The fatty acids found in fish may slightly lower a man’s risk of prostate cancer, but another type of fatty acid found in a range of foods may raise the risk, a large study suggests.

Researchers found that among nearly 48,000 U.S. men followed for 14 years, those with the highest intakes of two fatty acids found in oily fish were 26 percent less likely than men with the lowest intakes to develop advanced prostate cancer.

The opposite was true, however, when it came to alpha-linolenic acid, or ALA. Like the two fish-oil fats, ALA is an omega-3 unsaturated fatty acid that is thought to promote heart health; it is found in vegetable sources such as soybeans, canola oil, walnuts and flaxseed, and to a lesser extent in meat and dairy products.

In this study, men with the highest intake of ALA were about twice as likely as those with the lowest intakes to develop advanced prostate cancer. And the risk was increased regardless of whether the ALA came from vegetable or animal sources, according to findings published in the American Journal of Clinical Nutrition.

The role of dietary fat in prostate cancer is not yet fully clear, but a number of studies have suggested that a diet heavy in meat and saturated fat raise the risk. Eating a lot of animal fat seems a likely risk factor, study co-author Dr. Edward L. Giovannucci told Reuters Health, but not enough is known about the effect of overall ALA intake to give men any hard and fast diet rules.

“We wouldn’t want to recommend completely avoiding ALAs,” said Giovannucci, a researcher at the Harvard School of Public Health in Boston.

He noted that although there is a “potential downside” of ALA intake where prostate cancer is concerned, the fatty acid has been shown to benefit cardiovascular health. Limiting animal fat, which is also a rich source of saturated fat, is one way for men concerned about prostate cancer to cut back on ALAs, according to Giovannucci.

On the positive side, he pointed out, the findings add to evidence that the omega-3 fats found in fish - known as EPA and DHA - have important health benefits. Oily fish such as salmon, tuna and mackerel are high in the fats.

Prostate cancer is exceedingly common among elderly men, but because it is typically slow-growing it will never cause problems for most. Where dietary fat is thought to come in, Giovannucci said, is in promoting the growth of early prostate tumors. Lab research suggests that EPA and DHA inhibit prostate tumor progression, whereas one study showed that ALA can spark cancer cell growth in a lab dish.

The current study included 47,866 male health professionals between the ages of 40 and 75 who were cancer-free in 1986 and were followed for 14 years. Periodic food surveys were used to gauge the men’s intake of different fats, as well as fish-oil supplements.

When other factors were considered - such as age, family history of prostate cancer and overall diet - EPA and DHA, but not fish-oil supplements, seemed to modestly reduce the risk of prostate cancer. According to the researchers, this suggests that additional substances in fish, such as vitamins D and A, may contribute to the anti-cancer effect.

SOURCE: American Journal of Clinical Nutrition, July 2004.