Fishing for mercury: who's at risk? (Special Feature).

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.

Heart Disease

The omega-3 fats in seafood are good for your heart. Among the 84,000 women in the Nurses Health Study, for example, those who ate fish at least once a week were 30 percent less likely to die of heart disease than those who ate fish less than once a month. (2) But a handful of reports suggests that the mercury in some fish may blunt some of that benefit.

* In 1995, Finnish researchers found that men with at least two parts per million (ppm) of mercury in their hair had twice the risk of suffering a heart attack as men with lower levels of mercury. (3) (Mercury levels in hair or toenails reflect long-term exposure to the metal from food and other sources.) That's not a lot of mercury. The National Academy of Sciences, for example, recommends that pregnant women protect their fetuses by keeping the levels of mercury in their hair below 12 ppm.

* Researchers at Johns Hopkins in Baltimore found that men with the most mercury in their toenails were twice as likely to suffer a heart attack as men with the least. (4)

But another study appears to disagree. Researchers at the Harvard School of Public Health found that men with the most mercury in their toenails were no more or less likely to suffer a heart attack, to undergo cardiac surgery, or to die from heart disease than men with the least mercury. (5)

The Harvard and Johns Hopkins studies "may not be as contradictory as they seem at first glance," says Johns Hopkins researcher Elisio Guallar.

"Sixty percent of the Harvard study's group was dentists, who inhale metallic mercury during their work." If only methylmercury (the form found in fish) raises the risk of heart disease, the high levels of metallic mercury in the dentists might have obscured a link between methylmercury and the heart.

"When the Harvard researchers looked just at the non-dentists, whose major exposure to mercury is seafood," adds Guallar, "they found a risk from mercury similar to what we found." But it's hard to know if those results were due to chance, because there weren't enough non-dentists in the study. The bottom line: It's too early to say whether the mercury in fish can harm the heart.

"Until we know more, consumers should follow the American Heart Association's advice to eat at least two servings of fish a week," says Guallar. "But they should eat high-mercury fish like swordfish or shark rarely or not at all, and they should eat a variety of fish, not just canned tuna."

(1) Environ. Health Perspect. (in press).

(2) J. Amer. Med. Assoc. 287: 1815, 2002.

(3) Circulation 91: 645, 1995.

(4) New. Eng. J. Med. 347: 1747, 2002.

(5) New Eng. J. Med. 347: 1755, 2002.

Sorry, Charlie

Canned tuna doesn't have nearly as much mercury as swordfish, shark, tilefish, king mackerel, or fresh tuna (which comes from larger fish than canned tuna). Even so, it contributes more mercury to the average person's diet than any other food, simply because we eat so much of it.

How much canned tuna can you safely eat? It depends on who you are:

* If you're pregnant, nursing, a woman of childbearing age, or a young child, the chart below shows your weekly limit, assuming you eat no other seafood. Those limits are based on studies of children in the Faroe Islands (in the North Atlantic, near Iceland) who were exposed to high levels of mercury in the womb. So they are stringent enough to protect fetuses. It also makes sense to apply the limits to young children, since their brains are still developing.

(We include "women of childbearing age" in the same group as pregnant women because roughly half of all pregnancies are unplanned. However, levels of mercury in the body drop substantially within a year if people consume low-mercury species. So only women who could become pregnant within a year need to follow the advice for pregnant women.)

* If you're a man, an older child, or a woman beyond childbearing age, you can probably safely consume two or three times the tuna limits in our chart. We say "probably" because the evidence isn't clear.

Our chart is based on mercury intake limits for the entire population set by the Environmental Protection Agency (EPA) using studies on pregnant women. But the Food and Drug Administration (FDA) says that people (other than young children or pregnant women) can safely consume four times more mercury than the EPA recommends. The FDA's advice is based on severe mercury poisoning episodes in Japan and Iraq, where people were exposed to levels of mercury far higher than what most Americans get from fish.

So the best advice may lie somewhere between the EPA's limits (see chart below) and the FDA's looser limits.

Your Weight    White (albacore) Tuna       Light Tuna
(pounds)       (ounces per week) *     (ounces per week) *

33 or less              1                    1 1/2
34-49                 1 1/2               3 (1/2 can)
50-60                   2                      4
61-77            2 1/2 (1/2 can)           5 (1 can)
78-99                 3 1/2                    6
100-121                 4                      8
122-143             5 (1 can)             10 (2 cans)
144-165                 6                      11
166-187                 7                      13
188-209                 8                      14
210-230                 9                      16

* Numbers are for drained tuna. When you drain the water or oil, a
"6-ounce" can of tuna ends up with about 5 ounces of tuna meat.

Note: White tuna has about twice the mercury of light tuna. That's
because larger predatory fish like albacore (white) contain more
mercury than the smaller skipjack and yellowfin that are sold as light.

Tracking Mercury

Mercury is spewed into the air from coal-burning plants and facilities that incinerate medical waste. It's carried by the wind, and is deposited into bays, lakes, rivers, and the oceans, where it settles to the bottom. There, bacteria transform it into methylmercury, the form that plants and animals can absorb, store, and pass on to others. Plankton absorb mercury from the bacteria, small fish absorb it from the plankton they eat, and bigger fish absorb it when they eat the smaller fish.

In that way, greater and greater amounts of methylmercury are passed up the food chain to large predator fish like shark, swordfish, and barracuda. In bays, rivers and lakes, predators like pike, walleye, white croaker, and largemouth bass accumulate the most mercury. Ultimately, the toxic metal gets passed on to the biggest predators of all--humans.

Fortunately, mercury doesn't hang around for long. Every 50 days, on average, we eliminate half the mercury that's in our bodies. That means that more than 99 percent of the mercury you eat today will be gone within a year.

The Bottom Line

* It's too early to say whether the amount of mercury that some people get from fish can cause neurological or other health problems.

* Seafood's omega-3 fats are good for your heart. Your best bet is to get them from fish other than swordfish, shark, tilefish, king mackerel, and fresh tuna, which are high in mercury.

* The fish with the least mercury, according to the Food and Drug Administration and the non-profit Environmental Working Group: blue crab (mid-Atlantic), catfish (farmed), croaker (not white), fish sticks, flounder, haddock, salmon (farmed or wild), shrimp, and trout (farmed).

* For men, older children, and women above childbearing age: Limit high-mercury fish to no more than one serving a week. There is no need to limit other fish. (For advice on canned tuna, see "Sorry, Charlie.")

* For women of childbearing age and young children: Don't eat high-mercury fish at all. Eat up to 12 ounces a week from a variety of other fish. (For advice on canned tuna, see "Sorry, Charlie.")

* For people who eat locally caught fish: Most states have advisories that warn against eating fish that are caught in mercury-contaminated bays, lakes, rivers, and streams. California, for instance, tells adults not to eat more than two meals a month of sport fish caught in the San Francisco Bay. You can find a (not terribly user-friendly) link to the state advisories at www.epa.gov/waterscience/fish.