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University of Rochester

Brain food, they used to call it. Down enough codfish, or salmon, or some other fortifying denizen of the deep, and maybe someday, like Camus or Curie, you, too, will get the summons to Stockholm.

The association between "brain" and "fish" goes way back in folklore, and moreover has some scientific credibility: There's a theory afloat which holds that the evolution of humans from apes in Eastern Africa was due in part to an influx of fish into our ancestors' diet.

"What could be more nutritious than fish?" asks Thomas Clarkson, a Rochester professor of environmental medicine, who can cite "a slew of scientific reports" showing that eating fish helps protect against cardiovascular disease and enhances brain development before and after birth.

"Fish," he points out, "is a rich source of low-fat protein and is full of fatty acids known to lower cholesterol."

In recent years, though, what with the exhalations of newly active volcanoes like Mounts St. Helens and Pinatubo, augmented by the ceaseless puffings of utility plants, incinerators, and even human cigarette smokers, a disturbing question has arisen.

Could fish, as efficient collectors of mercury--a product of these effluvia--actually be a brain poison?

Mercury vapor enters the atmosphere from both natural and human-generated sources (e.g., volcanoes and utility plants) and falls in rainwater to the seas, where microbes transform it into a substance that accumulates in the fish that end up on our dinner plates.

Fish are the primary source of mercury exposure around the world. Mercury vapor in the atmosphere comes from both natural and synthetic sources and falls in rainwater to the Earth, where microbes in the oceans and other bodies of water transform it into a substance known as methyl mercury. This substance moves up the food chain and accumulates in virtually all fish around the globe, no matter how far their waters may lie from utility plants, incinerators, volcanoes, and other such sources of this airborne pollutant.

So, should you avoid fish like, well, like the plague?

The answer, according to a 10-year study by researchers at the University's Medical Center: a resounding No. If your concern is mercury poisoning, their advice is not to worry. Eat up, they say, and enjoy the benefits from the commercial fish that's sold in supermarkets, fish shops, and restaurants. (Caveat: Note the words "commercial fish." The same green light does not apply to the fish you or your friends may catch from your local waterways, but more on that point later.)

"We know that mercury is a deadly neurotoxin," says Philip Davidson, an expert in developmental disabilities and lead author of the Rochester team's paper published earlier this year by the Journal of the American Medical Association. It has in fact been recognized for years that toxic levels of mercury can ravage the brain or, as one investigator says, "It chromes your dome."

Take, for instance, the hat makers of yesteryear. The phrase "mad as a hatter" derives from the bizarre behavior of those who sustained brain damage from the mercury compounds once used in the manufacture of felt headgear. Earlier this year, the dangers of mercury poisoning were dramatically brought to the nation's attention through news stories about teenagers contaminating whole neighborhoods while playing with stolen mercury.

At toxic levels, mercury kills nerve cells, causing blurry vision, loss of coordination, slurred speech, and even death. Children prenatally exposed to high levels can suffer slowed development, blindness, and cerebral palsy, among other birth defects.

The news that mercury-tainted fish are OK to eat may therefore be surprising. But, in fact, we ingest many poisons daily, with no ill effects: chlorine and fluoride, for example, which we gulp down with every sip from the tap. These two dangerous chemicals are routinely added to the drinking supply--chlorine to zap dangerous microbes and fluoride to prevent our teeth from rotting.

It's the level of the toxic element that is the key. And the effects of low levels of mercury have been vigorously debated.

"When it comes to the question of mercury and fish, people want to know, 'Is there or is there not a poison in my food?' " says Rochester's Clarkson, who has spent the last 40 years analyzing the effects of mercury on the body. (Once, during a trip to Northern Canada, he ate a fish high in mercury and measured his blood every few hours to confirm his knowledge of how mercury enters the bloodstream. He was right on track.)

"So, in answer to the poison question --if you look hard enough, you can say, well, yes, there is a poison in your food." Clarkson says. "But will it harm you? No, it won't, because the level of the mercury is so low."

Clarkson is the first person governments and institutions around the world turn to when a mercury-poisoning event occurs. Such a disaster befell Iraq in 1972, when 459 people died and more than 6,500 others were sent to hospitals as a result of eating bread made from wheat treated with a mercury-based fungicide. Fifteen years earlier, a similar incident had occurred in Minamata Bay, a small Japanese coastal town, where 43 residents died and nearly 2,000 fell ill from eating contaminated fish.

While the deaths and deformed babies resulting from these two tragedies revealed the danger of mercury in stark detail, they highlighted a troubling question: How much mercury does it take to do harm? Or, to put it another way, just how tiny is the amount that can be safely ingested?

In an attempt to determine that crucial fact, Clarkson led a team that spent much of the 1970s examining the victims of the Iraqi wheat poisonings and poring over the resultant data. Their findings held open the possibility of negative effects from mercury at very low levels--a possibility that has since been widely taken up by environmental groups seeking to curb the spread of mercury worldwide.

But the researchers themselves had doubts about just how pertinent these findings were to human health generally.

"There are many reasons why you shouldn't determine risk to human health based on the Iraqi data," says Clarkson. "That was a poisoning event, which is very different from a low-level exposure. Further, the source of the mercury was contaminated grain--not fish, which is how most people are exposed. Moreover, the possible risk at low levels was determined by just a few cases."

What was needed was a new, more extensive investigation.

Where to conduct such a long-term study? The researchers scoured the globe looking for an area far from any sources of mercury pollution, where fish is the mainstay of the diet, and where the population would be easy to track over a number of years.

They found their answer in a speck of land an exhausting three plane flights and 10,000 miles away from the University's high tech laboratories: the Republic of the Seychelles, a steamy island nation lying along the equator a thousand miles off the coast of Kenya. This bit of Eden, as Davidson calls it, has become ground zero in the world's most comprehensive study of the biological effects of low levels of mercury.

