Facing facts about fish.

It is well established that the polyunsaturated omega-3 fatty acids found in dark-fleshed, coldwater fish are healthful in a variety of ways. Consuming fish oil regularly can help ward off the serious heart rhythm disturbances associated with sudden cardiac death. Omega-3s also reduce the risk of heart attack and stroke, and may stave off prostate cancer and mental decline in old age. More than one study has shown them to even possess promising antidepressant effects.

Bone health and beyond

Still more evidence indicates that eating fish contributes to bone health too. Polyunsaturated fatty acids (PUFAs) diminish the production of cytokines, pro-inflammatory compounds in the body that can activate the production of bone-degrading osteoclasts. Furthermore, dark-skinned, oily fish is the only significant food source of vitamin D, which aids in the absorption of calcium. Vitamin D is essential for cell rejuvenation as well. The skin manufactures vitamin D when it is exposed to sunlight (see sidebar). In older adults, for whom bone strength becomes of increasing concern, the skin's ability to manufacture vitamin D is compromised, making it even more important to consume fish regularly. Vitamin D's daily adequate intake (AI) for people ages 14 to 50 is 200 International Units (IU) or 5 mcg; it is three times that for those over age 71, and many health care professionals believe that number is closer to 1,000 IU (or 25 mcg).

Fish and shellfish also contain a good deal of protein, antioxidants, and other nutrients and are low in saturated fat (see "Nutrition Facts"). Some studies have shown that people who eat fish tend to consume less meat and cheese, and may tend toward eating other healthy foods like vegetables and brown rice.

The catch

Unfortunately, fish absorb contaminants--specifically, methyl mercury and polychlorinated biphenyls (PCBs)--found in the waters where they swim. Mercury occurs naturally in the environment but can be released into the air through industrial pollution. It then accumulates in streams and oceans, where it becomes methyl mercury, a harmful type. Levels of methyl mercury in fish differ; larger fish that have lived longer have had more time to absorb it. For most healthy adults, methyl mercury is not a serious concern. However, if you regularly eat types of fish that are high in methyl mercury, it can accumulate in your blood over time. Methyl mercury is removed from the body naturally, but it may take over a year for the levels to drop significantly. This is the reason why women who are trying to become pregnant should avoid eating certain types of fish. Fish with the highest levels of methyl mercury can harm an unborn baby or young child's developing nervous system.

Gulf of Mexico tilefish, shark, swordfish, and king mackerel contain the most methyl mercury, and should be eaten sparingly--and not at all if you are a young child, nursing mother, are pregnant, or are trying to become pregnant. The FDA Food Safety Web site lists the methyl mercury content of many types of fish at www.cfsan.fda.gov/~frf/sea-mehg.html. The list reveals the sometimes subtle differences in fish by region; note, for example, that North Atlantic mackerel is not nearly as high in methyl mercury as king mackerel.

As it happens, one of the best sources of omega-3 fatty acids is also the lowest in mercury content: salmon. Whether fresh, frozen, or canned, salmon's mercury levels are barely detectable. Salmon is also high in vitamin D, as are mackerel, tuna, and sardines.

The downside is that salmon and other farmed fish can contain PCBs. These toxins are chlorinated compounds in the form of oily liquids or solids that are colorless to light yellow. There are no known natural sources of PCBs. PCBs were used as coolants and lubricants in electrical equipment until 1977, when evidence emerged that they build up in the environment and can cause harmful health effects. Though they are no longer used, PCBs persist in contaminated water sediments and in fish.

How worried should we be?

As recently as 2004 the Environmental Protection Agency (EPA) recommended limiting consumption of farmed (and even wild) salmon, but there is some controversy over the recommendation.

The EPA guidelines say ingesting PCB-contaminated fish once a month increases the risk of cancer by one in 100,000. The FDA says the contamination levels aren't high enough to be concerned about, and encourages consumers to keep eating farmed salmon.

In the FDA's view, the EPA standards are designed to protect the environment, not people. The FDA guidelines allow 40 times more contamination. The researchers who published the salmon scare in January 2004 in the journal Science maintain that the FDA standards, drafted in 1984, are out of date and too lax. So while it's true that farmed salmon contain 10 times more toxins than wild salmon, there is disagreement over whether the health benefits from increasing your salmon consumption outweigh the modest cancer risk. While the EPA Web site currently has no specific advisory asking consumers to limit salmon consumption, in general they recommend that fish with PCB levels between 0.024 and 0.048 parts per million (ppm) be eaten only once a month. The average level of PCBs in salmon is 0.027 ppm. The current FDA limit of PCBs in all fish is 2 ppm. Health Canada's guideline for PCBs in fish is also 2 ppm. Canned salmon is almost always wild, and therefore of much less concern.

What to do?

The Harvard Men's Health Watch recommends removing the skin and fat from salmon before cooking to minimize exposure, since that's where the contaminants are stored. The American Heart Association's recommendation to eat two servings of fatty fish per week (up to 12 ounces total) is likely a healthy move for most of the population.

The FDA and the EPA are in agreement with each other in their co-authored methyl mercury advisory from March 2004. Again, women who may become pregnant, pregnant women, nursing mothers, and young children should avoid shark, swordfish, king mackerel, and tilefish. Five of the most commonly eaten fish that are low in methyl mercury are shrimp, canned light tuna, salmon, pollock, and catfish. Albacore tuna has more methyl mercury than canned light tuna.

Check local advisories about the safety of fish caught in your area by family and friends. If no advice is available, eat up to six ounces (one average meal) per week of fish you catch from local waters. For local fish advisory programs, go to www.epa.gov/ost/fish.

In the final analysis, the wide range of benefits to health most likely outweighs the risk of cancer from eating farmed salmon. It's still a good idea to mix and match the different types of fish and shellfish you eat. Consume canned tuna one day, broiled salmon another. You can also enjoy a diet rich in PUFAs--the "good fats"--by cooking with canola oil, and adding walnuts and flax seed to your regular food consumption.

Remember that on top of heart health and other benefits, the only excellent food source of vitamin D is oily fish. The vitamin is otherwise scarce in the food supply. Vitamin D is essential for bone health and controlling the life cycle of cells in many different tissues. Egg yolks and fortified milk, orange juice, and cereal contain only modest amounts.

Harvard Health Publications, "Health Benefits of Fish," www.health.harvard.edu/press_releases/health_benefits_of_fish_oil.htm

FDA Food Safety Web site, U.S. DHHS and U.S. EPA, "What You Need to Know About Mercury in Fish and Shellfish," March 2004, www.cfsan.fda.gov/~dms/admehg3.html

FDA Food Safety Web site, U.S. DHHS and U.S. EPA, "Mercury Levels in Commercial Fish and Shellfish," FDA Surveys, 2003, www.cfsan.fda.gov/~frf/sea-mehg.html

EPA Web site, www.epa.gov/ost/fish

USDA Nutrient Data Laboratory, www.nal.usda.gov/fnic/foodcomp/cgi-bin/list_nut_edit.pl

ACE FitnessMatters, 2005, Vol. 11, No. 5, p. 4-5

Science, 2004, Vol. 303, No. 5655, 226-229

Vitamin & Nutraceutical Information Service, American Dietetic Association Food & Nutrition Conference Release, 2005

Online Newshour with Jim Lehrer, program transcript, "Salmon Scare," Jan. 26, 2004, PBS.org/newshour/bb/health/jan-june04/salmon_01-26.html

www.healthcastle.com

www.webmd.com/content/article/78/95751.htm