Despite these apparently contradictory results, both groups of researchers, who reported their findings in the Nov. 28 New England Journal of Medicine -- as well as the U.S. Food and Drug Administration -- said people should continue to eat fish because the benefits far outweigh any risks from mercury for most people.
"I don't really see anything in these papers that indicates we need to change our advice on these things," David Acheson, FDA's chief medical officer in the Center for Food Safety and Nutrition, told United Press International.
However, pregnant and lactating women should avoid fish known to have high mercury levels -- which includes swordfish, king mackerel, tilefish and shark -- Acheson said. This is due to concerns that mercury could harm the brain development of fetuses and infants rather than any concern about its effects on the heart, he said.
The first study found men who had suffered a heart attack and survived tended to have high mercury levels in their blood, an indicator of fish consumption, lead study author Eliseo Guallar of Johns Hopkins University told UPI. The study of more than 1,400 men in Israel and eight European countries found those with the highest mercury levels had about twice the risk of a heart attack as men with the lowest mercury levels.
Guallar said, however, he agrees with the FDA this does not suggest people should stop eating fish.
"People should eat fish," he said. "Actually we would not like people to interpret our study that eating fish is bad."
Although mercury might play a role in damaging the heart, the omega-3 fatty acids lower the risk, Guallar explained. Indeed, his study also found men with low levels of a heart-protective omega-3 fatty acid, called docosahexaenoic acid, which indicating low fish consumption, had an increased risk of heart attack.
"The overall benefit of the fish is going to depend on what are the relative proportions (of mercury and omega-3 fatty acids) so people should try to avoid fish with high mercury and at the same time try to eat fish with high omega-3," he said. "Salmon would be a great example."
In the second study, male dentists in the United States were found to have higher levels of mercury than other healthcare professionals but this was not associated with an increased risk for heart disease.
"The bottom line is very clear for people in the U.S.," co-author of the study Walter Willett of Brigham and Women's Hospital in Boston told UPI. "We shouldn't stop eating fish because of concerns about mercury. On balance, fish is healthy and actually does reduce the risk of" fatal heart attacks.
Willett noted one reason for the apparent contradictory findings between the two studies is that the Johns Hopkins study focused on non-fatal heart attacks.
"The omega-3 fatty acids in fish are most strongly protective against fatal heart disease," he said, so if the Hopkins study had included fatal heart attacks, they might have obtained different results.
Willett pointed out that Finland is the only place in the world where fish consumption has been associated with an increased risk of heart disease. So another reason for the differences in the study findings could be fish in other parts of the world might be contaminated with different substances other than mercury that play a role in causing heart disease.
"With all food issues, balance is important," FDA's Acheson said. "Everything in moderation is the message here," he said.
(Reported by Steve Mitchell, UPI Medical Correspondent, in Washington)