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"The small increase of blood pressure due to methylmercury will never outweigh the benefits of omega-3 fatty acids," said Dr. Eric Dewailly, a professor in the department of social and preventive medicine at Laval University in Quebec and lead author of a report in the Oct. 5 issue of Hypertension.
Diets rich in omega-3 fatty acids from oily fish, such as fatty sardines, herring, trout and salmon, are associated in many studies with a reduced risk of death from heart attack, stroke and other cardiovascular disease. The American Heart Association recommends eating two meals a week containing four to six ounces of such fish.
But because fish can contain high levels of methylmercury, which can interfere with the normal development of the nervous system and brain in fetuses and newborns, the U.S. Food and Drug Administration advises pregnant women, those trying to get pregnant, nursing women and children to limit their fish intake.
FDA guidelines limit intake of low-mercury fish for those individuals to 12 ounces a week and high-mercury fish to three 6-ounce servings a month. The FDA also advises avoiding fish most like to carry the highest levels of mercury -- shark, swordfish, king mackerel and tilefish.
At first glance, the Canadian study appears to add high blood pressure to the list of problems linked to methylmercury in fish. Dewailly and his colleagues conducted a survey of Inuit residents of 14 Nunavik communities in northern Quebec, where the traditional diet is based on fish and marine mammals.
It found an average blood mercury level of 50 nanomoles per liter of blood, much higher than the 4-nanomole level of the general U.S. population. It also found a relationship between blood mercury levels and blood pressure after adjusting for other factors, such as smoking and physical activity.
Studies have shown that exposure to environmental mercury can affect the endothelium, the delicate lining of blood vessels, and decrease the ability of smooth muscles to relax, which could explain the slight increase in blood pressure seen in the study, Dewailly said.
It was not a great effect, he said. "For every 10% increase in blood mercury level, there is a 0.2 millimeter increase in blood pressure," Dewailly said. "Even if you apply that to an entire population, that is a small effect."
So, a 10% increase in blood mercury would raise a blood pressure reading from 120/80 to 120.2/80, Dewailly indicated. That is not a reason to avoid fish "if you look at the fish nutrients that are reported to be associated with so many benefits," he said.
But it's important to eat the right kind of fish, the oily species, Dewailly said. Anyone worried about blood pressure should avoid fish that have low levels of omega-3 fatty acids and high mercury content, such as big predator fish, including swordfish, marlin and shark, he said.
Another heart expert concurred.
"Many Americans can safely enjoy eating fish as a regular part of their diet to achieve the health benefits of omega-3 fatty acids," said Penny Kris-Etherton, distinguished professor of nutrition at Pennsylvania State University and a member of the American Heart Association Council on Nutrition Metabolism and Physical Activity Committee.
"And this includes canned light tuna, which is significantly lower in mercury than white tuna," she said in a statement.
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SOURCES: Eric Dewailly, M.D., professor, preventive medicine, Laval University, Quebec, Canada; Oct. 5, 2009, Hypertension
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