The research adds to a growing body of evidence that fish oils are good for the heart.
Both studies are based on long-term monitoring of the health of medical professionals and the findings are important because half the people who suddenly collapse and die of a heart attack do not have a history of heart problems.
In one of the studies, published in the New England Journal of Medicine, researchers found that the risk of sudden death from a heart attack decreased by as much as 81 percent for people who ate fish regularly.
The new results from the Physicians Health Study, an ongoing evaluation of 22,000 male doctors that began in 1982, looked at men with no known heart problems.
The New England Journal released the study a day early because the Journal of the American Medical Association was publishing similar research.
In that study, researchers found that women can significantly reduce their risk of heart disease by eating fish at least twice a week.
The study of almost 85,000 women found those who ate fish two to four times weekly cut their risk of heart disease by 30 percent, compared with women who rarely ate fish. Women who ate fish five or more times weekly reduced their risk 34 percent.
Past studies showed similar benefits for men, but this was the first to look specifically at the effect in women, lead researcher Dr. Frank Hu said. A study last year found that women who ate fish two to four times weekly cut their risk of ischemic, or clot-related, strokes by 48 percent.
"It was very important to confirm that the beneficial effects of fish are also tied to women," said Hu, assistant professor of nutrition in the Harvard School of Public Health.
The findings add to the rapidly growing evidence of the health benefits of omega-3 fatty acids, which are especially plentiful in dark, oily fish such as mackerel, salmon and sardines.
Researchers examined 16 years of data on 84,688 participants in the Nurses' Health Study. The women were ages 34 to 59 and had no sign of heart disease at the outset. In the following 16 years, researchers documented 1,513 cases of heart disease, including 484 deaths and 1,029 heart attacks.
While eating fish reduced the risk of nonfatal heart attacks, it appeared even more protective against sudden death from heart failure.
Besides preventing clotting — much as aspirin can — omega-3 fatty acids help prevent irregular heartbeats, which can lead to sudden death, Hu said.
The New England Journal study, led by Dr. Christine Albert, an epidemiologist at Brigham and Women Hospital in Boston, measured levels in the blood of the long-chain n-3 fatty acids, which are the type found in fish, especially fatty fish like salmon, mackerel, bluefish, swordfish and arctic char.
"The men with the higher fatty acid levels had a markedly lower risk of dying of heart disease. Specifically, they had a lower rate of sudden death," Albert told Reuters.
The study "is yet another addition to the growing body of evidence" that fish consumption "may provide protection against sudden death from cardiovascular causes," said Dr. Irwin Rosenberg of Tufts University in a Journal commentary.
Albert said the doctors, on average "ate about 1 to 4 meals of fish per week. There's a lot of evidence from other studies that only a small amount of fish is needed. The American Heart Association recommends two meals per week."
She said previous studies have shown people with heart disease are less likely to die suddenly if they consume fish oil capsules, which can cause stomach upset.
Another study currently under way is following people with implanted defibrillators, which constantly check the heart, to see if people who take fish oil capsules are less likely to develop a deadly heart rhythm.
Albert and her colleagues said if fish oil turns out to guard against sudden death, "increasing the intake of n-3 fatty acids by eating more fish or by taking supplements is an intervention that could be applied to this segment of the population at low cost and little risk."