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NEW YORK - Eating more heart-healthy omega-3 fats provided no additional benefit in a study of heart attack survivors who were already getting good care, Dutch researchers report.

After nearly 3 1/2 years, there was no difference in deaths, heart attacks and other heart problems between those who ate margarine with added omega-3 fatty acids and those who didn't, the study found.

The results don't mean that getting more of the essential nutrient has no value. Several studies have offered evidence that the fats -- mostly from fish oil -- reduce heart disease.

But for heart patients who are carefully treated "adding a little bit of omega-3 fatty acid does not seem to make a difference," said Alice Lichtenstein, a Tufts University nutrition professor, who was not involved in the research.

Omega-3 fatty acids are thought to help reduce the risk of abnormal heartbeats, slow the growth of plaque that can clog arteries and lower harmful fats called triglycerides.

In recent years, omega-3 has been added to some foods such as margarine and eggs, or labels highlight the omega-3 content of foods like tuna fish.

"Now they're popping up in the most unexpected places," including trail mix, said Lichtenstein.

Two kinds of omega-3s come from wild oily fish such as salmon, mackerel and tuna. A third type comes from plants; sources include walnuts, flaxseed, soybeans and canola oil.

It's generally recommended that people eat one or two servings of fish a week, said the study's leader, Daan Kromhout of Wageningen University in the Netherlands.

"The results of this trial do not change that," he said, noting that there were no harmful side effects. "It's still a good thing to eat fish once or twice a week."

The study's participants were taking the best medicines to prevent future heart trouble, and that could be why adding a low-dose of omega-3s offered no extra protection, the researchers said. The volunteers were also older and entered the study years after their heart attacks, in contrast to heart patients in earlier research who did benefit by taking fish oil pills.

The findings were presented Sunday at the European Society of Cardiology meeting in Stockholm and published online by the New England Journal of Medicine.

For the study, Kromhout and his colleagues recruited 4,837 heart attack survivors in the Netherlands who were 60 to 80 years old. They were randomly assigned to eat one of four kinds of margarine -- regular margarine or ones with added omega-3s derived from fish, plants or both.

Kromhout said they used margarine because it was easier than fish oil supplements to make all versions look and taste identical.

On average, the patients ate about 4 teaspoons (18.8 grams) of margarine a day, spread on bread at meals, Kromhout said. During the 3 1/2 years they were followed, 671 patients, or 14 percent, had a heart problem or died. There was no difference between the groups, no matter what kind of margarine they ate.

Dr. Stephen Kopecky, a cardiologist at the Mayo Clinic in Rochester, Minn., said it may have been a matter of too little, too late — the dose was small and the patients were enrolled many years after their initial heart attack -- on average four years.

"If you wait too long, sometimes you miss that window to benefit them," Kopecky said.

He said he tells his heart patients to take 1,000 milligrams of fish oil daily.

"The benefit potentially is so great, that we tend to put patients on it quite often," he said.

Since the Dutch study was in heart attack survivors, Lichtenstein said it still isn't known whether omega-3s can protect against a first heart attack or help those patients who don't get such good care.

The study was funded by the Netherlands Heart Foundation and the U.S. Institutes of Health. Unilever, which makes an omega-3 enriched margarine, provided the margarines.