Patients who took fish-oil supplements did no better than those who did not. And those with one kind of arrhythmia had more problems with irregular heartbeats.

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.

Irregular Heartbeats

The theory had been that the fatty acids helped stop irregular heartbeats 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 Raitt of the Portland Veterans Affairs Medical Center and Oregon Health and Science University.

More Problems with Irregular Heartbeats

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, Raitt said, doctors should consider advising patients with defibrillators to avoid the supplements.

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