'Drugs that affect rhythms are double-edged swords. The people they're most likely to hurt are the ones with the sickest hearts, with recurrent arrhythmias, the ones we had in our study.'

The finding, from a study of 200 patients with implanted defibrillators that shock the heart back to a normal rhythm, surprised researchers who expected to find that fish oil guards against rhythm abnormalities in patients who need protection most.

The heart gets its pumping power from cells that fire in sequence as electrical charges race from one cell to another. Rhythm disturbances occur when these circuits misfire, causing the heart to beat out of sync and lose its pumping power. The result can be death caused by a kind of electrical storm rather than the clogged arteries that cause "standard" heart attacks.

Four previous studies have shown that fish oils cut the risk of fatal heart rhythm problems, called arrhythmias, in patients who had had standard heart attacks.

Patients Prone to Arrhythmias Tested

The research team chose to test fish oil in patients who were prone to arrhythmias because any benefits probably would be obvious and apply to a lot of people. About 150,000 people each year receive implanted defibrillators.

"Our initial thought was that this was a great population in which to show that fish oil is anti-arrhythmic," says study leader Merritt Raitt of the Portland VA Medical Center. His team's report appears in today's Journal of the American Medical Association.

The research was carried out at six major medical centers from 1999 to 2003. Half of the patients were given fish oil. The rest were given olive oil, a placebo.

Researchers found that 65% of the patients who took fish oil supplements developed rhythm disturbances over the next six months, compared with 36% of those in the placebo group.

Nearly two-thirds of subjects taking fish oil who had a type of rapid heartbeat known as tachycardia experienced episodes, compared with 37% of those taking placebo.

'Double-Edged Swords'

"This is a really interesting study," says Alice Lichtenstein, a Tufts University cardiovascular nutritionist. "It's telling us that just popping a pill doesn't always lead to the expected result. It may seem like a quick and easy way of treating a problem, but usually things are a lot more complex."

Dietary omega-3 fatty acids, the active ingredients in fish oil, repeatedly have been linked to a decreased heart disease risk because of their beneficial properties, Lichtenstein says.

"They're anti-inflammatory, they minimize clot formation, they may have a mild antihypertensive effect, and, if people are eating fish, they're not eating steak," she says. "That decreases their fat intake."

Raitt says he can only speculate why fish oil hurt rather than helped. But he says other drugs shown to prevent rhythm disturbances also can cause them.

"Drugs that affect rhythms are double-edged swords," he says. "The people they're most likely to hurt are the ones with the sickest hearts, with recurrent arrhythmias, the ones we had in our study."