CHICAGO - The latest study to explore the slippery question of whether eating fish reduces the risk of heart disease found that it does - in modest quantities and for a certain type of the illness.
The study found that people who ate the equivalent of three ounces of salmon a week were only half as likely to be stricken with cardiac arrest as those who ate no fish.
Results are published in The Journal of the American Medical Association.
The findings may seem to conflict with a well-publicized study this spring by Harvard researchers, who found that men who ate fish several times a week were just as likely to have heart trouble as those who ate fish once a month.
"(But) we view these results as complementary and not in conflict with earlier findings," said the lead author of the new study, Dr. David S. Siscovick of the University of Washington in Seattle.
The studies differed in two key ways, Siscovick said. The Seattle study focused on cardiac arrest rather than overall heart disease, which was the chief concern of the Harvard study.
Also, the Seattle study explored the value of eating some fish compared to eating none, rather than the Harvard report's focus on eating more versus eating less.
In the new study, "modest" amounts of seafood containing two key omega-3 oils were sufficient, he said.
The oils are unique to fish, and are especially plentiful in salmon, herring, mackerel and anchovies. To a lesser degree, they are present in oysters, sardines, rainbow trout, albacore tuna and other fish and shellfish.
Two three-ounce servings of albacore tuna provide as much of the oils as one three-ounce serving of salmon.
No one knows why omega-3 oils might prevent cardiac arrest, but one theory is that they may help regulate the movement of chemical compounds called electrolytes - calcium, potassium, sodium and others - in and out of cells.
Cardiac arrest is different from what most people regard as a heart attack. In a heart attack, a blocked artery prevents the heart muscle from getting enough blood, and part of it dies. Most victims survive.
In cardiac arrest, the problem isn't clogged arteries, but rather a scrambling of electrical impulses that regulate the heart's rhythm. Instead of pumping, the organ just quivers. Most victims die before emergency workers can jump-start their hearts.
Siscovick's team measured the levels of the two key oils in red blood cells in their subjects and found that higher levels were associated with lower risk of cardiac arrest. That strengthens the findings, he said.
Cardiac arrest among apparently healthy people is a rare event, Siscovick noted, estimating that it strikes about two people in 10,000 annually.
But on a nationwide scale, the problem claims more than 250,000 lives a year, representing almost half of people who die of heart problems, the American Heart Association estimates.
Dr. Alberto Ascherio, who headed the Harvard study, agreed that the Seattle research complements rather than contradicts his own.
"It's a good study," he said, adding that his own research also found an advantage in eating some fish compared with eating none.
Previous studies of fish and fish oil supplements have yielded confusing findings. Consuming fish has been found to have positive effects on blood, blood pressure and arteries. Fish oil capsules have boosted levels of "bad" cholesterol in some people and lowered other blood fats and blood pressure in others.
Ascherio and Siscovick said more research is needed.
The Seattle study analyzed all cases of cardiac arrest among apparently healthy people in King County, Washington, from 1988 to 1994. Researchers compared eating habits of the 334 people whose deaths met study criteria with 493 similar living people. The researchers controlled statistically for other factors that could have affected risk, such as family history, smoking habits, high blood pressure, diabetes, weight and physical activity. By The Associated Press