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Health Benefits of Fish Weighed Against Mercury Risk

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Knight Ridder Newspapers


PHILADELPHIA - Fish has long been touted as a near-perfect health food. High in protein and brain-boosting nutrients, fish appears to offer protection against heart disease, Alzheimer's disease and depression.

But now, some doctors and environmental groups are warning that many fish also serve up a dangerous dose of mercury. Some fish fanciers in California claim they have gotten sick from eating too many mercury-laden fish, and the National Academy of Sciences warns that thousands of children are at risk for neurological damage because their mothers ate too much of the wrong fish while pregnant. As a result, some activists are pushing the Food and Drug Administration to tighten mercury standards.

All of which leaves consumers in a quandary. Should they eat fish? If so, which fish? And how much? Is any amount of mercury safe? How much is dangerous?

The FDA and the Environmental Protection Agency had for years been using different safe levels, further confusing the issue, but they have recently aligned.

The agencies say one should consume, on average, no more than 0.1 microgram of mercury per kilogram of body weight per day. If you weigh 130 pounds, then one 6-ounce serving of swordfish, fresh tuna, shark or eight ounces of canned tuna in a given week could send you over that limit (though mercury levels vary from fish to fish within a species). Not that this mercury would stay with you forever; it lingers for a month or two before the body flushes it out.

The standards are designed to protect children and women who are pregnant or may become so, but the FDA has decided to use that standard for everyone.

Still, uncertainty looms behind that number. Asking pregnant women to give up fish is not the answer. A growing body of evidence suggests that the human brain, especially when it is developing, thrives on the omega-3 oils in fish.

Some anthropologists have suggested that eating fish helped prompt humanity's intellectual leap beyond our Neanderthal competitors about 30,000 years ago. And a recent study found that pregnant women who eat fish suffer less often from postpartum depression.

The FDA recommends that children and pregnant women avoid the four worst mercury sources - tilefish, shark, swordfish and king mackerel - and eat no more than 12 ounces a week of a variety of other fish.

That recommendation assumes that 12 ounces of fish would contain 42 micrograms of mercury - the safe limit for a 132-pound person.

Environmental groups such as the Mercury Policy Project criticize this recommendation because it is based on the average amount of mercury in fish and therefore assumes people will choose low-mercury fish such as sardines as well as more risky choices such as white albacore tuna, project spokesman Michael Bender said. If a woman gets her 12 ounces from canned albacore or red snapper, she could get 150 to 200 micrograms of mercury.

That 42 micrograms is safe is based on limited science and some educated guessing.

David Acheson, chief medical officer for the FDA, suggests that women who are pregnant or plan to become so need to find out which seafood items are lowest in mercury - for example, sardines, flounder, scallops and wild salmon - and stick with those. Canned light tuna has about a third as much mercury as white tuna.

Mercury is not new to the oceans. It comes naturally from volcanic activity and probably contaminated fish for millions of years. Today, coal burning and other human activities have increased the load.

The mercury itself is not so deadly, but once it gets into bodies of water, microscopic organisms convert it into an extraordinarily toxic form, called methylmercury. It gets concentrated in the flesh of the biggest, longest-living fish.

Methylmercury seems to cross the protective blood brain barrier by latching onto an amino acid and disguising itself as a substance the brain needs. In 1997, a 48-year-old chemist at Dartmouth got a drop of methylmercury on her gloved hand. A trace of the substance got through the glove, penetrated her skin, and killed her slowly over several months.

Most of what scientists now know about mercury's toxic effects derives from two cases. First, in the 1950s, dozens of fishermen living on Japan's Minamata Bay reported numbness and "pins and needles" as well as staggering and tunnel vision. Eventually the symptoms were linked to mercury poisoning; local factories were found to be dumping methylmercury into the bay.

Though no death count was taken, University of Rochester toxicologist Tom Clarkson estimates that hundreds probably died and those who survived sustained permanent blindness or other neurological damage.

In the 1970s, thousands of people in Iraq ate bread mistakenly made from grain treated with methylmercury to reduce fungal infestations. Hospitals were overflowing and the stricken populace begged for nonexistent antidotes, said Clarkson, who went to Iraq to study the aftermath.

He pinpointed a blood level below which people had no apparent symptoms and above which they suffered irreparable harm: 200 micrograms of mercury per liter of blood.

The FDA originally deemed anything below 20 micrograms per liter to be safe, calculating a safety factor of 10.

In the late 1980s, seeking better information on the safety of fish consumption, scientists went to the Seychelle Islands, northeast of Madagascar, where people eat about 12 meals of fish a week. Many people had blood levels close to the FDA standard of 20 micrograms per liter.

Researchers were able to analyze the hair of women with infants and small children to estimate how much mercury they had in their bloodstreams while pregnant. Surprisingly, Clarkson said, the more mercury women ingested when pregnant, the better their children performed on various tests of cognitive skills. That effect might be connected to other beneficial nutrients in fish that overshadow the negative effects of the mercury.

But another study done around the same time in the Faroe Islands, north of Scotland, produced the opposite result - the more mercury women ingested while pregnant, the worse their children performed. But those women got their mercury from eating whale meat and blubber, which also was contaminated with PCBs and may also damage the developing brain.

The Faroe study found that children started scoring lower if their mothers had blood-mercury levels around 58 micrograms per liter. That would suggest a safety standard of 5.8 micrograms per liter, to provide the same 10-fold level of safety assumed by the current standard. The EPA and FDA base their current consumption standard on this study.

The agencies estimate that a person can stay under that limit by ingesting no more than the 0.1 microgram per kilogram of body weight per day - not a particularly useful number for most people.

FDA's Acheson said this number was used to arrive at the more user-friendly recommendation of 12 ounces of fish a week.

Many fish lovers far exceed that. San Francisco physician Jane Hightower discovered that a number of people eat fish nearly every day.

Some of her patients had mercury levels far above the government standard. She recently reported that she had some patients with blood-mercury levels as high as 89 micrograms per liter. They reported symptoms, including hair loss, difficulty concentrating or remembering, and fatigue.

When these people stopped eating fish, their mercury measurements went down over the course of a month or two, she said.

Clarkson said the reported symptoms were not consistent with mercury poisoning. He added that people have to keep in mind the safety factor built into these standards.

"It's very misleading to say that if you're three times higher than the EPA limit, you're going to be poisoned," he said.

Hightower has not discounted the possibility that something else in the fish could be affecting her patients. Whatever the culprit, she said, they reported feeling better when they temporarily cut fish from their diets.

She would like to see more doctors test people for mercury, which could help people decide how much fish to continue to eat.

But what should fish fanciers substitute? Beef with its saturated fat? Starchy foods that raise blood sugar? Eggs? Chicken? Hightower said a little beef or a few eggs are fine, that people should eat a wide variety of foods: "I tell people to rotate their poison."


(c) 2003, The Philadelphia Inquirer. Distributed by Knight Ridder/Tribune News Service.

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