries — but it is far less frequent in Asian countries, according to the study authors. Observational research suggests that this geographic variation might be explained by lower soy consumption in the United States as compared to Asian countries.
“Previous epidemiologic and animal studies have suggested that eating soy may protect against prostate cancer, but this is the first randomized clinical trial to test the hypothesis with prostate cancer as the endpoint,” says Maarten Bosland, professor of pathology at the UIC College of Medicine and lead author of the study.
Researchers enrolled 177 men who had undergone radical prostatectomy at seven U.S. centers between 1997 and 2010. Participants were randomly assigned to receive a daily serving of a beverage powder containing either soy protein isolate or a placebo. The men began taking the supplement powder within four months after surgery and continued daily for up to two years. Blood tests measured prostate-specific antigen (PSA) levels every two months during the first year of the study and every three months thereafter.
The trial was stopped early when it became clear the treatment had no effect. A total of 159 participants (81 in the soy protein group and 78 in the placebo group) were evaluated after completing the baseline visit and at least one follow-up visit.
Overall, 28.3 percent of participants showed biochemical evidence of cancer (defined by a PSA level above 0.07 ng/mL) within two years of entering the trial. Detectable PSA levels after prostatectomy are commonly used to identify recurrent prostate cancer.
Twenty-two participants in the soy protein group and 23 participants in the placebo group tested positive for cancer. The difference in time to develop recurrence beytween the two groups was not statistically significant.
“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 trials,” wrote the authors. “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.”
A possible explanation for the discrepancy between previous results and the new study is that soy may be protective against prostate cancer if consumption begins early in life but not once prostate cancer is already present, the researchers said.
The research was supported in part by National Institute of Health grants (U01 CA072290, R01 CA166195, P50 CA16087, and UL1 TR000050) , with minor support from the Prevent Cancer Foundation and the United Soybean Board. Solae, LLC provided the intervention materials.
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