Fish is mostly good for you.
It's an excellent source of omega-3 fatty acids called eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). A diet rich in fish oil may help reduce inflammation and decrease your risk of cardiovascular disease. The American Heart Association recommends eating at least two servings of fatty fish each week.
Fish is also an excellent source of lean protein, plus most types of fish contain various vitamins and minerals.
When Fish Isn't Good
Almost all fish is contaminated with trace amounts of mercury and other contaminants. While most healthy adults have no problem eliminating the mercury from their bodies, children and women who are pregnant or breastfeeding should avoid some types of fish and shellfish to reduce their risk of mercury exposure.
Fish that contain the highest level of mercury are larger and older sharks, swordfish, king mackerel, and tilefish. It is probably a good idea for most people to eat less of these fish. They can be replaced with other fish and shellfish such as shrimp, pollock, canned light tuna, salmon and catfish, which all contain much less mercury.
Most other fish fall somewhere in between. The United States Environmental Protection Agency has a complete listing of the mercury levels in commercial seafood and fish. It is also interesting to note that deep-frying fish may increase the concentration of mercury in fish.
Besides mercury, fish can be a problem if it isn't prepared properly. Deep fried or served with a heavy, fat- and calorie-dense sauce will turn a healthy fish dish into an unhealthy meal.
Another potential problem is eating undercooked fish, which may lead to a parasitic infection. Make sure you cook your fish until it is flaky and tender; the meat should show no signs of translucency. And do not cross contaminate raw fish with uncooked or ready to serve foods.
What About Fish Oil Supplements?
Most people can get all the omega-3 fatty acids they need from fish, but EPA and DHA are also available as dietary supplements.
The DHA supplements may be the most beneficial for babies. The developing brain accumulates large amounts of DHA during the third trimester of pregnancy through the first three months of infancy. Women can take DHA supplements during their pregnancy and in the initial months of breastfeeding to be sure their babies receive enough DHA for normal brain development.
Burger J, Dixon C, Boring CS, Gochfeld M. "Effect of deep-frying fish on risk from mercury." J Toxicol Environ Health A. 2003 May 9;66(9):817-28.
Cetin I, Koletzko B. "Long-chain omega-3 fatty acid supply in pregnancy and lactation." Curr Opin Clin Nutr Metab Care. 2008 May;11(3):297-302.
FDA/Center for Food Safety & Applied Nutrition. "What You Need to Know About Mercury in Fish and Shellfish." Updated February 2005.