It may be a red herring to worry over whether people who eat lots of fish may lose whatever heart benefits they might have gained because of an increased exposure to mercury, a new study shows.

Rich in omega-3 fatty acids, fish is thought to rank high on the list of heart-healthy foods. But it has a potential dark side: many fish species that wind up on the plate have high levels of mercury, a known neurotoxin.

The scientists studied more than 900 Swedish men and women who answered questionnaires about the amount of fish in their diet. The researchers also analyzed the subjects’ red blood cells for levels of mercury and selenium, another element that has been tied to heart health.

Mercury levels in the subjects were generally low by Scandinavian standards, the Swedish team found, but higher than much of the U.S. population. But people whose red blood cells showed elevated amounts of mercury did not have a higher risk of cardiac problems than those whose red cells had less of the toxin.

In other words, “the protective nutrients in fish override any harmful effect of mercury at these low levels of mercury,” says Maria Wennberg, a public health researcher at Ume University and a member of the study team.

The American Heart Association recommends that people consume at least two servings of fish a week. Salmon, mackerel and albacore tuna are particularly high in omega-3 fatty acids.

One expert questioned the findings, which appear in the American Journal of Clinical Nutrition. Dr. David O. Carpenter, director of the Institute for Health and the Environment at the University at Albany, in Rensselaer, New York, said the Swedish researchers assumed that the mercury in the subjects’ blood cells came from fish.

“That’s just not a legitimate conclusion,” said Carpenter, who noted that many other sources of environmental mercury, including coal-fired power plants and dental fillings, might account for the presence of the element.

What’s more, he said, fish in the Baltic Sea, which borders Sweden, are high in other toxic compounds such as PCBs, which complicates the picture.

“The fact that you don’t have a significant effect (on heart risks) with measured levels of omega-3 fatty acids, and no effect of fish consumption, says to me that this study is totally inconclusive,” Carpenter said.

The researchers did not call their study the last word on the subject. Although they found no association between the amount of fish the study subjects reported eating every month - or their blood levels of two omega-3 fatty acids - and their risk of heart disease or stroke, that likely reflects the drawbacks of relying on subjects’ memory rather than a real effect, they said.

Wennberg cautioned that her study “does not discard the need of restrictions in consumption of fish high in mercury” - species including predators such as perch, shark, swordfish and halibut.

The FDA, for example, advises women who are pregnant or who might become pregnant to avoid eating shark, swordfish, king mackerel or tilefish because of their high mercury content. The agency says these women can safely eat one meal a week of albacore tuna, which has more mercury than other types of tuna.

The study did raise one potential red flag: people whose red blood cells had elevated traces of selenium appeared to be at increased risk of sudden cardiac death. The number of such cases was small, however, so the researchers called for more investigation into the possible link.

SOURCE: American Journal of Clinical Nutrition, online November 3, 2010.