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Mercury and Fish

Many women of child bearing age are wondering which fish to buy to get those beneficial omega-3 fatty acids without poisoning themselves or eating the last of some endangered species. Higher intakes of the omega-3 fatty acids, eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), appear to decrease the risk for hypertension, atherosclerosis, type 2 diabetes, and some inflammatory diseases. DHA decreases the likelihood of premature birth, and is key to normal brain, retinal and possibly testicular development in fetuses. Mackerel, herring, salmon, halibut and tuna have the greatest amounts of EPA and DHA, but caution is advised. Some seafood contains significant amounts of methylmercury (meHg), which is toxic to the nervous system and may negate the cardiac benefits of fish. Large scale mercury poisonings 30-40 years ago in Japan and Iraq resulted in infants with cerebral palsy, mental retardation, developmental delay, seizures, blindness, and hearing impairment. While some mothers of affected infants were asymptomatic others showed toxic effects including fatigue, muscle and joint pain, headaches, hair loss, impaired memory and concentration, numbness, loss of peripheral vision, blindness, decreased coordination, difficulty walking, kidney failure, and death. Research in monkeys has revealed that the reproductive effects of meHg include sperm toxicity, decreased pregnancy rates and increased miscarriages and stillbirths. Human studies describe higher mercury levels in couples experiencing infertility than in fertile couples. In ongoing studies, measurable decreases in intelligence and evidence of learning disabilities have been tied to methylmercury in children of some, but not all fish eating populations. Toxic amounts of mercury rain down from skies polluted by the burning of coal and leach into waterways from old gold and mercury mines, including one in Marin near Tomales Bay. Bacteria convert the inorganic mercury to meHg, which then increases in concentration in organisms as it moves up the food chain. The human intestine absorbs 95% of ingested meHg, and then the body slowly excretes it over a period of months. Unfortunately, ingested methylmercury can show up in breast milk. In 2000 the National Academy of Sciences (NAS) set the maximum acceptable daily meHg intake at 0.1 mcg/kg of body weight although some scientists have proposed an even lower threshold. Others have used a weekly or monthly intake guideline, which permits higher intake on any individual day, but limits the amount of fish eaten per week. The San Francisco Chronicle recently sponsored an analysis of locally purchased fish, which revealed that a 120 lb. person could easily ingest 4 to 40 times her daily allowable intake of meHg by choosing popular fish including swordfish, halibut, Chilean sea bass, and ahi tuna. In separate tests white albacore tuna exceeded the NAS standard by 11 fold and chunk light by 3 to 4 fold. Consistent with these findings is the report published last year by San Francisco physician Jane Hightower. She found that 66 of her female patients had an average blood meHg level three times the maximum recommended by the National Academy of Sciences (NAS). Many were symptomatic as were some of the children she studied. Agreement on what constitutes "safe" levels of exposure for pregnant women is still pending the outcome of ongoing studies. New data indicate that blood mercury concentrations are higher in the umbilical cord than in the mother and consequently, that 16% of infants are exposed to excessive mercury levels before birth. At an EPA conference in January a new maximum daily meHg intake for pregnant women of 0.07 mcg/kg was proposed. The FDA has issued a warning that women who might become pregnant should avoid shark, swordfish, king mackerel and tilefish and PFC would add white albacore tuna. By not eating swordfish, shark and tuna, you're not only protecting yourself but also these threatened species (www.montereybayaquarium.org). Also, many fish from Northern California waterways should not be eaten by women of childbearing age because of mercury or PCB contamination (www.oehha.ca.gov/fish.html). Beth Schriock, MD

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