Keck School of Medicine of USC researchers and colleagues from the National University of Singapore were the first to find in a prospective investigation that eating plenty of fish abundant in omega-3 fatty acids can cut the risk of developing breast cancer.

Writing in the Nov. 3, 2003 issue of the British Journal of Cancer, the Keck School’s Manuela Gago-Dominguez, M.D., Ph.D., reported that women who regularly consumed fish rich in omega-3 fatty acids could cut their risk of breast cancer by a quarter.

“We believe this is important, because it would be a modifiable risk factor for breast cancer—a disease in which most risk factors cannot easily be changed,” says Gago-Dominguez, assistant professor of preventive medicine at the Keck School and lead author of the study.

The research blossomed from work spearheaded by Mimi C. Yu, Ph.D., professor of preventive medicine, who has conducted extensive studies on diet and disease in the United States and Asia. Besides Gago-Dominguez and Yu, study authors also included Keck School assistant professor of research Jian-Min Yuan, research associate Canlan Sun and the National University of Singapore’s Hin-Peng Lee.

In this case, findings came from Yu’s Singapore Chinese Health Study, a prospective study of more than 35,000 Singapore Chinese women ages 45 to 74.

Scientists carefully followed the daily diet of these women and monitored their health. Of all participants, 314 were diagnosed with breast cancer during the years after they were recruited to the study.

Analyzing participants’ diets, researchers found that the overall amount of saturated, monounsaturated or polyunsaturated fat was unrelated to breast cancer risk. But when they looked specifically at the amount of fish and shellfish women consumed, the data turned interesting.

The researchers divided the women into four groups depending on the average amount of seafood they ate each day. Women in the top group ate about 81 grams (g) of seafood a day, while women in the other three groups ate about 58 g, 44 g and 25 g of seafood a day, respectively. (For comparison, a 3-ounce salmon steak would equal about 84 g of seafood.)

When scientists compared breast cancer cases in the study, they found that women in the three highest-consuming groups had a 26 percent lower risk of developing breast cancer than those in the lowest-consuming group. This jibes with earlier laboratory research, which showed that marine omega-3 fatty acids can inhibit breast cancer tumors in rodents.

They also looked at consumption of omega-6 fatty acids, and found that overall, eating more of these acids had no effect on risk. But when they looked at the balance between omega-3 and omega-6 fatty acids in the diet, they found that women who consumed little omega-3 fatty acids from fish and who ate a lot of omega-6 fatty acids—found in soybean and other plant oils, as well as some processed foods—actually had an increased risk of breast cancer.

Interestingly, they also found that of the seven women with breast cancer in the study who had a family history of the disease, five women were in the lowest-consuming omega-3 fatty acid group. They suggest further research into whether omega-3 fattyacids might be therapeutic for certain women with a genetic mutation linked to breast cancer risk.