Mercury in Fish Affects Adults' Brains

Low Levels May Have Cumulative Effect

June 5, 2003 -- If you're eating more fish these days, here's something to consider. A new study raises questions about the safety of low levels of mercury in fish -- pointing to bad effects on adult brain function.

"This is the first time anybody has applied neurological tests to people who have been exposed to methyl mercury, to find out how adults are affected," says lead researcher Ellen K Silbergeld, PhD, an environmental scientist with Johns Hopkins University.

"Adults may be as sensitive as children to methyl mercury [in fish]," she tells WebMD. Silbergeld's study was published yesterday by Environmental Health: A Global Access Science Source.

The FDA advises that women avoid four fish -- shark, swordfish, king mackerel, and tile fish -- if they are pregnant or of childbearing age and may become pregnant. The FDA also advises that young children avoid eating those fish.

It's an issue that's become as murky as the water, with contradictory findings from various studies. Earlier this year, a nine-month study -- published in The Journal of the American Medical Association -- showed that mercury in fish isn't a problem during pregnancy. The study looked at pregnant women with high levels of mercury in their blood from eating a lot of fish. However, the researchers found no signs of mercury poisoning in the babies born to the moms.

New Evidence in Adults

In the new study, Silbergeld and colleagues focused on a fishing community -- six villages total -- along the Cuiaba River in Brazil. "We have been studying this area for a long time," she tells WebMD. "Because of heavy use of mercury to extract gold from the earth, it has resulted in contamination of rivers and fish in those rivers."

First, they tested hair samples from 129 adults, a standard measure of mercury contamination. "We found a range of exposures, from hardly any to four times levels found in the U.S.," she tells WebMD.

Then, each of the Brazilians took several tests to measure the mercury's effects on nerve cells -- tests like bead threading and remembering strings of numbers -- which measure fine-motor speed and dexterity, concentration, and memory.

"At that point, no one knew what [the Brazilians'] mercury levels were," Silbergeld tells WebMD. "It was only at the end, when we put it all together, that we saw associations between increased exposures of methyl mercury and decreased performance on tests."

In fact, the degree of the effects increased with mercury levels in the hair, she adds. "The effects may be cumulative. We just looked at one point of time. We don't know, but from what we know of mercury poisoning, we suspect they probably are."

Experts Weigh In

There's no question that mercury exposure is "a bad thing," concurs Richard Weisman, MD, a toxicologist at the University of Miami School of Medicine and director of the Poison Control Center for South Florida.

However, Silbergeld's study falls short of showing that people should alter eating habits because of mercury in fish, he tells WebMD.

"There is no doubt that mercury is one of the worst [toxins affecting the brain]," says Weisman. "We should expose ourselves to as little mercury as possible. But I don't think that means you should limit intake of fish. ... Swordfish is known to contain some mercury, but I don't think it's any level that should concern people. ... I think more energy should be mounted against protecting our water and our fish."

From the FDA: Mercury in fish is an issue "of ongoing concern," says an FDA spokesman. In fact, the FDA held a public hearing last year to determine whether the advisory should be expanded. One of the agency's goals is to review all existing research and revise the advisory by the end of 2003, if necessary, he says.

"Adults can safely eat 12 ounces of fish per week and should choose a variety of species including shellfish, canned fish, smaller ocean fish, or farm-raised fish," the FDA source says.

From the CDC: Fish is high in protein and nutrients and low in saturated fatty acids and cholesterol and should be considered an important part of the diet, according to the CDC. Adults should take care to avoid eating fish known to have high levels of methyl mercury, the report states.

The EPA web site provides state-based fish advisories and bans of fish species contaminated by mercury and their locations, the CDC report adds.

SOURCES: Environmental Health: A Global Access Science Source, June 4, 2003. Ellen K Silbergeld, PhD, environmental scientist, Johns Hopkins University. Richard Weisman, MD, toxicologist, University of Miami School of Medicine; director, Poison Control Center for South Florida. Morbidity and Mortality Weekly Report, March 2, 2001. WebMD Medical News: "Mercury in Fish No Problem During Pregnancy."