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"These anti-inflammatory omega-3s were associated with a 43 percent increased risk for prostate cancer overall, and a 71 percent increased risk in aggressive prostate cancer," said study lead author Theodore Brasky, a research assistant professor at Ohio State University Comprehensive Cancer Center in Columbus, who was at Hutchinson at the time of the study.
Aggressive prostate cancer is often fatal, he added.
Omega-3 fatty acids, found in fish such as salmon, trout and fresh tuna and in fish oil capsules, are widely reputed to have health benefits because of their anti-inflammatory properties.
But this new research, published online July 11 in the Journal of the National Cancer Institute, confirms damaging evidence reported in two prior studies.
Just why are these omega-3 fatty acids associated with prostate cancer? "That's the million dollar question," Brasky said.
Omega-3 fatty acids may have properties that aren't well understood and in high doses could cause oxidative stress, which can lead to DNA damage, possibly increasing the risk for prostate cancer, he speculated.
Although omega-3 fatty acids have been touted as beneficial to heart patients, results of a study published May 9 in the New England Journal of Medicine found they didn't live up to the claims.
"These fatty acids have been promoted as a blanket anti-chronic disease fatty acid," Brasky noted. "But nutrition is more nuanced, as is disease occurrence. It's about time we stop talking about foods as good or bad and no gray area," he said.
Based on these and other findings, Brasky thinks men should probably moderate their intake of fatty fish and fish oil supplements.
One expert cautions that these new findings don't show a cause-and-effect relationship between prostate cancer and omega-3 fatty acids.
"All of these studies on associations, which is what this is, are hypothesis-generating because they are looking back in time," said Dr. Anthony D'Amico, chief of radiation oncology at Brigham and Women's Hospital in Boston. "It's not a cause and effect."
The study would have to account for other risk factors for prostate cancer before it could be considered definitive, he said. These include family history, age and race, among others, D'Amico explained.
The researchers compared blood levels of omega-3 fatty acids in more than 800 men later diagnosed with prostate cancer with blood samples from nearly 1,400 men who did not develop the disease.
The difference in blood levels of omega-3 fatty acids between the lowest- and highest-risk groups was about 2.5 percent (3.2 percent versus 5.7 percent) -- a gap larger than achieved by eating salmon twice a week, the researchers noted.
The investigators found that men eating the most fatty fish and taking the most fish oil supplements had an overall 43 percent increase in risk for all prostate cancer, compared with men eating the least fish or taking the fewest supplements. The risk for aggressive prostate cancer was 71 percent higher; for non-aggressive prostate cancer, the risk was 44 percent greater.
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