Mercury content in the hair is a marker for the amount of methyl mercury, a toxic form of the element, accumulated in the body by eating contaminated fish. Some scientists believe that the amalgam in dental fillings may also be a significant source of mercury, but questions remain about whether the mercury in dental fillings, which is inorganic, is absorbed into the body.

“Although consumption of fish may be healthy in general, some fish may contain methyl mercury in amounts harmful for humans,” says study author Jukka T. Salonen, M.D., Ph.D, MScP.H., professor of epidemiology at the Research Institute of Public Health at the University of Kuopio in Finland.

In the Kuopio Ischemic Heart Disease Risk Factor (KIHD) study, a total of 2,005 men without heart disease, between 42 and 60 years old were divided into four groups based on the mercury content of their hair, and tracked for an average of 12 years.

Heart disease was defined as a history of an acute coronary event, like a heart attack, or angina pectoris, stroke or other cardiovascular event. The researchers controlled for other risk factors that could have affected their results, including age, levels of high-density lipoprotein (HDL, “good” cholesterol), low-density lipoprotein (LDL, “bad” cholesterol), triglycerides, family history of coronary heart disease, systolic blood pressure, weight and intake of fatty acids and antioxidants.

The men who scored in the top 25 percent for hair mercury content had a 60 percent increased risk of death from CVD compared to the men in the lower mercury content. Those same men had a 70 percent increased risk of coronary heart disease alone, says Salonen. The amount of mercury in the hair was determined by flow injection analysis-cold vapor atomic absorption spectrometry and amalgamation, one of several tests available to determine mercury content.

“Men who consumed 30 grams or more of fish daily – had 56 percent higher average hair mercury than those whose daily consumption was less than 30 grams. Those same men also tended to consume certain types of ‘predatory’ fish,” says Salonen. Fish higher in the food chain – i.e., those who eat smaller contaminated fish – tend to have the highest levels of methyl mercury.

“The results also showed that men whose hair mercury levels were in the top 20 percent had a 32 percent faster increase in the thickness of the inner walls of their arteries, a measure of atherosclerosis, compared to men in the rest of the group.

Atherosclerosis is the build-up of fatty plaque in arteries and is the underlying process that causes cardiovascular disease. Previous studies have shown that increasing dietary levels of fish containing omega-3 fatty acids benefits people with cardiovascular disease, as well as healthy people.

The American Heart Association currently recommends that individuals consume two servings of fish weekly, both for the benefits of omega-3 fatty acids, and because fish tends to be low in saturated fats, which contribute to elevated cholesterol levels.

“These results from Kuopio are intriguing, but preliminary, and should be viewed in the context of many other studies that have shown a clear cardiovascular benefit to consuming fish on a regular basis,” says Barbara V. Howard, Ph.D., chair of the American Heart Association’s Nutrition Committee and president of MedStar Research Institute in Washington, D.C.

“It is important to note that this is an observational study, and the conclusions do not prove a direct relationship between the amount of mercury in the hair and heart attacks. There may be factors such as the socio-economic status of the men or other dietary factors that are hard to measure, that account for the higher risk,” says Howard.

Researchers became interested in looking at an association between mercury and cardiovascular disease because mercury has been shown to promote the oxidation of low-density lipoproteins in the arteries. Oxidation is a major component in the development of atherosclerosis. In addition, mercury can interfere with the antioxidant effects of selenium, an essential trace element found mainly in plant foods, and in the U.S., in grains and meat.

The KIHD study is an ongoing, population-based study designed to investigate risk factors for cardiovascular diseases and their outcomes among men in Eastern Finland. Previous studies with shorter follow-up periods from the same research group found a strong association between high hair mercury content and an increased risk of death. Researchers wanted to retest these results over a longer follow-up period.

“It should be noted that we are not against eating fish per se,” adds Salonen. “What these results mainly say is that one should avoid regular consumption of old, large predatory fish, in which mercury levels are high, especially when caught from sources that are known to have a high mercury content. Our best advice is to consume a variety of fish, preferably young and small, from different lakes and seas in order to avoid possible high local levels of mercury.”

The American Heart Association suggests consuming fish such as mackerel, lake trout, herring, sardines, albacore tuna and salmon twice a week.

Co-authors include Jyrki K.Virtanen M.S.c., R.D.; Sari Voutilainen Ph.D., R.D.; Tiina H. Rissanen, M.Sc, R.D.; Jaakko Mursu, M.Sc, R.D.; Meri Vanharanta, M.Sc, R.D.; Kari Seppanen; and Jari Laukkanen, M.D.

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