Mercury is directly associated with the risk of heart attack, according to a study by researchers from the Johns Hopkins Bloomberg School of Public Health, in collaboration with an international team. They say it may be advisable to eliminate fish with high mercury content from the human diet. "Mercury, Fish Oils and the Risk of Myocardial Infarction" appears in the Nov. 28, 2002, issue of The New England Journal of Medicine.
Eliseo Guallar, assistant professor of epidemiology at the Welch Center for Prevention, Epidemiology and Clinical Research of the Johns Hopkins Bloomberg School of Public Health, said, "Although it is believed that fish intake may reduce the risk of cardiovascular diseases, current epidemiologic studies of fish intake or fish oil levels and coronary heart disease in the general population are contradictory. Our findings suggest that mercury found in fish may counteract the benefits of the omega 3 fatty acids, also present in fish."
Mercury, a highly reactive heavy metal, exists in several forms, but fish intake is the main source of exposure to methylmercury. Fish with relatively high methylmercury content include swordfish, shark, tilefish, king mackerel and fish from locally contaminated areas, while fresh or frozen tuna, marlin and red snapper have intermediate concentrations of mercury. Although the consequences of longtime exposure to low concentrations of mercury are poorly understood, past research has suggested that fish from mercury-contaminated sources may predispose people to cardiovascular disease. The study by Guallar and co-workers indicates that this effect also may be present at lower mercury concentrations, more typical of the levels of people in Western countries.
School of Public Health researchers used toenail clippings from research participants to measure their exposure to mercury and adipose tissue samples to measure their levels of fish oils. Then, they correlated the levels of both mercury and fish oils with the risk of a heart attack. Mercury was positively associated with risk of heart attack, fish oils were associated with a reduction of risk, and these two effects counterbalanced each other.
Toenail mercury has been found to be a reliable biomarker of long-term exposure to mercury, just as adipose tissue fish oils are a marker of fatty fish intake. In addition, the correlation between mercury and fish oils indicates that fish is likely the main source of toenail mercury in these populations. The case-control study was conducted in eight European countries and Israel. Cases included 684 men with a first diagnosis of myocardial infarction. Controls were 724 men representative of the populations originating the cases. In this study, mercury levels in patients with myocardial infarction were 15 percent higher than in healthy controls living in the same areas.
"Exposure to methylmercury is currently a concern in specific high-risk groups, such as pregnant women and women of childbearing age who may become pregnant, but this warning should perhaps be extended to the general adult population," the researchers wrote in their study.
The researchers do not advise people to stop eating fish, and their analyses are consistent with a protective effect of dietary fish, as long as it is not heavily contaminated by mercury. Guallar said, "A weekly intake of two to four servings of fish from a variety of species, with special emphasis on fatty fish with low mercury content, such as salmon and small oceanic fish, is consistent with current advice for reducing cardiovascular risk."
Additional researchers were M. Inmaculada Sanz-Gallardo, Pieter van't Veer, Peter Bode, Antti Aro, Jorge Gomez-Aracena, Jeremy D. Kark, Rudolph A. Reimersma, Jose M. Martin-Moreno and Frans J. Kok.
The Heavy Metals and Myocardial Infarction Project was supported by a BIOMED-2 Concerted Action from the European Commission and was an ancillary project to the EURAMIC Study. National studies were financed by grants from the British Heart Foundation, the Dutch Ministry of Health, the Spanish "Fondo de Investigaciones Sanitarias," the German Federal Health Office, the Norwegian Research Council, the Russian Ministry of Science, the Swiss National Science Foundation, the Yrjo Jahnsson Foundation and the Israel Science Foundation.