"We in the West are just not eating enough omega-3s," says William Harris, a University of South Dakota nutritionist who is one of the leading experts on these fats. He notes two of three big clinical trials of fish oil show a reduction in deaths. "That's what sets omega-3s apart" from numerous other hyped vitamin supplements.

Omega-3s' benefits are mainly proven in heart disease, where big clinical trials have shown that lots of fish or doses of fish oil supplements can help people who already have heart disease. In patients with heart disease, trials have found that fish oil pills can modestly decrease a person's chances of dying. The fats may act as a mild blood thinner, and may even reduce the risk of sudden cardiac arrest by improving the electrical conductivity of heart cells.

A 1989 study of 2,000 men who had just had heart attacks found that those assigned to eat a lot of fish or take fish oil pills were 30% less likely to die during two years of treatment. A 19,000-patient study in Japan found that purified EPA fish oil pills reduced major heart problems by 19% in heart patients already taking cholesterol-lowering statins like Lipitor and Zocor.

The benefits in healthy people are murkier. But epidemiology studies indicate that fish oil may protect the hearts of healthy people as well. The American Heart Association recommends that healthy people eat oily, omega-3 containing fish twice a week. It recommends that patients with heart disease get a gram of the oil from fish or supplements each day.

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