Fish oil may fight breast cancer

A large survey of postmenopausal women has found that fish oil may guard against breast cancer. Although the study wasn’t designed to show a cause-and-effect relationship, it sets the stage for an upcoming trial of fish oil consumption that may clarify the issue.

Meanwhile, 14 other over-the-counter dietary supplements had their hopes dashed, showing no apparent benefit against breast cancer, researchers report in the July Cancer Epidemiology, Biomarkers and Prevention.

While other studies have found that fish oil supplements or a diet high in fish shows promise against cardiovascular ailments, (SN: 2/15/97, p. 101) the new study is the first to suggest a link between fish oil and a lower risk of breast cancer, says study coauthor Emily White, an epidemiologist at the University of Washington and the Fred Hutchinson Cancer Research Center in Seattle.

White and her colleagues used data from a massive survey of women in western Washington who filled out questionnaires between 2000 and 2002 regarding their diet, supplement intake, exercise habits and overall health and lifestyle. The analysis included more than 35,000 postmenopausal women ages 50 to 76 who didn’t have breast cancer at the study outset. By the end of 2007, 880 of these women had developed breast cancer.

Women who reported taking fish oil at the start of the study were roughly half as likely to develop ductal carcinoma of the breast, the most common form of breast cancer, during the follow-up years. Women taking fish oil showed no reduced risk of the less-common lobular breast cancer.

The scientists accounted for factors that might have influenced the women’s cancer risk such as age, body weight, fruit and vegetable consumption, aspirin use, smoking status, age at which they first gave birth and age at menarche.

“It seems to me that this is not a fluke or a false positive finding, as least with respect to the methods — it’s pretty solid work,” says Timothy Rebbeck, an epidemiologist at the University of Pennsylvania in Philadelphia. “Short of a randomized trial, this is as about as well as you can do. This is really something that has to be followed up.”

Researchers at Harvard Medical School are now beginning a five-year randomized trial of 20,000 people to examine the effects of fish oil and vitamin D on the risks of cancer, heart disease and other ills.

Other supplements showed no anticancer benefit in the new study. These included glucosamine, chondroitin, grapeseed, black cohosh, soy, dong quai, St. John’s wort, coenzyme Q10, garlic pills, ginkgo biloba, ginseng, melatonin, acidophilus and methylsulfonylmethane.

How fish oil might prevent cancer remains unknown, but inflammation — linked to cancer in many studies — may play a central role. Fish oil contains omega-3 fatty acids, which impede a compound called nuclear factor kappa-B, White notes. “Fish oil inhibits this major inflammatory molecule,” she says.