A small piece of fish each day may help keep the heart doctor away.

That's the finding of an extensive study of Dutch men in which deaths from heart disease were more than 50 percent lower among those who consumed at least an ounce of saltwater fish per day than those who never ate fish.

Researchers from the University of Leiden in the Netherlands concluded that "the consumption of as little as one or two fish dishes per week may be of preventive value in relation to coronary heart disease."

The Dutch research is one of three human studies published today in the New England Journal of Medicine that give strong scientific backing to the long-held belief that eating fish can provide health benefits, particularly to the heart.

Heart disease is the number-one killer in the United States, with more than 550,000 deaths occurring from heart attacks each year. But researchers previously have noticed that the incidence of heart disease is lower in cultures that consume more fish than Americans do. There are fewer heart disease deaths, for example, among the Eskimos of Greenland, who consume about 14 ounces of fish a day, and among the Japanese, whose daily fish consumption averages more than 3 ounces.

For 20 years, the Dutch study followed 852 middle-aged men, 20 percent of whom ate no fish. At the start of the study, the average fish consumption was about two-thirds of an ounce each day, with more men eating lean fish such as cod than fatty fish such as herring, tuna fish and mackerel.

During the next two decades, 78 of the men died from heart disease. The fewest deaths were among the group who regularly ate fish, even at levels far lower than those of the Japanese or Eskimos. This relationship was true regardless of other habits -- such as age, high blood pressure, or blood cholesterol levels -- that may influence a person's risk of heart disease, said Dr. Daan Kromhout and his colleagues.

An editorial in the journal by Dr. John A. Glomset of the University of Washington said this research and the other two studies "support the possibility that the consumption of fish may be of special benefit to human health," offering a "substantial protective effect" against heart disease. But Glomset cautioned that "an important question that has yet to be answered is whether the consumption of fish also correlated, perhaps unfavorably, with mortality from cancer and other diseases."

Experts are not sure how fish helps the heart. But Glomset noted that one strong possibility focuses on so-called "omega 3" polyunsaturated fatty acids -- particularly one, eicosapentaenoic acid or EPA -- found in high amounts in edible fish tissues. EPA appears to differ in chemical effect from the polyunsaturated fatty acid known as linoleic acid that is found in vegetable oil and is more common in the American diet.

A second study in the journal by researchers at the Oregon Health Sciences University found that patients who ate a four-week diet rich in salmon fish oil had a marked reduction in dangerously high levels of triglycerides in their blood and blood cholesterol, both of which have been associated with a higher risk of heart disease.

Researchers found that fatty fish, such as salmon, as well as lower-fat white-meat fish were beneficial. In contrast, a diet high in polyunsaturated vegetable oils was less effective. In addition, this Oregon group reported at a recent scientific meeting in Washington on new studies with primates showing that a diet rich in fish and fish oils also seems to be important to good vision and brain function, particularly in early stages of development.

The third study in this week's journal, conducted by Harvard University scientists, also found other biological effects of diets rich in fish-oil fatty acids. They found that seven normal men who ate fish supplements over a six-week period underwent changes in their white blood cells that helped guard against inflammation.

Glomset speculated that the fish fatty acids may protect against heart disease by affecting both blood platelets and white blood cells in a way that helps keep blood vessel walls unclogged. It is known that the blood of Eskimos takes longer to clot when they eat large amounts of fish.

Most coronary heart disease is linked to hardening of the arteries, or atherosclerosis, a progressive disease in which excessive fats and cholesterol in the blood clog the inner walls of these vessels. When a blood clot gets stuck in a narrowed artery, a heart attack can occur.