People who have diets high in fish oil have been shown to be at lower risk of dying from heart attacks. But a new study has found that fish oil supplements may pose a risk to some patients who use implanted defibrillators to control irregular heart rhythms.

The researchers said the results did not mean that the omega-3 fatty acids found in fish do not help fight heart disease. But the study, which appeared in the June 15 issue of The Journal of the American Medical Association, does suggest that fish oil does not help in the way that many researchers had thought.

The theory had been that the fatty acids helped stop the irregular heart beats known as arrhythmia, which can lead to cardiac arrest. So for this study, the researchers took a group of 200 patients with defibrillators, gave half of them fish oil supplements and monitored them over a period of up to two years.

"We thought that it would be a great population to prove that fish oil is preventing arrhythmia," said the lead author of the study, Dr. Merritt H. Raitt of the Portland Veterans Affairs Medical Center and Oregon Health and Science University.

Instead, the researchers found that patients who took the supplements did no better than those who did not. And those with one kind of arrhythmia had more problems with irregular heartbeats.

As a result, Dr. Raitt said, doctors should consider advising patients with defibrillators to avoid the supplements.

Researchers began looking into omega-3 fatty acids after Greenland Eskimos, who eat a diet heavy in fish, were found to have a low risk of cardiac death. Later studies found that heart attack patients fared better with diets high in fish oil or supplements.

One explanation for the findings in the new study, Dr. Raitt said, is that the fatty acids may work differently in people who have had heart attacks.