Epidemiological studies have suggested that groups of people who consume diets high in omega-3 fatty acids, found in certain fish and vegetables, may experience a lower prevalence of some types of cancer, according to background information in the article. Many small trials have attempted to assess the effects of omega-3 fatty acids on cancer treatment by adding omega-3 fatty acid to the diet either as omega-3 fatty acid-rich foods or as dietary supplements. Because of the results of some studies, a number of omega-3 fatty acid-containing dietary supplements have appeared on the market claiming to protect against the development of a variety of conditions including cancer, even though studies have reported mixed results.

Catherine H. MacLean, M.D., Ph.D., of RAND Health, Santa Monica, Calif., and colleagues assessed the validity of claims that omega-3 fatty acids prevent cancer by systematically reviewing the literature for studies that evaluated the effect of omega-3 fatty acids on the incidence of cancer. Using several databases and other sources, the researchers identified 38 articles, published between 1966 to October 2005, which met the study criteria. Reviewers independently abstracted detailed data about the incidence of cancer, the type of cancer, the number and characteristics of the patients, details on the exposure to omega-3 fatty acids, and the elapsed time between the intervention and outcome measurements.

The researchers found: "Among 65 estimates of association calculated across 20 different cohorts for 11 different types of cancer and 6 different ways to assess omega-3 fatty acid consumption, only 10 are statistically significant. Significant associations between omega-3 fatty acid consumption and cancer risk were reported for breast cancer in 4 studies; for colorectal cancer in 1; for lung cancer in 2; for prostate cancer in 2; and for skin cancer in 1. However, for each breast, lung, and prostate cancer, there were significant associations for both increased risk and decreased risk and far more estimates that did not demonstrate any association. The study that assessed skin cancer risk found a significantly increased risk. Hence, no trend was found across many different cohorts and many different categories of omega-3 fatty acid consumption to suggest that omega-3 fatty acids reduce overall cancer risk."

"Omega-3 fatty acids appear not to affect a mechanism of cancer development that is common across the different types of cancers evaluated in this report. Likewise, there is little to suggest that omega-3 fatty acids reduce the risk of any single type of cancer," they write.

"A large body of literature spanning numerous cohorts from many countries and with different demographic characteristics did not provide evidence to suggest a significant association between omega-3 fatty acids and cancer incidence. Dietary supplementation with omega-3 fatty acids is unlikely to reduce the risk of cancer," the researchers conclude.

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