High blood levels of omega-3 fatty acids have been linked to an increased risk of prostate cancer, according to a new study from the Fred Hutchinson Cancer Research Centre (Fred Hutch) in Seattle.

Published in the online edition of the Journal of the National Cancer Institute, the latest findings indicate 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 per cent increased risk of high-grade prostate cancer. The study also found a 44 per cent increase in the risk of low-grade prostate cancer and an overall 43 per cent increase in risk for all prostate cancers.

The study’s authors said the increase in risk for high-grade prostate cancer is important because those tumours are more likely to be fatal.

“We’ve shown once again that use of nutritional supplements may be harmful,” said Dr Alan Kristal, the paper’s senior author and member of the Fred Hutch Public Health Sciences Division.

The researchers said the newest findings confirm their 2011 research, which reported a similar link between high blood concentrations of DHA and a more than doubling of the risk for developing high-grade prostate cancer. A large European study also recently found a similar link.

“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 of the US study wrote.

Findings surprising

The authors said the findings in both the latest study and the 2011 research were surprising because omega-3 fatty acids are believed to have a host of positive health effects based on their anti-inflammatory properties. Inflammation plays a role in the development and growth of many cancers.

Researchers said it was unclear from this study why high levels of omega-3 fatty acids would increase prostate cancer risk, however, the replication of this finding in two large studies indicated 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, according to the researchers. Whether these effects impact cancer risk is not known.

“What’s important is that we have been able to replicate our findings from 2011 and we have confirmed that marine omega-3 fatty acids play a role in prostate cancer occurrence,” said Theodore Brasky, corresponding author and Research Assistant at the Ohio State University Comprehensive Cancer Centre. “It’s important to note, however, that these results do not address the question of whether omega-3’s play a detrimental role in prostate cancer prognosis,” he said.

Benefit of omega-3 supplementation for cardiovascular disease also questioned

Researchers from the latest study noted that another recent analysis, published in the Journal of the American Medical Association, questioned the benefit of omega-3 supplementation for cardiovascular diseases. That analysis, which combined data from 20 studies, found no reduction in all-cause mortality, heart attacks or strokes.

Study method

The current study analysed data and specimens collected from men who participated in the Selenium and Vitamin E Cancer Prevention Trial (SELECT), a large randomised, placebo-controlled trial to test whether selenium and vitamin E, either alone or combined, reduced prostate cancer risk. 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 omega-3 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.

The difference in blood concentrations of omega-3 fatty acids between the lowest and highest risk groups was about 2.5 percentage points (3.2 per cent vs 5.7 per cent), which is somewhat larger than the effect of eating salmon twice a week, according to the study’s authors.

The National Cancer Institute and the National Centre for Complementary and Alternative Medicine funded the research. Also participating in the study were additional Fred Hutch scientists and researchers from the University of Texas, University of California, University of Washington, National Cancer Institute and the Cleveland Clinic.

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