TUESDAY, June 14, 2005 (HealthDay News) -- Fish oil -- either from a diet rich in fish or from supplements -- is generally considered heart-healthy.
But a new study found that for some people, fish oil supplements may have no effect or may even be dangerous.
In people who have an implantable cardiac defibrillator because they have a history of irregular heartbeats, fish oil supplements may increase their risk of life-threatening heart rhythm abnormalities.
These findings, which appear in the June 15 issue of the Journal of the American Medical Association, came as a surprise to the study's authors.
"We would very likely have come to the conclusion that fish oil supplements would be beneficial. But, you really never know until you try," said study author Dr. Merritt Raitt, a physician at the Portland VA Medical Center, and an associate professor of medicine at Oregon Health & Science University in Portland.
"There are clearly patients who do benefit from fish oil supplements, predominantly people with recent heart attacks, but we found a population with no effect from fish oil supplements, or even an adverse effect," he added.
Because of the findings, Raitt said people with implantable defibrillators who have a history of the irregular heart rhythms called ventricular tachycardia or ventricular fibrillation shouldn't take fish oil supplements.
Ventricular tachycardia (VT), which is sometimes called V-tach, is a very rapid heartbeat that begins in the ventricles, the heart's two blood pumping chambers. In ventricular fibrillation (VF), the ventricles contract in a disorganized manner. In either case, blood isn't pumped from the heart to the rest of the body as it's supposed to be. Both conditions are common causes of sudden cardiac death, according to the National Institutes of Health.
Treatment for VT and VF is often with an implantable cardiac defibrillator, a device that senses the abnormal heart rhythm and can literally shock the heart back into a normal rhythm if necessary.
Raitt and his colleagues recruited 200 people who had implantable cardiac defibrillators (ICDs) because of a history of ventricular tachycardia or fibrillation. He said this group was chosen for the study because the ICDs keep track of the heart rhythms, and if there were any problems the devices would protect the study participants.
Half of the study volunteers were randomly assigned to receive either 1.8 grams of fish oil daily or a placebo.
After six months, 46 percent in the fish oil group had experienced VT or VF, while only 36 percent in the placebo group had. After 24 months, 65 percent in the fish oil group had experienced VT or VF, compared to 59 percent of those receiving a placebo.
The effect was even more pronounced in a subset of patients (133) who had a history of VT before the four-year study began. In this group, 61 percent of those receiving fish oil supplements experienced VT or VF within the first six months, compared to just 37 percent in the placebo group.
"Much to our surprise, patients assigned to fish oil had a trend toward a higher risk of arrhythmia," said Raitt.
Raitt said the researchers don't know why fish oil doesn't have an effect -- and may even have an adverse effect -- in this group of people. But, he said, there are definitely drugs that work differently depending on the group of patients being treated, and that may be the case here.
He said he doesn't want people to think that fish oil is dangerous for the general population.
Dr. David Haines, director the heart rhythm center at William Beaumont Hospital in Royal Oak, Mich., said he "was surprised by the study's findings."
"There's a body of evidence out there that fish oil has an anti-arrhythmic effect. This is a great study that illustrates the fact that just because things seem to make sense about the way biology should work, it can be just the opposite."
Haines said it's important to note that not everyone with an implantable cardiac defibrillator needs to be concerned about fish oil supplements, only those with a history of VT or VF.
Also, he said, while the study found an increase in the risk of irregular heartbeats, there wasn't an increase in the risk of death for people on fish oil. In fact, of 14 total deaths during the study period, only four were in the fish oil group.
"The trade-off may be more arrhythmias, but maybe the overall risk of dying is lower. It may be that if they repeated this study with 1,000 patients in each group, they might see more non-fatal arrhythmia episodes in the fish oil group," Haines said.
Still, he said, the conclusion that people with an implantable defibrillator and a history of VT or VF shouldn't take fish oil supplements is a prudent one. But, he added, he wouldn't advise those same patients to avoid fish in their diet.
Read what the American Heart Association has to say about fish oil.