Studies question role of omega-3, soy in PCa

High blood concentrations of omega-3 fatty acids are associated with an increased risk of prostate cancer, results of a recently published study show.

In a second study, researchers found that adding soy to a diet does not increase risk of prostate cancer recurrence after radical prostatectomy.

Researchers reported that high concentrations of EPA, DPA, and DHA-the three anti-inflammatory and metabolically related fatty acids derived from fatty fish and fish-oil supplements-are associated with a 71% increased risk of high-grade prostate cancer. The study also found a 44% increase in the risk of low-grade prostate cancer and an overall 43% increase in risk for all prostate cancers.

“The consistency of these findings suggests that these fatty acids are involved in prostate tumorigenesis and recommendations to increase long-chain omega-3 fatty acid intake, in particular through supplementation, should consider its potential risks,” the authors wrote online in the Journal of the National Cancer Institute (July 10, 2013).

“We’ve shown once again that use of nutritional supplements may be harmful,” said senior author Alan Kristal, DrPH, MPH, MS, of Fred Hutchinson Cancer Research Center, Seattle, whose research team previously reported a similar link between high blood concentrations of DHA and a more than doubling of the risk for developing high-grade prostate cancer. Dr. Kristal also noted a recent analysis from Greece published in JAMA that questioned the benefit of omega-3 supplementation for cardiovascular diseases (2012; 308:1024-33).

It is unclear from the current study why high levels of omega-3 fatty acids would increase prostate cancer risk, according to the authors; however, the replication of this finding in two large studies indicates the need for further research into possible mechanisms. One potentially harmful effect of omega-3 fatty acids is their conversion into compounds that can cause damage to cells and DNA, and their role in immunosuppression. Whether these effects impact cancer risk is not known.

The study analyzed data and specimens collected from men who participated in the randomized Selenium and Vitamin E Cancer Prevention Trial (SELECT). That study showed no benefit from selenium intake and an increase in prostate cancers in men who took vitamin E.

The group included in the current analysis consisted of 834 men who had been diagnosed with incident, primary prostate cancers (156 were high-grade cancer) along with a comparison group of 1,393 men selected randomly from the 35,500 participants in SELECT.

Separately, researchers from the University of Illinois, Chicago found that adding soy to a diet does not reduce the recurrence of prostate cancer after prostatectomy. Their findings were published last week in JAMA (2013; 310:170-8).

“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,” said lead author Maarten Bosland, DVSc, PhD.

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 4 months after surgery and continued daily for up to 2 years. Blood tests measured PSA levels every 2 months during the first year of the study and every 3 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% of participants showed biochemical evidence of cancer (defined by a PSA level above 0.07 ng/mL) within 2 years of entering the trial. 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 between 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.

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