Fish are a great source of protein -- low in saturated fat and high in omega-3 fatty acids, which are essential to a healthy heart and brain, among other things. But during their life cycle, some fish become contaminated with high levels of toxic heavy metals, such as methyl mercury. Mercury from natural sources (volcanic activity) and man-made ones (power plants, incinerators, boilers, and mines) reaches lakes, rivers, and oceans through rain, runoff, and dumping. Fish ingest it, bigger fish eat the smaller fish, and the mercury gets more concentrated all the way up the food chain. That's especially worrisome for nursing mothers, children, and pregnant women (methyl mercury can pass from mother to child during pregnancy, when the child's nervous system is especially vulnerable -- and, confoundingly, when it can benefit most from omega-3s).
A buildup of methyl mercury in the body can create neurotoxicity, causing brain and nervous system damage. Besides mercury, fish can also be a source of persistent organic pollutants, or POPs. These include carcinogenic endocrine disrupters and neurotoxins such as dioxins and polychlorinated biphenyls (PCBs), which were banned in the United States in 1977 but remain in many water systems. Toxins in shellfish are a concern as well, because these creatures are bottom-feeders and effectively filter biological and industrial pollutants out of the surrounding water. Freezing or cooking shellfish does not remove these toxins.
The most contaminated fish are usually the largest predators. The FDA recommends -- at the very least -- that children and pregnant women avoid shark, swordfish, king mackerel, and tilefish, all of which have high levels of mercury.
The FDA also monitors commercial producers to make sure the fish they sell is relatively clean. But fish you catch in your local lake, river, or coastal area is not regulated; check with your state's department of environmental protection or health department to see if it's safe to eat fish from your area.
When deciding what fish to eat, there are two issues to balance: your own health and that of the planet. Many fish are endangered, overfished, or farmed in ways that are harmful to the environment. Some experts recommend eating smaller fish (sardines and anchovies, for example) and fewer fish in general.
The helpful charts available at oceansalive.org and thegreenguide.com indicate which fish are safest to eat -- for you and the environment -- and how much of them to eat. You can also print excellent national and regional wallet-size charts from the Monterey Bay Aquarium website, seafoodwatch.org.
Almost all fish and shellfish contain some mercury. You can cut your consumption even more radically and try to get omega-3 from flaxseed (however, this oil contains the less beneficial ALA, or alpha-linolenic acid); vegetarian omega-3 capsules made from algae; or fish-oil capsules labeled as "molecularly distilled" to remove impurities such as mercury, dioxins, and PCBs. These last two types of capsules can be found with both DHA (docosahexaenoic acid) and EPA (eicosapentaenoic acid); these omega-3s have the most compelling research to back up claims for reduced risk of heart attacks, strokes, and hardening of the arteries.
Some people experience gastrointestinal discomfort when taking fish oil. Start with the lowest dose and work your way up to the amount recommended on the label, or consult your physician.