Researchers in this latest study found that women with certain common DNA patterns experienced more breast cancer protection from omega-3 fatty acids than women with other common patterns.
Women whose bodies do a poor job of getting rid of the fish oils' byproducts are the ones who benefit most from consuming the oils, according to the study carried out by scientists at the Keck School of Medicine of the University of Southern California (USC) and the National University of Singapore.
They propose that the protective effects of omega-3 fatty acids are linked to the cancer-fighting properties of these byproducts.
Findings from the Singapore Chinese Health Study, a prospective investigation of diet and cancer risk in more than 63,000 Chinese men and women in Singapore, had already shown that postmenopausal women in this group, who ate the most omega-3 fatty acids were 34 per cent less likely to be diagnosed with breast cancer than women who ate the least.
The investigators suspected that lipid peroxidation products - that is, substances produced when the fatty acids break down - were behind the protection.
Manuela Gago-Dominguez, assistant professor of preventive medicine at the Keck School of Medicine of USC and the study's lead author, explained that certain enzymes in the body known as glutathione S-transferases (GSTs) help the body flush out and get rid of these lipid peroxidation products.
Each person has certain genes that carry the recipe for making GST, but these genes can be found in slightly different varieties. This can mean the difference between GST that clears substances efficiently out of the body and GST that works a little slower.
The researchers found that postmenopausal women who had low-activity versions of genes associated with GSTs (known as GSTM1, GSTT1 and GSTP1) had a lower risk of breast cancer. On the other hand, women with a combination of the lowest-activity forms of GSTM1 and GSTP1 had 64 per cent lower risk of the cancer, and women with a combination of the lowest-activity forms of GSTT1 and GSTP1 had a 74 per cent lower risk of the cancer.
Among women with high-activity versions of GST-related genes, though, they saw no evidence that fish oils reduced breast cancer risk. That held true for both pre- and postmenopausal women.
Laboratory studies have shown that cancer growth is suppressed by omega-3 fatty acid byproducts, and the suppression is enhanced by drugs that increase lipid peroxidation. When antioxidants are introduce to battle the effects of peroxidation, though, the cancer continues to grow.
"Our findings may have practical implications in treatment and prevention strategies for breast cancer," says Gago-Dominguez. "Omega-3 fatty acids have been shown to enhance the cancer-killing effect of certain chemotherapy drugs and radiotherapy in experimental studies. Since these anti-cancer agents may act through similar oxidative mechanisms as fatty acids understanding the anti-cancer effect of marine n-3 fatty acids may be important to finding the mechanisms for killing cancer cells."
The study was published in an early online version of Carcinogenesis.