WEDNESDAY, March 23 (HealthDay News) -- Though repeatedly linked to neurological deficits in children and unborn babies, Americans' level of exposure to mercury from sources such as fish is not associated with a higher risk of heart disease, stroke or other cardiovascular disease, a new study suggests.
Building on prior research that produced inconsistent results, scientists from Harvard School of Public Health and Brigham and Women's Hospital in Boston evaluated data from two separate studies on more than 173,000 men and women who answered questions about their medical history, risk factors, disease incidence and lifestyle.
The researchers also measured mercury concentrations in the stored toenail clippings -- a reliable storehouse of long-term mercury exposure -- of nearly 7,000 participants, an equal number of whom had or had not suffered a cardiovascular event during the study follow-up period.
The team found no sign that the mercury levels hiked cardiovascular risk.
"Basically, what we found was very simple and very clear," said study author Dr. Dariush Mozaffarian, an associate professor of epidemiology at Harvard School of Public Health and division of cardiovascular medicine at Brigham Women's Hospital and Harvard Medical School.
"I think this is the most definitive study, and I'm not sure more studies are actually needed," Mozaffarian added. "It's nice to be able to answer an important research question. This is observational, so there's possibly some subtle effect we missed. But I think this provides the most definitive evidence available."
The study, which was funded by the U.S. National Institutes of Health, is published March 24 in the New England Journal of Medicine.
Health experts have long advised the public to balance the health benefits of fish (which often contains healthy omega-3 fatty acids) with the potential of mercury exposure from doing so. Also known as methylmercury, the heavy metal settles into bodies of water and can be absorbed by various species of fish, according to the U.S. Environmental Protection Agency.
Some fish, such as swordfish, tilefish and king mackerel, tend to store more of the toxin in their flesh. Due to the risk for neurodevelopmental issues, the U.S. Food and Drug Administration has long advised pregnant women, women who may become pregnant, nursing mothers and young children to limit their intake of these and other fish and shellfish.
Among study participants in the top 20 percent of mercury exposure, average toenail mercury levels measured 0.7 micrograms per gram. Current U.S. advisories for sensitive subgroups aim at keeping mercury exposure below a level correlated with toenail levels of 0.4 micrograms per gram.
Not only was no link between mercury and higher cardiovascular disease found, the study said, but participants with higher mercury levels actually experienced slightly lower heart disease rates. Mozaffarian and his team attributed this to the other beneficial effects of fish consumption.
The authors noted that their research should not change advisories for eating fish with higher mercury levels among women who are or may become pregnant.
"There is no strong or moderate evidence that mercury has any effects in adults," Mozaffarian said. "So it's important and helpful that people can feel comfortable with fish as a normal part of their diet."
"It doesn't mean we can stop worrying about mercury in the environment," he added. "But for the individual consumer making a decision about eating fish, they can take this worry off the table."
One expert agreed that the study shouldn't cause Americans to feel any easier about mercury in the environment.
Elena Craft, a health scientist at the Environmental Defense Fund, acknowledged that the study looks at a much larger sampling than prior research.
"But for me, it doesn't alter the primary health concerns about mercury exposure -- the neurobehavioral effects on children and developing fetuses," Craft said. "That's always been the most sensitive endpoint. In some ways, it's an interesting study, but in regards to protecting public health it just doesn't alter that bottom line."
For guidelines on mercury in fish and shellfish, visit the U.S. Food and Drug Administration.
SOURCES: Dariush Mozaffarian, M.D., associate professor, epidemiology, Harvard School of Public Health, and division of cardiovascular medicine, Brigham and Women's Hospital and Harvard Medical School, Boston; Elena Craft, Ph.D., health scientist, Environmental Defense Fund, Austin, Texas; March 24, 2011, New England Journal of Medicine
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