Fish in moderation is safe for most

A San Francisco Superior Court judge ruled earlier this month that tuna canners don’t have to provide label warnings about mercury in canned fish. In the ruling, he pointed to the fact that the Food and Drug Administration has said mercury levels in canned tuna are not high enough to merit a warning. Still, many consumers were left wondering if they consume -- or have consumed -- too much of the heavy metal and what can be done about it.

Janet Cromley

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When eaten, methylmercury -- the mercury compound found in canned tuna -- is absorbed into the bloodstream from the gastrointestinal tract. Attaching itself to red blood cells, the methylmercury circulates through the body -- primarily affecting the kidneys, heart, brain and central nervous system. Researchers believe the mercury binds with sulfhydryl groups (structures containing a sulfur atom and a hydrogen atom) found in proteins. This process disrupts nerve conduction and tissue respiration, eventually killing neurons.

The severity of symptoms depends on the level of methylmercury buildup in the tissues, which in turn is affected by the amount consumed and absorbed over time. Factors in how mercury is absorbed include the age, metabolism and general health of the person consuming it, says Irva Hertz-Picciotto, a public health professor at UC Davis and an expert in the field of environmental metals exposure.

Although scientists are debating the health consequences of chronic long-term exposure to low doses of methylmercury, acute poisoning is extremely rare. Symptoms may include, to varying degrees, numbness and tingling in the extremities, hand tremors and loss of coordination, nausea, abdominal pain, skin irritation, respiratory and memory problems, blurred vision and headaches.

Methylmercury is most dangerous to developing brains and nervous systems. High levels of maternal methylmercury have been linked to deficits in learning, language skills, memory and emotion in children, says Dr. Ashokkumar Jain, a medical toxicologist at USC’s Keck School of Medicine.

Methylmercury poisoning is typically diagnosed by testing blood and sometimes urine, says Jain. Long-term exposure can be tested through hair.

In most cases, the best treatment is to eliminate exposure. Chelation therapy, used for other types of mercury poisoning, tends not to be particularly effective in eliminating methylmercury. “The best course of action is to let it leave the system over time,” Jain says. That process can take months, particularly at higher levels of exposure. Symptoms generally improve, however, as levels are reduced.

Researchers, physicians and nutritionists don’t agree on how much mercury-contaminated fish can be safely consumed by healthy adults. And contamination levels vary even within species, depending on the size of the fish, its location and how it was raised.

“The vast majority of people don’t need to worry about this,” says Hertz-Picciotto. The possible exception, she says, might be populations that rely exclusively on fish diets or people who have a particular sensitivity to mercury.

Pregnant women and children are a slightly different story. The FDA recommends that pregnant women, nursing mothers and young children continue to eat a variety of seafood, but eliminate fish with known high levels of methylmercury and limit consumption of other fish, including canned light tuna, to 12 ounces a week and canned Albacore or white tuna to 6 ounces a week. For information, go to www.cfsan.fda.gov/dms/admehg3.html.