Fatty fish, fish-oil linked to increased prostate cancer risk

SEATTLE -- Scientists at Fred Hutchinson Cancer Research Center have confirmed previous studies demonstrating there is a link between high blood concentrations of omega-3 fatty acids, such as those found in fish-oil supplements and salmon, and an increased risk of prostate cancer.

"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.

A study published July 11 in the "Journal of the National Cancer Institute" reports 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-percent increased risk of high-grade prostate cancer. The study also found a 44-percent increase in the risk of low-grade prostate cancer.

The increase in risk for high-grade prostate cancer is important because the five-year survival rate of those cancers is less than 50 percent, Kristal said.

These findings confirm a 2011 study published by the same Fred Hutch scientific team that reported a similar link between high blood concentrations of DHA and a more than doubling of the risk for developing high-grade prostate cancer. The latest study also confirms results from a large European study.

Kristal was so surprised by the 2011 results he asked his assistant to run the data again. Omega-3 fatty acids are widely believed to have a host of positive health effects based on their anti-inflammatory properties.

"The expectation is that omega-3 fatty acids reduce cancer risk," Kristal said. "It's part of a generational story - 25 years ago we were very naive and we thought large doses of micronutrients could prevent cancer. Now we're learning many have no impact or cause the cancers we were trying to prevent."

Kristal suggests the public should consider the risks of omega-3 fatty acid supplements like fish oil, especially if they have other risk factors for prostate cancer such as a family history of the disease. He also noted a recently published analysis which questioned the benefit of omega-3 supplementation for cardiovascular diseases. The analysis, which combined the data from 20 studies, found no reduction in all-cause mortality, heart attacks or strokes.

"Supplement use, except in very specific cases, is not a good thing," Kristal said. "Taking large doses of anything, evidence shows that it is harmful or not beneficial."

Still, Kristal said eating fatty fish like salmon a couple times a week is not likely to increase your cancer risk.

The study did not determine why high levels of omega-3 fatty acids would increase prostate cancer risk; Kristal said further research is needed to find such answers. But, 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. Still, whether these effects impact cancer risk is not known.

Researchers are also not sure whether omega-3 fatty acids are linked to risks of other cancer types.

The current study analyzed data and specimens collected from men who participated in the Selenium and Vitamin E Cancer Prevention Trial - a large randomized, 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.