Fishing for mercury: who's at risk?

Nutrition Action Healthletter, March, 2003 by David Schardt

San Francisco physician Jane Hightower was baffled. A growing number of her patients were complaining about memory problems, headaches, slurred speech, hair loss, tremors, being tired and depressed, or having a metallic taste in their mouths.

On a hunch, she started asking them how much fish they ate. Bingo. They were eating a lot, especially swordfish, shark, and ahi tuna (which is served as tuna steaks or sushi).

"The symptoms my patients were reporting are consistent with symptoms reported in medical literature from around the world when it comes to mercury poisoning," says Hightower.

The omega-3 fats in seafood help protect against heart attacks, sudden death, and stroke. But fish is also our major source of the toxic metal mercury. Although Hightower's findings are preliminary, they suggest that some fish-eaters are getting more mercury than health authorities recommend, and that those higher levels might lead to neurological problems. But the picture is anything but clear.

Mercury is a neurotoxin, which means that it can damage the brain and nervous system. "A developing brain is the most vulnerable to the harmful effects of mercury," says Vas Aposhian, a toxicologist at the University of Arizona in Tucson.

"That's why we've been most concerned about mercury in pregnant and nursing women and in children." Aposhian serves on a panel that advises the Food and Drug Administration (FDA) about mercury in seafood.

Fortunately, exposure to extremely high levels of mercury in food is rare. In one of the two worst poisoning incidents, women who ate seafood from the mercury-polluted Minamata Bay in Japan in the 1950s gave birth to severely retarded infants. And in the 1970s, Iraqi women who mistakenly ate grain that was not intended for human consumption (it had been treated with a mercury fungicide) gave birth to brain-damaged children.

It's not just children who are vulnerable. "The adults in Japan and Iraq who consumed very large amounts of mercury also suffered permanent brain and nerve damage," says Alan Stern of the New Jersey Department of Environmental Protection.

But that was far more mercury than people normally get from eating fish. As for the effects of more moderate levels: "We don't know," says Stern, "because we haven't looked into it."

"There has been a belief that mercury has been around for years and no one has really been hurt by moderate levels," explains Aposhian. What's more, the symptoms from too much mercury are hard to trace back to the toxic metal. "Headache, no energy, not wanting to do anything--people report that about colds, the flu, and a lot of other things," Aposhian points out.

Unfortunately, Jane Hightower's study, which is about to be published, doesn't show that symptoms were linked to changes in mercury levels. (1) What she did was measure mercury in the blood of 89 of her patients who either reported eating enough fish to supply high levels of mercury or who reported symptoms that are often associated with too much mercury.

"When we pulled fish out of the diet, the mercury levels came down and the symptoms went away," she says. That's Hightower's interpretation of what happened. But her study doesn't say how many people had which symptoms, either before or after their mercury levels fell. So it's impossible to be sure that changes in symptoms were linked to the drop in mercury.

Also, it's not clear from the study exactly how Hightower asked about symptoms. If she inadvertently led her patients to expect to feel better when they stopped eating fish, that might have influenced what they reported.

"It's difficult to determine whether mercury is causing or exacerbating these symptoms," says Hightower. "And since only mercury has been tested in these individuals, other contaminants responsible for symptoms can't be ruled out."

The bottom line: Hightower's study can't say whether mercury can cause symptoms like fatigue, memory loss, and headaches. But it does show that some fish-eaters have higher levels of mercury, and that cutting back on fish can lower those levels. So do similarly inconclusive, but troubling, reports from New Jersey, Massachusetts, and Wisconsin.

"They're a whole new ball of wax about the potential harm from moderate levels of exposure to mercury," says Alan Stern. But, he cautions, "the symptoms have been subjective and self-reported, and they could be due to a lot of other things besides mercury."

If future studies do nail down a link between neurological symptoms and mercury, says Stern, "it suggests that subtle health effects may be occurring in adults at levels of mercury intake we didn't previously think were a problem."

Fortunately, he adds, "the preliminary evidence from San Francisco and elsewhere suggests that if mercury in seafood does cause symptoms in adults, the effects seem to be temporary. They eventually go away if the exposure is discontinued." But it's hard to know for sure, because these studies weren't large enough or long enough to answer the question.

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