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How Bad for Your Heart Is Mercury in Fish?

Two new studies offer seemingly conflicting views

WEDNESDAY, Nov. 27, 2002 (HealthDayNews) -- When it comes to mercury, fish and the heart, you can be forgiven if it's hard to make heads or tails out of the mess.

A new study says mercury in fish negates the heart-friendly benefits of fatty acids in seafood by increasing the risk of heart attacks. But a second new paper seems to suggest that the heavy metal doesn't promote cardiovascular disease after all.

Yet the science may not be as mercurial as it seems, said Dr. Eliseo Guallar, an epidemiologist at Johns Hopkins University's Bloomberg School of Public Health and lead author of the first paper. "If you go deep into the papers they may show very similar things," Guallar said.

Most of the people in the second study, led by Harvard University researchers, were dentists -- and dentists are often exposed to metallic mercury on the job when they handle fillings. This group didn't have a higher risk of heart problems. However, the rest of the people in the study probably got most, if not all, of their mercury exposure from eating fish. And while their numbers were few, those with the most mercury intake had a 70 percent greater risk of heart disease than those with the lowest intake of the toxin.

"It may be that the effect of mercury in fish is different from the effects of metallic mercury," said Guallar, who was not involved in this study. Seafood builds up methylmercury, a form of the poison that's initially modified by plankton and works its way up the food chain.

Both studies appear in tomorrow's issue of the New England Journal of Medicine.

The Harvard study looked at mercury levels in toenail clippings of more than 33,700 men in the health professions who had no prior history of heart disease. Over five years of follow-up, 470 developed heart conditions, including heart attacks or clogged arteries that required surgery.

Mercury levels, which corresponded well with fish consumption, didn't predict a man's risk of heart problems. Yet when dentists were excluded -- leaving people whose mercury presumably came from fish meals -- a link between the two did seem possible, though it was not statistically significant.

"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," the scientists wrote.

For his study, Guallar and his colleagues compared mercury levels and heart attack risk in 1,408 European and Israeli men. Of those, 684 had suffered one heart attack. They also looked at fat stores of a substance called DHA, an omega-3 fatty acid from fish that's believed to promote cardiac health.

Men with the most mercury in their toenail clippings had about twice the risk of a heart attack as those with the least. That effect washed out the benefits of DHA, which lowered heart attack risk by more than 40 percent in men with the highest levels of the substance in their body.

But Guallar said the washout shouldn't discourage people from eating fish. Rather, people should consume species that are low or modest in methylmercury, like tuna, salmon, and trout. Fish to avoid include swordfish, shark, tilefish, and king mackerel.

The U.S. Food and Drug Administration cautions pregnant women to minimize their consumption of those species over concerns that eating mercury could harm unborn babies. The FDA also recommends that expectant mothers regularly eat no more than 12 ounces of cooked fish each week.

Dr. David Acheson, chief medical officer for the FDA's Center for Food Safety and Applied Nutrition, said agency scientists would review the latest evidence of a possible connection between mercury and heart problems.

"What this illustrates is that FDA needs to be looking at this issue, examining the science behind it and determining what should be done based on these papers," he said. No changes in its public stance on the heavy metal are imminent, Acheson added.

The American Heart Association suggests people eat at least two servings of fatty fish a week. The group also urges people to seek other sources of omega-3 fatty acids, including tofu, walnuts, canola oil and other vegetables.

What To Do

For more on mercury and pregnancy, visit the FDA. You should also try the Environmental Protection Agency for more information about mercury, including contacts of state fisheries that post advisories about local fish.

For more on fish oils, click on the American Heart Association.

SOURCES: Eliseo Guallar, M.D., Dr.Ph., assistant professor of epidemiology, Johns Hopkins University Bloomberg School of Public Health, Baltimore; David Acheson, M.D., chief medical officer, Center for Food Safety and Applied Nutrition, U.S. Food and Drug Administration, Rockville, Md.; Nov. 28, 2002, New England Journal of Medicine
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