Baked or broiled but not fried, fish helped reduce the risk of atrial fibrillation, Dr. Dariush Mozaffarian of Brigham and Women's Hospital and Harvard Medical School in Boston and colleagues found.

"The results suggest that regular intake of tuna or other broiled or baked fish may be a simple and important deterrent to atrial fibrillation among older men and women," Mozaffarian said in a statement issued by the American Heart Association.

More than 2 million Americans are affected by atrial fibrillation, a chronic condition that causes fatigue, shortness of breath and an inability to exercise.

The heart's two upper chambers, called the atria, quiver instead of beating effectively. Blood is not pumped out properly and may pool and clot.

These clots cause about 15 to 20 percent of strokes.

Writing in the journal Circulation, Mozaffarian and colleagues said they studied 4,815 people over the age of 65.

They asked them to describe what they ate, beginning in 1989, and then watched them for 12 years.

Doctors discovered 980 cases of atrial fibrillation in the volunteers. Those who reported eating more baked or broiled fish were the least likely to have atrial fibrillation.

Those who said they ate fish one to four times per week had a 28 percent lower risk, compared to those who ate fish less than once a month.

The researchers credit the omega-3 fatty acids found in many types of fish as well as in walnuts, flaxseed and many green leafy vegetables. Omega-3's are also believed to reduce the risk of a range of heart disorders, and are important to brain development and function.