Heart-healthy fish oil supplements may not be so healthy in people with implantable cardioverter defibrillators (ICDs), a new study shows.

In these people, fish oil supplements may not reduce the risk of heart rhythm problems and may promote irregular heartbeats.

That does not rewrite the book on the heart benefits of fish oil. Their study "does not call into question the potential benefits of fish oil or dietary fish intake in patients who have had a [heart attack]," the researchers write in the June 15 issue of The Journal of the American Medical Association.

Researchers working on the study included Merritt Raitt, MD, of Oregon Health Sciences University. The study was funded in part by Hoffmann-La Roche Inc., which supplied the supplements. Grants also came from the National Institutes of Health and the Public Health Service.

Fish Oils and Heart Health Recommendations

The American Heart Association and a host of other health organizations have recommended eating fish as part of a healthy diet. Raitt's study doesn't question that or dismiss fish oil supplements for most people.

The study included 200 people with very specific heart conditions. Each person had an ICD and a recent history of irregular or rapid heartbeats (ventricular fibrillation, or VF, and ventricular tachycardia, or VT). ICDs are devices implanted in the chest that are designed to shock the heart back into rhythm if it beats out of control.

Most of the patients had coronary artery disease. Their hearts' left ventricles were working at a significantly reduced level. However, they had not had a recent heart attack.

Patients were randomly assigned to take fish oil pills or a placebo for about two years. None took fish oil pills or ate more than one weekly serving of fish per week before the study. They were told not to change their fish intake and to follow the American Heart Association's step 1 low-fat diet during the study.

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Surprising Results

More people in the fish oil group had irregular heartbeats that were treated by their ICDs during the study. The devices handled 45 VF and 901 VT events during follow-up.

Six months after the pills were assigned, 46 percent of fish oil group had had ICD therapy for VT/VF, compared with 36 percent with placebo. After a year, that rose to 51 percent and 41 percent, respectively. After two years, the numbers were 65 percent of the fish oil group and 59 percent of the placebo takers.

The fish oil group also tended to need an ICD shock for VT or VF sooner than the placebo group, says the study.

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No Major Differences in Serious Side Effects

The pattern observed in the fish oil group was not significant, the researchers write. In addition, there were no significant differences in serious side effects between the two groups, with the possible exception of more hospitalizations for neurological events in the placebo group.

Though an increase in irregular heartbeats was seen in the fish oil group, the researchers stress that the findings don't mean that fish oil increases the risk of sudden death.

The findings were "unexpected," write Raitt and colleagues. They note evidence that omega-3 fatty acids reduce heart-related deaths by protecting against irregular heartbeats. Omega-3 fatty acids are found in fish oils but not in olive oil.

They write that they can't rule out the possibility that chance or other factors played a role.

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Results From Past Research

In May 2004, a study in The Lancet showed that infusions of fish oil appeared to prevent irregular heartbeats in people with implanted defibrillators and a history of VT. That study only included 10 people.

A year earlier, a report in Circulation showed that adding omega-3 fatty acids to animals' heart cells prevented potentially fatal heart rhythms. That study was not done on humans.

Raitt and colleagues say other researchers who studied more than 3,000 men with coronary artery disease found a higher risk of sudden death among those assigned to eat more fish or fish oils. But those men could have veered from other heart health recommendations, thinking the fish oils would make up for lapses, say Raitt's team.

By Miranda Hitti, reviewed by Charlotte Grayson, MD

SOURCES: Raitt, M. The Journal of the American Medical Association, June 15, 2005; vol 293: pp 2884-2891. WebMD Medical News: "Fish Oil Prevents Deadly Heart Rhythms." WebMD Medical News: "Fish Oil Prevents Deadly Irregular Heartbeats." News release, JAMA/Archives.