MONDAY, April 25 (HealthDay News) -- High levels of heart-healthy omega-3 fatty acids in the blood may be associated with an increased risk for developing aggressive prostate cancer, while elevated levels of unhealthy trans-fatty acids may lower the risk, a new study suggests.

Researchers examined data from a U.S.-wide study of more than 3,400 men, and found that those with the highest blood percentages of docosahexaenoic acid (DHA) were two-and-a-half times more likely to develop aggressive prostate cancer than those with the lowest DHA levels.

DHA is an inflammation-reducing omega-3 fatty acid commonly found in fatty fish. In the study, the men consumed the DHA primarily from fish rather than fish oil supplements.

The study also found that the risk of aggresive prostate cancer was 50 percent lower in men with the highest blood levels of trans-fatty acids, which are abundant in processed foods and associated with inflammation and heart disease.

There was no link between prostate cancer risk and omega-6 fatty acids, which are found in most vegetable oils and are thought to promote inflammation, according to the researchers at the Fred Hutchinson Cancer Research Center in Seattle.

None of the three types of fats were associated with the risk of low-grade prostate cancer.

The study appeared online April 25 in the American Journal of Epidemiology.

The researchers noted that "the most striking aspect of our findings is that they were not in the directions hypothesized."

"We were stunned to see these results, and we spent a lot of time making sure the analyses were correct," Theodore M. Brasky, a postdoctoral research fellow in Hutchinson's Cancer Prevention Program, said in a Hutchinson news release. "Our findings turn what we know -- or rather what we think we know -- about diet, inflammation and the development of prostate cancer on its head and shine a light on the complexity of studying the association between nutrition and the risk of various chronic diseases."

But, Brasky and his colleagues don't believe men who are concerned about heart disease should stop using fish oil supplements or eating salmon or other fish that are rich in omega-3 fatty acids.

"Overall, the beneficial effects of eating fish to prevent heart disease outweigh any harm related to prostate cancer risk," Brasky said. "What this study shows is the complexity of nutrition and its impact on disease risk, and that we should study such associations rigorously, rather than make assumptions."

Dr. Elizabeth Kavaler, a urology specialist at Lenox Hill Hospital in New York City, added that, "Nutritionally, studies are very difficult to assess, since most of the participants take large amounts of the supplement being studied. Most physicians would recommend eating a healthy diet of fish, vegetables and fiber without overdoing supplements, as their benefit in one area may outweigh the risks in another."

More information

The U.S. National Cancer Institute has more about prostate cancer.