The American Diabetes Association, the American Heart Association, and the federal government's dietary guidelines all recommend that we eat fish twice a week. Some nutritional experts recommend even more—up to five servings per week. Eating oily fish regularly provides health benefits that affect our hearts, as well as possibly making stroke, depression, and Alzheimer's disease less likely. But many are concerned about contaminants in fish. It's important to know the risk-benefit ration when it comes to incorporating more fish into your diet.
Eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), the two omega-3 fats found in fish, have been shown to reduce the risk of death from coronary artery disease by about 35% when consumed in modest amounts. Modest means a daily average of about 500 milligrams (mg) of EPA and DHA together (not each), which you can get by eating two servings of fish a week — provided, of course, it's oily fish. Two servings provides from 3.0 to 4.5 grams of EPA and DHA, and because much of it is stored, that's equivalent to ingesting about 500 mg daily.
There's some evidence from epidemiologic studies that omega-3 fats may protect against such widely disparate disorders as depression, metabolic syndrome, Alzheimer's disease, some kinds of cancer and, in high doses, inflammatory disorders such as rheumatoid arthritis and inflammatory bowel disease. And research shows that it helps people who are critically ill from conditions like adult respiratory distress syndrome or who are recovering from major surgery.
The American Heart Association recommends a daily average of 500 mg of EPA and DHA for prevention of coronary artery disease, and twice that much — a gram a day — for people with established heart disease.
As for contaminants, methyl mercury and toxic organic compounds such as polychlorinated biphenyls (PCBs) and dioxin are the ones that have caused the most concern. (Meat and dairy products also contain methyl mercury and toxic organic compounds.) In 2006, Harvard researchers Dariush Mozaffarian and Eric Rimm wrote a comprehensive review about the risks and benefits of eating fish that was published in The Journal of the American Medical Association, and some of the following information is taken from their review.
Methyl mercury is found in highest concentrations in four types of fish: swordfish, shark, king mackerel, and tilefish (sometimes called golden bass). Because methyl mercury can impair neurologic development and function, these species should be avoided by pregnant women, those who may become pregnant, those who are breast-feeding, and infants. However, it's still important for the people in these groups to consume two servings of fish per week with high EPA and DHA content, because DHA is an essential nutrient for optimal brain development, which occurs during gestation and early infancy. The dangers of methyl mercury aren't an issue for adults unless they eat more than five servings of fish a week. Even then, the risk can be managed by limiting intake of the four species with high mercury levels.
The risks of PCBs and dioxin are essentially below the level of detection when consuming store-bought fish, and because these compounds are also found in similar amounts in meat and dairy products, there doesn't seem to be any disadvantage from swapping one good protein source for another. However, there may be advisories about contamination of freshwater fish in certain areas, and these local recommendations should be heeded.June 2009 update