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Many Americans have been passing up the steak at dinner in favor of what has been deemed a healthier choice-fish. What they don't know is that there is growing concern among health organizations that consuming large amounts of fish may be detrimental to your health.
Fish has long been promoted as a health food because it is low in calories and full of vitamins and omega-3 fatty acids. However, because pollutants are so widespread, most fish contains at least some toxic methylmercury.
A recent study found that among patients at a California medical practice, those who ate a lot of fish had high levels of toxic mercury, or methylmercury, in their systems. They were also suffering from symptoms of low-level mercury poisoning, including hair loss, depression, difficulty concentrating, headaches and fatigue.
The Food and Drug Administration (FDA) permits 480 micrograms of methylmercury per one pound of fish. However, commercial fish is not regulated and there is no way to know just how much mercury is in any given fish. Moreover, people tolerate mercury very differently. Much like a bee sting can make one person extremely ill while hardly affecting another, varying amounts of mercury can cause reactions ranging from severe to mild.
Mercury is especially dangerous to children and can cause birth defects during pregnancy. For this reason, the FDA recommends that women who are pregnant or may become pregnant limit their intake of fish to 12 ounces per week, and 10 states warn pregnant women to limit their consumption of seafood.
Methylmercury is a potent neurotoxin, causing axonal demyeliniation. Adults can experience symptoms months after an acute exposure consisting of ataxia, blurred vision, hearing deficits and paraesthesias.
Fetuses are particularly sensitive to methylmercury, as shown by the more than 1,400 infants from the Minimata area of Japan who were acutely exposed to the toxin in utero when their mothers ate fish contaminated by a factory discharge. The children, while often normal at birth, developed abnormal reflexes, problems with suckling, swallowing, gait, and speech, and mental retardation.
Stop Eating Fish BEFORE Becoming Pregnant
It is critical that women of childbearing age stop eating all fish for six months to one year before becoming pregnant. Currently, it is estimated that about eight percent of women of childbearing age have enough mercury in their systems to cause potential learning disabilities in their unborn babies.
Elemental mercury from rocks and soil exists naturally in background levels in lakes and streams but is concentrated in the environment by emissions from hydroelectric projects, smokestacks, the burning of garbage and fossil fuels, municipal waste facilities, other hazardous waste combustors and industrial pulp and paper and mining processes.
Microorganisms in lake and stream sediments convert elemental mercury to organic methylmercury, which binds tightly to the proteins in fish tissue and is concentrated in fish higher up the food chain.
Mercury is converted into methylmercury, an organic form, by bacteria in water. The substance is about 95 percent absorbed when consumed and is toxic to humans because it is very hard for the body to eliminate. This allows it to build up in the system where it can eventually affect the central nervous system.
Humans are exposed to methylmercury largely through fish consumption. The toxin accumulates in fish, as it does in humans, therefore big fish that eat other fish typically contain high levels of mercury. This includes meatier fish such as swordfish, shark, mackerel, and tuna. Swordfish had the highest correlation with mercury levels out of the 30 fish used in the study. Fish that generally have low levels of mercury include salmon, flounder, cod, catfish and trout.
When ingested by humans, methylmercury is easily absorbed and retained by the body; it has a half-life in blood of about 44 days, which makes blood tests useful measures of acute exposure. It concentrates in new hair, and consecutive hair segments indicate a person's exposure history. Methylmercury is eliminated fecally as inorganic mercury.
Although practices such as trimming off fat can reduce the intake of organic pollutants such as polychlorinated biphenyls, pesticides, insecticides and dioxins, mercury intake is unaffected by this because it is deposited uniformly throughout fish tissue.
Environmental Health Perspectives November 1, 2002
Canadian Medical Association Journal October 15, 2002;167(8):897
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