Mercury in fish may not increase heart disease risk: Study

By Dr. Ananya Mandal, MDMar 24 2011

According to a new study 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. This is in spite of the repeated proofs of neurological deficits in children and unborn babies due to mercury exposure.

For this study, 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 of nearly 7,000 participants, an equal number of who 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.

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 said, “Basically, what we found was very simple and very clear…I think this is the most definitive study, and I'm not sure more studies are actually needed…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.

Fish, of course, has been touted as heart-healthy food for the omega-3 fatty acids contained in fish oil. Specifically, that is DHA (docosahexaenoic acid), ALA (alpha-linolenic acid) or EPA (eicosapentaenoic acid). Some fish rich in these fatty acids, such as swordfish, tilefish and king mackerel, also tend to store more of mercury 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.

Surprisingly 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…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…But for the individual consumer making a decision about eating fish, they can take this worry off the table,” he said.

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…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,” she added.