THURSDAY, June 27 (HealthDay News) -- Regularly eating oily fish such as salmon, tuna or sardines may help reduce the risk of breast cancer, a new report suggests.
These fish contain a type of fatty acid known as n-3 polyunsaturated fatty acids (PUFAs).
"Increased n-3 PUFA intake has a protective effect for breast cancer," said researcher Duo Li, a professor of nutrition at Zhejiang University in Hangzhou, China.
Li and his team reviewed 21 different studies that looked at the intake of fish and PUFAs. The previously published research included more than 800,000 women in the United States, Europe and Asia, and 20,000 cases of breast cancer. The follow-up time varied, from four years to 20.
The new report is published online June 27 in the journal BMJ..
Fish include several types of PUFAs that are involved in chemical messaging in the brain, helping to regulate both blood vessel activity and the immune system. The fatty acids also have been linked with other health benefits, such as lower risk of heart problems.
Earlier studies have shown conflicting results about the protective effects of PUFAs that are found in fish and breast cancer risk. So Li decided to pool the results of the 21 studies and reanalyze them.
In his analysis, consumption of most types of PUFAs -- but not fish itself -- was linked with a lower risk. Women with a high intake of PUFAs had a 14 percent reduction in breast cancer risk. For every 0.1-gram-per-day increase in the intake of the fatty acids, there was a 5 percent lower risk of breast cancer, the study found.
So how much fish should you eat? "One to two servings of oily fish per person per week is suggested," Li said.
Li said he can't explain with certainty the association between PUFAs and lowered breast cancer risk. Among other possibilities, he speculated that the fatty acids may help regulate the activities of molecules involved in cell growth and in the spread of cancer cells.
Two U.S. experts who reviewed the new findings saw pros and cons to the report.
Although the number of women studied was large, the link found between fatty acid intake and breast cancer risk reduction "is not necessarily cause-and-effect," said Dr. Stephanie Bernik, chief of surgical oncology at Lenox Hill Hospital in New York City.
Dr. Joanne Mortimer, director of women's cancer programs at the City of Hope Comprehensive Cancer Center in Duarte, Calif., agreed. "My take on this is it may be more than just what they eat" that helps reduce breast cancer risk, she said. "To make an assumption that the lower risk is due entirely to diet may be a false one."
The women with a high intake of PUFAs also may be more apt to exercise and follow other healthy habits, Mortimer said.
Both Mortimer and Bernik cautioned against focusing too much on fish for risk reduction or on eating too much of it. "It's no cure-all," Bernik said. And, if eaten in excess, the mercury content of some fish can be unhealthy, she added.
Besides eating oily fish, Bernik tells patients to exercise regularly, eat plenty of fruits and vegetables, and not smoke to reduce the risk of breast and other cancers.
SOURCES: Duo Li, Ph.D., M.Sc., professor, nutrition, Zhejiang University, Hangzhou, China; Joanne Mortimer, M.D., director, women's cancer programs, City of Hope Comprehensive Cancer Center, Duarte, Calif.; Stephanie Bernik, M.D., chief, surgical oncology, Lenox Hill Hospital, New York City; July 27, 2013, BMJ, online