Headlines linking fish oil to both higher and lower risk for cancer in two contradictory new studies have left millions of Americans confused and even alarmed about the safety of the nation’s most popular supplement.
First, Chinese researchers reported that high intake of fish oil reduced women’s risk for breast cancer by 14 percent. The analysis, published in the British Medical Journal, pooled data from 21 studies involving nearly 900,000 women.
Then another study, published a month later in the Journal of the National Cancer Institute, ignited a media firestorm, with such scary headlines as, "Men who take omega-3 supplements at 71 percent higher risk of prostate cancer,” and “Hold the Salmon: Omega-3 Fatty Acids Linked to Higher Risk of Cancer.”
However, some critics have attacked the prostate cancer study as “very fishy,” “flawed” or even “junk science.” In addition, media coverage of the findings has been lambasted as “disgraceful, incompetent, and scientifically illiterate.”
Earlier research reports that the fatty acids in fish (such as DHA and EPA) reduce risk for heart attacks, sudden cardiac death, heart failure, and arrhythmias, and may help ward off Alzheimer’s disease.
So should you shun fish oil or pop it as a panacea against killer diseases? Here are some surprising things you should know about the new research.
The headlines mentioned above might sound as if the researchers randomly assigned men to either take fish oil supplements or a placebo, then tracked rates of prostate cancer in both groups.
Actually, none of the men were given fish oil, nor did the study collect any dietary data, says senior study author Alan R. Kristal, Dr.P.H., associate head of the cancer prevention program at Fred Hutchinson Cancer Research Center.
Instead, the study compared the blood levels of various long-chain fatty acids in 834 men with prostate cancer and 1,393 randomly selected cancer-free men of the same age and race. All of the men had previously participated in the SELECT Trial, designed to evaluate if vitamin E and selenium prevented prostate cancer risk. The SELECT (Selenium and Vitamin E Cancer Prevention Trial) study was halted in 2008 after no benefits were found. In fact, taking vitamin E raised prostate cancer risk by 17 percent.
The mean combined percentage of omega-3s (DHA, EPA and DHA, found in both fish and fish oil) in the blood of the cancer group was 4.66 percent, compared to 4.48 percent in the cancer-free group.
“That means that the association between omega-3 fatty acids and prostate cancer in this study is based on a 0.2 percent difference,” says Bradley Bale, MD, medical director of the Heart Health Program at the Grace Clinic in Lubbock, Texas, who was not involved with the study. “To me, that’s weak scientific evidence.”
However, Dr. Kristal reports, “the difference between the lowest to the highest intake group is about what you’d get from eating fish two to three times a week.” Of the three fatty acids, he adds, “DHA had the most effect in terms of increased prostate cancer risk associated with higher blood levels.” The study found no impact from the type of fatty acid in flax oil.
When the men were divided into four groups based on the amount of EPA, DPA and DHA in their blood, those with the highest levels were 71 percent more likely to develop high-grade prostate cancer—which is more aggressive—and 44 percent were more likely to develop any form of the disease.
Trans fats—the much discussed dietary villains that raise bad cholesterol and lower good cholesterol—are found in many fried foods, such as donuts and French fries, as well commercially baked goods, including cookies and cake.
Since trans fats increase inflammation, which has been linked to everything from cancer to heart attacks, strokes, type 2 diabetes and even Alzheimer’s, the study notes that, “the findings were counter to expectations.”
Kristal theorizes that inflammation may have both positive and negative effects. “In some ways, inflammation, which omega-3 fatty acids decrease, promotes cancer by fueling cell growth, but it also clears away damaged tissue. So if a cell is on its way to becoming cancer, inflammation can help repair the damage.”
Men in the prostate cancer group also tended to be smokers (53 percent), had a close relative with prostate cancer (30 percent), regularly drank alcohol (64 percent) and were obese (80 percent), notes Dr. Bale. “Yet these confounders weren’t taken into account in the study design.”
“There was also a strong correlation between increased risk and higher levels of education, but nobody would suggest that men drop of out of school to prevent prostate cancer,” Dr. Bale adds.
Dr. Kristal, however, points out that two earlier studies, one of which was conducted by his team, also show a link between omega-3s and prostate cancer risk. “I’d advise people to toss their fish oil supplements. There’s no evidence that they’re helpful and we found evidence of harm.”
Dr. Kristal also questions the American Heart Association’s recommendation to eat fatty fish (such as salmon, tuna, herring, lake trout, and sardines) at least twice a week. “I take issue with these nutritional beliefs because they’re not based on solid science,” he adds.
In a Swedish study of 6,272 men, published in Lancet, those who did not eat fish were two to three times more likely to develop prostate cancer than those who consumed moderate to high amounts.
An even larger Harvard study involving 47,882 men reported that over a 12-year period, those who ate fish more than three times a week had a significantly lower risk of prostate cancer, compared with eating it less than twice a month. For each additional 0.5 grams of marine fatty acids consumed daily, risk of metastatic disease fell by 24 percent.
Lastly, a 2010 Canadian analysis of four studies of nearly 50,000 men reported a 63 percent drop in fatal prostate cancer.
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