(HealthScoutNews) -- Eating fish just once a month can reduce a man's risk of stroke by more than 40 percent, a study finds.

The omega-3 polyunsaturated fatty acids in fish prevent blood clots from forming and thus reduce the incidence of ischemic strokes, the kind that happen when a clot blocks an artery, says a report in the current Journal of the American Medical Association.

A number of studies have documented the beneficial effects of eating fish for prevention of heart disease, but this is one of just a few to look at the effect on stroke.

The information comes from the Health Professional Follow-Up Study, which has followed more than 43,000 men for 12 years, chronicling their dietary habits and illnesses. It finds a 43 percent reduction in strokes for men who ate one to three fish meals a month compared to those who ate no fish at all.

"The overall findings are not surprising," says Dr. Ka He, a researcher in the departments of nutrition and epidemiology at the Harvard School of Public Health. What is somewhat surprising is that there was no evidence of major risk reduction of stroke by consuming fish more often. The incidence of stroke was reduced just 46 percent for men who ate fish six times a month.

The reduction was entirely confined to ischemic strokes. There was no effect on hemorrhagic strokes, which happen when a blood vessel bursts. There is even a theoretical possibility that eating fish increases the risk of hemorrhagic stroke, He says, although there is no solid evidence of that from the study.

"We weren't surprised to find that adding fish to one's diet would prove beneficial," He adds. "But we were surprised to see how small amounts of fish and omega-3 fatty acids, eaten regularly, can significantly reduce the risk of ischemic stroke for men. The message is clear for men: incorporate fish, whether it's lobster, canned tuna or salmon, into your diet and reduce the risk for stroke."

But don't think that just eating fish once a month can justify ignoring other risky behaviors that can lead to heart disease and stroke, says Alice Lichtenstein, director of the cardiovascular nutrition research program at Tufts University and vice chairwoman of the American Heart Association's nutrition committee.

Looking at the study closely shows that men who ate fish also tended to: have lower blood levels of trans-fatty acids and saturated fats, eat more fruits and vegetables, engage more in physical activity and smoke less, Lichtenstein says.

"There is a constellation of factors, all taken together, that dramatically cut risk," she says. "Eating a tuna fish sandwich once a month will not do the trick."

The kind of fish that is eaten can be important. Light-meat fish, such as flounder or whiting, have lower levels of the omega-3 fatty acids compared to dark-meat fish such as salmon, mackerel, bluefish or sardines. And frozen fish dinners can be prepared with a lot of saturated fats and trans-fatty acids.

Still, Lichtenstein says, the study is "another example of how eating fish is associated with better health."

What To Do

You can learn more about healthy eating from the American Heart Association. And read this MSNBC story on fish and heart disease.

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