Fish oil may help lower the risk of breast cancer, but the benefit may depend largely on a woman’s genetic makeup, researchers have found.

Their study of middle-age and older women in Singapore found that those who carried “low-activity” versions of certain genes were less likely to develop breast cancer when they ate a diet rich in fish that contain omega-3 fatty acids.

In contrast, fish oils provided no apparent breast cancer protection to women who carried other forms of the genes, which code for enzymes called glutathione S-transferases (GSTs).

The findings were published online recently by the medical journal Carcinogenesis.

GSTs are believed to rid the body of certain byproducts that are produced when omega-3 fatty acids are metabolized. In women with low-activity versions of the enzymes, these omega-3 byproducts would be expected to linger in the body for a longer time.

Therefore, the new findings support the hypothesis that it’s the metabolic byproducts of omega-3 fats that confer breast cancer protection, said Dr. Manuela Gago-Dominguez, an assistant professor of preventive medicine at the Keck School of Medicine of the University of Southern California in Los Angeles.

The results, she told Reuters Health, build on evidence from lab experiments that such omega-3 byproducts, which are produced by a process called lipid peroxidation, can kill breast cancer cells.

For this latest study, Gago-Dominguez and her colleagues gathered data from a trial of more than 63,000 middle-age and older men and women in Singapore.

The researchers were able to analyze dietary reports and DNA samples from 399 women who developed breast cancer during the study, and 670 women who did not. Focusing on variations in three GST genes, they found that omega-3 fatty acids lowered breast cancer risk only in women with low-activity forms of the genes.

Among women with these gene variations, high intake of omega-3 fatty acids appeared to cut breast cancer risk by up to 74 percent, depending on which combination of low-activity variants a woman carried.

“These are women whose bodies do a poor job of getting rid of the beneficial byproducts of omega-3 fatty acids,” Gago-Dominguez explained.

Although the findings suggest that only some women may benefit from fatty fish when it comes to breast cancer risk, Gago-Dominguez said the study could also have implications for breast cancer treatment.

She noted that experimental research has shown that omega-3 fatty acids may enhance the effects of certain cancer drugs. Understanding the mechanism by which the fatty acids may fight breast cancer, she said, could help scientists find better ways to kill cancer cells.

SOURCE: Carcinogenesis; advance online edition.