Most of the 65,000 inhabitants eat nearly a dozen fish meals each week; throughout their lives they tend to stick close to their homeland; and they all receive free, high-quality medical care, thereby reducing the muddying element of illnesses from extraneous sources. (Such factors are a must for biostatisticians like Christopher Cox, who is responsible for analysis of all the data.) Blood mercury levels among the Seychelles people, thanks to their fish-laden diet, register 10 times higher than for most U.S. citizens.

In 1989, pediatric neurologist Gary Myers took off for this island community, spending about a year enrolling 779 newborns in the study--about half of the Seychelles births for that year. The team designed the project to examine the effects of fetal exposure to mercury, since infants in the womb are far more susceptible than children or adults to its damage. Among their first steps, researchers took hair samples from the children's mothers. (Hair, Clarkson had earlier discovered, preserves a record of an infant's mercury exposure during gestation.)

Over the succeeding years, Myers, Davidson, and other team members repeatedly returned to the islands, visiting the children's homes, talking to their parents, and performing a battery of nearly three dozen developmental and neurological tests. The analysis included noting when the children learned to walk and talk, and measuring reflexes, word recognition, and social behavior.

The findings? Eating lots of fish carries no detectable risk from low levels of mercury, even for very young children and pregnant women. In continuing to track the youngsters' mercury levels over the years, the scientists have found no harmful effects even in children exhibiting mercury at levels up to 20 times the U.S. average.

"We look at the Seychelles people as a sentinel population," says Myers. "If somebody who eats fish twice a day does not show effects from mercury exposure, it's unlikely that somebody who eats fish only twice a week will be affected. And the fish they eat in the Seychelles contains the same amount of mercury as the supermarket fish sold in the United States."

"What we found in the Seychelles is applicable to every woman, every man, and every child around the world who eats ocean fish," declares Davidson.

So far the team has published its results of studies of the children at 6, 19, 29, and 66 months of age. And the analysis is continuing--Davidson and Myers have just returned from the islands, where they're now studying the same children at 8 years of age. The tremendous scale of the study, the researchers say, is possible only because of the efforts and cooperation of more than 30 scientists, technicians, nurses, and other personnel both at the University and in the Seychelles, all of them on the lookout for even the slightest mercury-induced effect.

"When you don't find something, people always say, 'Did you not find it because it wasn't there--or because you didn't look thoroughly enough?' That's why," Myers says, "our tests in the Seychelles were the most sophisticated possible for these age groups."

The research has thrust the investigators into the political arena in a way that doesn't normally happen to scientists who look at, say, the genetics of yeast or the vagaries of the photon. But this is an issue that has broad implications not only in the field of child development, but also in the arenas of nutrition, the environment, and the utility and fishing industries. Following the Iraqi study, the scientists found themselves heroes, as Clarkson puts it, to those preaching the dangers of mercury. A decade later, he notes, they've become antiheroes to that faction--and the good guys to opposing voices who attest that low mercury levels are of little concern.

The experience can be disconcerting, Clarkson admits. "About the worst thing that can happen to a professor is that you can be quietly working away in your lab, never suspecting that some agency out there is using your paper as a basis for regulation. When that happens, my gosh, holy hell breaks loose as they pull your work apart from every direction."

So far, the current study's findings have withstood all serious scrutiny. There hasn't been a single peer-reviewed criticism of its results since publication began three years ago, the team reports. "The science speaks for itself," says Davidson.

Nevertheless, the Rochester findings have been taking center stage at a November White House summit called to hash out the risk from low levels of mercury. The issue has recently become acute through an action of the federal Environmental Protection Agency: Last year the EPA proposed slashing the amount of mercury that is acceptable for people to ingest.

If the Food and Drug Administration follows the new EPA guideline, it would take off the market a significant proportion of the fish now available--especially large predatory fish like swordfish, shark, and red snapper--and could even affect that family standby, tuna.

Scientists estimate that the average person would be able to eat only a few ounces of fish per week before bumping up against the proposed new limit.

"Eating lots of ocean fish isn't much of a hazard compared to missing out on their benefits by not eating fish," says Clarkson.

The University group's findings differ markedly from those of a Danish team that looked at a population in the Faroe Islands. The Faroese people, who live close to the Arctic Circle, are exposed to mercury mainly through eating whale as well as fish. In contrast to the evidence of the Seychelles study, the Danish investigators found that children who had been prenatally exposed to mercury exhibited slight abnormalities in development at age 7. The Rochester scientists, however, are not convinced by those results, since whale meat contains other pollutants, like PCBs, that are known to be toxic at low levels.

It's those other pollutants that ring alarm bells about some kinds of fish. Those that come from the hundreds of contaminated freshwater bodies in North America are dangerous to eat in abundance--usually because of pollutants other than mercury. Fish eaters still should follow advisories about their consumption.

The Seychelles study, the investigators reiterate, applies only to fish bought and sold at grocery stores, supermarkets, fish shops, and restaurants. These fish are already regulated by the FDA to help consumers avoid excessive exposure to mercury, a potential danger primarily for someone eating frequent meals of the aforementioned swordfish, shark, and red snapper.

The safety of seafood is near to the heart--and brain--of Tom Clarkson, who enjoys a fish dinner twice a week. He, for one, sees little need for regulatory bodies to take costly measures to lower the amount of mercury in our diet.

"Overstating the almost negligible risk of mercury could adversely affect millions of people who face the much greater risk of heart disease," he declares. "The guidelines now in place have protected our health for 30 years. Why try to fix something that's working?"

Tom Rickey is senior science editor for the Office of University Public Relations.

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