Current health advice on the heart health benefits of fish has recently been tempered by concern that heavy metal contaminants in fish - primarily mercury - may actually promote cardiovascular damage. The subject has received considerable media attention, and now consumers wonder whether they should eat fish to benefit from omega-3 fatty acids, or avoid fish to limit their exposure to mercury. Unfortunately, the relationship between mercury levels and cardiovascular risk is still not clear, as evidenced by conflicting reports published in the New England Journal of Medicine.

Analyses of mercury levels and fish intake

In one study, researchers from 8 European countries and Israel measured toenail mercury levels and adipose tissue levels of docosahexaenoic acid (DHA) - an omega-3 fatty acid found in fish - in 684 men with a diagnosis of first myocardial infarction (MI) and in 724 healthy controls. Their analysis showed mercury levels to be positively associated with DHA levels (p<0.0001).

After adjusting for the DHA level and multiple cardiovascular risk factors, the men in the highest quintile of mercury level (0.66 µg/g) were more than twice as likely as those in the lowest quintile (0.11 µg/g) to suffer an MI (Odds Ratio=2.16, 95% CI, 1.09-4.29). A dose-response curve showed an increase in risk associated with mercury levels above 0.25 µg/g, a trend that became more pronounced after adjustment for DHA levels.

In the second study, a nested case-control analysis, Harvard researchers assessed toenail mercury levels and coronary heart disease risk in 934 men in the Health Professionals Follow-Up Study. In this case, no significant association was found in a comparison of the men in the highest quintile of toenail mercury level (1.34 µg/g) with those in the lowest (0.15 µg/g).

Following the trail of mercury

Mercury exists in several forms. Contact with the inorganic form would most likely come through occupational exposure. The type at issue here, though, is methyl mercury, which filters from air pollution into ocean waters, where it is taken in and stored by fish. Mercury stays in the system for the lifetime of the animal, so large, long-lived fish like sharks and swordfish are particularly prone to mercury accumulation of an estimated 1µg/g. (By comparison, tuna, trout, pike, and bass have an average concentration of 0.1-0.5 µg/g, while shellfish generally have low levels.)

Some research suggests that heavy metals like mercury may contribute to arterial damage by enhancing the production of free radicals, or by countering the ability of antioxidants to inhibit free radical-caused damage.

Mercury levels in fish have become an increasing concern for consumers who, following the advice of health professionals, are eating less red meat and more fish. In both of these studies, the participants who ate the most fish tended to have the highest levels of mercury.

Why the lack of consensus?

It's not clear why one study showed an increased risk of CVD with escalating mercury levels and the other did not, but differing methodologies and study populations may at least partially explain the difference. The US study population was made up largely of dentists, many of whom have experienced long-term environmental mercury exposure. (The average mercury level of those in the highest quintile in the US study was 1.34 µg/g, more than twice the upper limit registered by participants of the European study.)

Advice to consumers

Current advice from the US Food and Drug Administration is for pregnant women to avoid eating fish known to carry high levels of mercury - tilefish, king mackerel, shark, and swordfish - or any fish caught in local areas known to be polluted. The authors of the European study profiled here question whether it is time to extend this warning to the entire fish-eating population. But, they say, their findings should not warn consumers away from other kinds of fish, since the omega-3s in fish appear to offer considerable health benefits.