People who eat fish with a raised mercury content run a greater risk of coronary heart disease than previously thought, an international team of researchers has found.
Scientists from Edinburgh University, Baltimore in the US and seven other European countries say that mercury, which is found in certain fish from environmentally contaminated areas, may counteract the health benefits from some of the fatty acids present in fish.
The findings of the study suggest warning on mercury exposure should not be confined to high-risk groups like pregnant women, or those of child-bearing age who may become pregnant, as previously thought.
The article published in the 28 November issue of the New England Journal of Medicine indicates that although it is believed that fish intake may reduce the risk of cardiovascular diseases, epidemiological studies of fish intake or fish oil levels and coronary heart disease in the general population have been contradictory. The researchers suggest that mercury found in fish may counteract the benefits of the omega 3 fatty acids, also found in fish.
Mercury exists in several forms but fish intake is the main source of exposure to methylmercury. Fish with a relatively high methylmercury content include swordfish, shark, tilefish, king mackerel, and fish from locally contaminated areas, while tuna, marlin and red snapper have intermediate concentrations of mercury.
Although the consequences of long-term exposure to low concentrations of mercury are poorly understood, one study suggested that fish from a mercury-contaminated lake in Finland predisposed people to cardiovascular disease. This new research indicates that this is a more general problem and may also occur at lower mercury concentrations, more typical of the levels in the diet of people in Western countries.
The level of mercury in toenail clippings reflects long-term dietary exposure to mercury, in the same way that docosahexaenoic acid (DHA) in body fat is a marker of fatty fish intake. The researchers used toenail mercury and DHA in body fat of patients with a first heart attack and also in healthy volunteers living in the same areas of eight European countries and Israel to see if there was a link with the risk of heart disease in men. Mercury and DHA levels were related to each other, indicating that fish is likely to be the main source of toenail mercury in these populations. In this study, mercury levels were 15 per cent higher in coronary patients than in the healthy volunteers.
Professor Rudolph Riemersma, of the University of Edinburgh Cardiovascular Research Unit, said: "We do not advise people to stop eating fish, and our analyses are consistent with a protective effect of dietary fish, as long as it is not heavily contaminated by mercury."
He continued: "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 a salmon and small oceanic fish, is consistent with current advice for reducing cardiovascular risk."
Funding for the research was drawn from various sources, including the British Heart Foundation.