Is there a difference in the amount of mercury in fish, whether you eat it raw or cook it?
Mercury in fish is tightly bound to protein and is not removed during cooking processes such as smoking, broiling, baking, boiling, pan frying, and deep frying. Nor does the addition of lemon juice release mercury from its bound state. On the other hand, the cooking method affects the health benefits of fish, and the mercury concentration is strongly dependent on the type of fish.
Mercury originates from natural sources (volcanoes) and human sources (coal-fired power plants, waste incineration, gold mining). Organisms do not readily absorb mercury in the form in which it is usually released into the environment—metallic or inorganic mercury. Once rainwater carries inorganic mercury into lakes and oceans, microbes convert it into methylmercury, or organic mercury. (In chemistry, “organic” refers to carbon-containing compounds and has nothing to do with organic agriculture.)
Organic mercury is readily absorbed by organisms and accumulates in their tissues. It bioaccumulates in the aquatic food chain. In other words, short-lived species low in the food chain (such as shellfish and salmon) have low concentrations of mercury, while longer-lived predators (such as swordfish and shark) have high concentrations. The levels in albacore tuna are lower than those in swordfish but higher than those in salmon.
Industrial catastrophes that have resulted in mass consumption of high levels of mercury reveal that it is toxic to nerve cells, especially in children exposed during their early development. Studies of the effects of exposure to lower levels of mercury have been conflicting, but based on the possible risks, the U.S. Environmental Protection Agency and the U.S. Food and Drug Administration have issued advisories for women of childbearing age, pregnant women, nursing mothers, and young children.
At the same time, studies suggest that intake of fatty acids in fish by pregnant and nursing women is beneficial for the development of brain cells in infants. In addition, fish, except deep-fried fish, has well-documented cardiovascular benefits. For example, omega-3 polyunsaturated fatty acids in fish decrease the risk of heart attack by improving the fluidity of heart cell membranes.
In response to the confusion about the role of fish in a healthy diet, a 2006 article in the Journal of the American Medical Association concluded that, with the exception of a few fish species, the benefits of moderate fish consumption (two servings per week) outweigh the risks. The article recommends that nursing mothers and pregnant women avoid shark, swordfish, golden bass, and king mackerel; limit intake of albacore tuna to 6 ounces per week; and consult advisories for locally caught fish. But they should get at least 12 ounces per week of other fish and shellfish. See http://www.epa.gov/waterscience/fish/ for a list of mercury levels in different species and local fish advisories.