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But a new study seems to turn that advice on its head for men: Researchers at the Fred Hutchinson Cancer Research Center in Seattle have found that men with high blood levels of fish oils have an increased risk of aggressive prostate cancer.
Men with the highest levels had a 71 percent increased risk of high-grade prostate cancer, the kind most likely to be fatal. The study also found a 44 percent increase in the risk of low-grade prostate cancer and an overall 43 percent increase in risk for all prostate cancers.
The difference in omega-3 blood levels between the highest and lowest groups is about what someone would get by eating salmon twice a week, the researchers said. The same fatty acids found in vegetable oil, flaxseed and other plant sources did not affect prostate cancer risk.
The results are startling because fish oil was thought to have an anti-inflammatory effect, which would seem to protect against cancer. But this is the second, large study the cancer researchers have done that finds that a high intake seems to have negative effects. A 2011 study found similar results. The latest study also confirms results from a large European study, the researchers said in a prepared statement.
For men who have been popping fish oil pills as added health insurance, this could be a warning to stop, Theodore Brasky of Ohio State University Medical Center, who worked on the study with the Seattle team, told CBS News.
"A 70 percent increased risk in high-grade prostate cancer, given it's the No. 1 cancer in men and fish is a commonly consumed thing and is thought to be a healthy food, I think it'd be a concern for people," he added.
The study, published today in the online edition of the Journal of the National Cancer Institute, analyzed blood samples from 834 men who developed prostate cancer compared with 1,393 men of the same age and race who did not.
Researchers cautioned that it's still unclear why high levels of omega-3 fatty acids would increase prostate cancer risk. It's possible that mega-doses of the oils could cause cell damage, researchers said -- another argument for moderation in diet and supplement use.
Edward Giovannucci of the Harvard School of Public Health, who was not involved in the study, told the Seattle Times that the research looked at whether cancer was diagnosed, but not at whether patients survived the typically slow-growing cancer.
The researchers also didn't determine what the biological mechanism was that would cause omega-3 oils to trigger prostate tumors. "If there were a compelling mechanism, that would make the findings more worrisome," Giovannucci told the Times.
Nevertheless, men might want to think twice about taking fish oil pills or eating more than two servings of salmon a week.
Photo: Mark Wiener/Alamy