Outcomes/Effectiveness Research

A diet high in omega-3 fatty acids is unlikely to reduce the risk of cancer

Taking dietary supplements containing omega-3 fatty acids or regularly consuming fish does not appear to reduce a person's risk of developing cancer, according to the findings of an in-depth analysis of large-scale U.S. and foreign population studies supported in part by the Agency for Healthcare Research and Quality (AHRQ) (290-02-0003). Some research indicates that people who consume diets high in omega-3 fatty acids are less likely to develop some types of cancer; however, researchers at the Southern California Evidence-based Practice Center in Santa Monica found very little evidence that omega-3 fatty acids reduce any one of 11 different types of cancer.

The researchers analyzed findings from a large body of literature spanning numerous groups from many countries with different demographic characteristics for the effects of omega-3 fatty acids on 11 different types of cancer—breast, colorectal, prostate, ovarian, lung, pancreatic, stomach, skin, and bladder—as well as aerodigestive cancer and lymphoma. In addition, the researchers evaluated the literature on the possible effect of omega-3 fatty acids in cancer treatment but did not find a significant association between omega-3 fatty acids and clinical outcomes after tumor surgery.

After analyzing data from prospective studies conducted in the United States and six other countries—Norway, Sweden, Denmark, the Netherlands, Japan, and China—that involved more than 700,000 patients and in some cases lasted up to 30 years, the researchers found no evidence that omega-3 fatty acids reduce overall risk of cancer. Fifty-five of the 65 analyses conducted found no effects at all. Only 10 studies yielded statistically significant results, and these were mixed. Omega-3 fatty acids appeared to increase the risk of developing some cancer, particularly breast, prostate, and lung cancer, while in other types it appeared to reduce the risk.

However, the data are not sufficient to rule out with certainty the possibility of an association between consumption of omega-3 fatty acids and cancer incidence, according to RAND Health's Catherine H. MacLean, M.D., Ph.D., who led the systematic review. RAND Health is a part of the Southern California Evidence-based Practice Center. Dr. MacLean also said that although a number of studies suggest that omega-3 fatty acids may play a role in inhibiting tumor growth in laboratory animals, it is not possible to form strong conclusions because of the quality of the studies.

For more details, see "Effects of Omega-3 Fatty Acids on Cancer Risk: A Systematic Review," by Dr. MacLean, Sydne J. Newberry, Ph.D., Walter A. Mojica, M.D., M.P.H., and others, in the January 25, 2006, Journal of the American Medical Association 295(4), pp. 403-415.

Editor's Note: This new study is part of a larger project supported by AHRQ and the National Institutes of Health's Office of Dietary Supplements which reviewed the scientific evidence of the health benefits of omega-3 fatty acids and found that that taking these supplements or eating fish has been shown to help protect against heart disease. Other reports in this series evaluated the effects of omega-3 fatty acids on cardiovascular outcomes, child and maternal health, cognitive function, asthma, and organ transplantation. All of the reports are available at AHRQ's Web site athttp://www.ahrq.gov/clinic/epcix.htm.

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