Everyone knows that fish oil is good for you, right? It's a rich source of omega-3 fatty acids, which are marketed to reduce the risk of just about everything from heart disease to Alzheimer's.
But a startling study shows men who have the highest levels of these compounds—the kinds found in fish but not in vegetable sources—have a higher risk of prostate cancer. Men with the very highest levels had a 71 percent higher risk of high-grade prostate cancer—the kind most likely to spread and kill, they report in the Journal of the National Cancer Institute.
It might be a sign that popping a pill is not only possibly a waste of money—it might be downright dangerous. And eating fish too often might be, also.
"These fish oil supplements in which some men are getting mega, mega doses … in our opinion that is probably a little bit dangerous," said Theodore Brasky of Ohio State University Medical Center, who worked on the study with a team from the Fred Hutchinson Cancer Research Center in Seattle.
The same team published a study in 2011 that showed men with the highest levels of one omega-3 fatty acid called docosahexaenoic acid, DHA for short, had double the risk of high-grade prostate cancer. Other studies have had similar findings.
To try to confirm their work, the team looked at data from a different prostate cancer trial called SELECT, for Selenium and Vitamin E Cancer Prevention Trial. That study showed 17 more cases of prostate cancer among men who took vitamin E alone for about five years compared to men taking placebos.
The effect was even stronger when they looked at omega-3 fatty acids—specifically, the kinds found in fish oil as compared to those found in vegetable oils.
Brasky's team looked at 834 of the men in the SELECT trial who developed prostate cancer, and 1,393 randomly chosen others from the trial who didn't have cancer. They divided the men into four groups based on their blood levels of three omega-3 fatty acids—EPA, DPA and DHA.
Those with the highest blood levels had a 71 percent higher risk of high-grade prostate cancer, compared to those with the lowest levels. Overall, their risk of any kind of prostate cancer was 44 percent higher.
The difference between the group with the highest levels of omega-3s in their blood and those with the lowest works out to about what someone would get by eating salmon twice a week, the researchers said.
Fatty acids found in vegetable oils, flaxseeds and other vegetable sources—including alpha-linolenic acid (ALA)–did not affect prostate cancer risk, the researchers found.
"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," Brasky said in a telephone interview.
The American Cancer Society projects that 240,000 U.S. men will be diagnosed with prostate cancer in 2013, and about 30,000 will die from it.
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"We've shown once again that use of nutritional supplements may be harmful," said Alan Kristal of Fred Hutchinson, who also worked on the study.
Brasky, who says he still eats fish "but in moderation", says the study cannot answer the question of how fish oil might cause cancer. They took into account other factors that might be associated with eating fish and Brasky notes that mercury, which can be found in fatty fish, doesn't cause prostate cancer.
The study also doesn't say anything about the effects of fish oil on men who already have cancer. "This study is not about men with prostate cancer," Brasky said, noting that some studies have suggested fish oil might be beneficial in men who already have cancer.
Men might be at a loss for what to do, as omega-3 fatty acids were also believed to lower the risk of heart disease, which is far more common than prostate cancer. The American Heart Association recommends that people with heart disease eat fish twice a week and people with heart disease might need fish oil capsules.
But the researchers point out that recent studies have shown taking extra omega-3 has little effect on heart disease–including a study published in the New England Journal of Medicine in May.
—By Maggie Fox, NBC News