A diet high in omega-3 fatty acids offers protection against advanced prostate cancer, even in men who carry a particular variant in the COX-2 gene that is known to raise the risk of the disease.

"Previous research has shown protection (by omega-3 fatty acids) against prostate cancer, but this is one of the first studies to show protection against advanced prostate cancer and interaction with COX-2," Dr. John S. Witte of the University of California, San Francisco noted in a statement from the American Association for Cancer Research.

Witte and colleagues studied 466 men diagnosed with aggressive prostate cancer and 478 healthy matched controls. They assessed diet using a "food frequency" questionnaire and genotyped the men for nine COX-2 variants.

The researchers report in the journal Clinical Cancer Research that increasing intake of long-chain omega-3 fatty acids — the kind found in dark fish, like salmon, and shellfish — was strongly associated with a decreased risk of aggressive prostate cancer.

Men who consumed the most long-chain omega-3 fatty acids had a 63 percent reduced risk of aggressive prostate cancer compared to men who consumed the least.

"Importantly," Witte and colleagues say, this protective effect was even stronger in men who carried the COX-2 variant, rs4647310, which is a risk factor for prostate cancer.

Specifically, men with low intake of long-chain omega-3 fatty acids and this particular variant had a more than fivefold increased risk of advanced prostate cancer. But men with high intake of omega-3 fatty acids had a substantially reduced risk, even if they carried the COX-2 rs4647310 variant.

In other words, the increased risk of prostate cancer associated with the COX-2 rs4647310 variant was "essentially reversed by increasing omega-3 fatty acid intake by a half a gram per day," Witte said.

"If you want to think of the overall inverse association in terms of fish, where omega-3 fatty acids are commonly derived, the strongest effect was seen from eating dark fish such as salmon one or more times per week," he added.