Omega-3 fatty acids have long been one of the most commonly used medical supplements. In the 1970s, researchers studying the Greenland Intuit Tribe noted that while they consumed large amounts of fat from fish, they displayed virtually no cardiovascular disease. This finding was attributed to the high amount of omega-3 fatty acids in their fish-rich diet.
The main reason I prescribe omega-3 fatty acids is to lower triglycerides, a type of bad cholesterol that can inflame the pancreas if their level in the blood gets too high. But omega-3s are taken to treat many other conditions, including dry eyes, depression, and attention deficit disorder (ADD). The evidence of their efficacy for these other indications is not robust. But since Omega-3s occur in nature and are not thought to have many serious side effects (aside from possibly fishy breath!), many doctors and patients ask themselves “why not try them?” Many people take Omega-3s not for any specific reason, but as a general measure to maintain good health. Such an attitude seemed validated by a 2010 study published in the Journal of the American Medical Association (JAMA) which showed that people with a higher concentration of omega-fatty acids in their blood seemed to be aging slower. The ends of chromosomes, called telomeres, are shortened with aging. The telomeres in people with higher serum levels of omega-3s were shortening at a slower rate than others.’
With all this omega-enthusiasm, it came as a surprise when a recent study in the Journal of the National Cancer Institute suggested a possible downside to the fishy pills. Studying men over age 50, it found that those with the highest levels of omega-3 fatty acids in the serum had a 43% higher risk of developing prostate cancer, and a 71% higher chance of developing high-grade prostate cancer, which is more likely to be fatal. The study did not examine whether the men with higher omega-3 levels had them due to supplements or their diet.
Does this mean Omega-3s cause prostate cancer? Not necessarily. We would need a different kind of study to answer this question. One possible partial explanation for the association between prostate cancer and higher omega- 3 levels is that men who ingest more omega-3 fatty acids may be the kind of guys who get regular check-ups and are screened for prostate cancer. More prostate cancer was detected in men with higher omega-3 levels, but maybe men with lower levels had prostate cancer that was never screened for and detected. A similar kind of study bias accounts for why taking hormone replacement therapy was long associated with a reduced risk of coronary artery disease in women after menopause. It turned out that women receiving hormone replacement therapy were more likely to being seeing their doctor and taking other steps (exercising, eating healthy, controlling blood pressure) that reduce heart disease risk. It was not until a later prospective, interventional trial (the Women’s Health Initiative) that we came to see that taking hormone replacement therapy actually increases the risk of coronary artery disease. We would need a similar type of study to see if ingesting more omega-3 fatty acids really increases men’s risk of developing prostate cancer.
One important take-away from this study is that nutritional supplements are not always as safe as they seem. It is wise to have a specific reason for taking a supplement that is supported by strong scientific evidence.