The dish on fish
Benefits outweigh risks, according to two recent reviews.
We know, we know: We're supposed to eat fish. Several guidelines say a healthful diet should include two servings (usually defined as 3 ounces) per week. Fish contains eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), two long-chain omega-3 fats that aren't found in appreciable amounts in any other sort of readily available food. Those omega-3 fats may have all kinds of benefits, but so far the best evidence is that they're protective against fatal cardiac arrhythmias and heart attacks. In addition, DHA may be important to early brain development and to healthy pregnancies.
But there are contaminants to consider. It's well known that mercury (more precisely, methylmercury) accumulates in the lean tissue of some species. Polychlorinated biphenyls (PCBs) and dioxins are found in fatty tissue. Mercury crosses the placenta, and high doses cause serious brain damage. In adults, the metal may harm the heart. PCBs and dioxin cause cancer in animal experiments, and there's evidence that they're human carcinogens, too. They may also throw the immune system out of kilter and cause neurologic defects.