Dec 02 (Reuters Health) - Will consuming mercury- contaminated fish increase a man's risk for heart disease? Maybe yes and maybe no, according to two new studies with essentially opposite findings published in Thursday's issue of The New England Journal of Medicine.

While experts know that exposure to high levels of mercury can cause neurologic and kidney damage, little is known about the long-term consequences of low levels of exposure.

Previously, Finnish researchers reported a link between heart disease and increased levels of mercury in men whose mercury levels were measured from hair samples. The authors of one of the current studies note that the men were likely exposed to mercury by eating locally contaminated fresh water fish.

While consumption of fish rich in heart healthy omega-3 fatty acids is believed to cut heart disease risks, the researchers, led by Dr. Eliseo Guallar of Johns Hopkins Medical Institutions in Baltimore, Maryland, questioned whether the mercury often contained in fish might offset such benefits or increase the risk of heart disease.

To investigate, Guallar and colleagues measured levels of DHA, a type of omega-3 fatty acid, in fat tissue and mercury levels from toenail clippings in 684 men who previously suffered a heart attack. Their results were compared to a similar group of 724 men with no history of heart disease ("controls").

Mercury levels in the men who had a heart attack were "15% higher than those in controls," Guallar and colleagues write. And those with the highest levels of mercury were more than twice as likely to have had a heart attack than the men with the lowest mercury levels, the authors add.

In addition, the researchers found that after adjusting for mercury levels, high DHA levels were "inversely associated" with heart attack risk. In other words, higher levels of DHA appeared to lower a man's risk of having had a heart attack and vice versa.

Guallar's team points out that the US Food and Drug Administration currently advises pregnant women and women who may become pregnant to steer clear of fish known to have higher levels of mercury, including tilefish, shark, swordfish and mackerel. In light of the new findings, they suggest that perhaps such advice should be "extended to the general adult population."

"However, our findings do not imply that people should stop eating fish," Guallar and colleagues write. "Our mercury-adjusted analysis is consistent with a protective effect of dietary fish, provided it is not heavily contaminated."

In the second study, Dr. Kazuko Yoshizawa of the Harvard School of Public Health in Boston, Massachusetts, and colleagues found no association between mercury levels and heart disease.

The researchers measured levels of mercury in toenail clippings from 33,737 male health professionals between the ages of 40 and 75. After 5 years follow-up, 470 cases of heart disease were diagnosed among the group.

While mercury levels were significantly correlated with fish consumption and dentists showed the highest mercury levels, there was no correlation between the amount of mercury in toenails and heart disease risk.

"Our findings do not support an association between total mercury exposure and the risk of coronary heart disease, but a weak relation cannot be ruled out," Yoshizawa and colleagues conclude.

The opposing findings of the two studies underscore the controversy of whether or not mercury, especially from eating fish, endangers the heart, note Drs. P. Michael Bolger and B. A. Schwetz of the US Food and Drug Administration in College Park, Maryland.

"The notion that methylmercury contributes to cardiovascular disease is certainly a testable hypothesis and one that warrants further testing," they write.

But evidence for such a link from large, well-designed studies of populations who rely on fish as a staple food would be needed to justify changes in dietary recommendations, Bolger and Schwetz conclude.