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THURSDAY, Aug. 28, 2003 (HealthDayNews) -- The mercury in swordfish, tuna and other underwater species isn't as toxic as the mercury scientists use to gauge the dangers of eating seafood, says a new study.
Mercury is a well-established toxin that in high doses can destroy nerve cells and trigger heart attacks. Yet the new study found the form of mercury most prevalent in certain food fish appears to be far less harmful than methylmercury chloride, the molecule upon which cautions about fish consumption are based.
"Now that we know what the mercury compound in fish is, we're well positioned to understand its toxicological implications," says study author Graham George, an X-ray spectroscopy expert at the University of Saskatchewan in Saskatoon. "Now we can really see what kind of a health risk fish poses" to people.
Dr. Gary J. Myers, a mercury researcher, says the findings point up the importance of studying mercury as it appears in the diet.
"We hope that other people around the world will study populations that actually consume fish as their source of mercury," says Myers, whose group has been doing just that in the inhabitants of the Seychelles Islands in Africa.
Some ocean fish are higher in mercury than others. The U.S. Food and Drug Administration recommends pregnant women avoid eating swordfish, shark, tilefish and king mackerel altogether. Other fish and shellfish should be limited to no more than 12 ounces per week, or between two and four servings. Most people in this country eat only one fish dish a week.
However, Myers' group has found no solid evidence that babies born to women in the Seychelles, where fish is a staple, suffer neurological deficits later in life.
The new study, reported in the Aug. 29 issue of Science, used X-ray spectroscopy to examine the nature of mercury in swordfish, orange roughy and sole purchased from a California fishmonger.
Molecular fingerprints, or spectra, weren't readily available for sole. But the spectra of the mercury in swordfish and roughy showed that the predominant form of mercury was methylmercury cysteine or its near relatives. Methylmercury chloride isn't present at all in the fish.
Unlike methylmercury cysteine, methylmercury chloride doesn't dissolve readily in water. That may explain its high toxicity to cells.
While the dangers of methylmercury cysteine aren't well understood, at least one study found that zebrafish larvae can tolerate 20 times more of the substance than methylmercury chloride.
The researchers acknowledge the human stomach may somehow convert methylmercury cysteine to methylmercury chloride, but that hasn't been demonstrated. "The best way to figure it out is to do the studies," says George, who conducted the research while at Stanford University.
George Gray, director of the Program on Food Safety and Agriculture at the Harvard Center for Risk Analysis in Boston, says the new findings are instructive. "In the world of human health risk assessment, if we don't have the information about how what we do in the lab is different from what people experience, the exposure scenarios in toxicology experiments won't reflect" real life, Gray says.
A good illustration of that split, Gray says, is the effect of chloroform on mice. Mice given a large dose of chloroform all at once develop liver tumors. But when the rodents sip the same dose in their water over the course of a day, no liver cancers appear. "We know that those things are out there," Gray says, and mercury exposure through fish may be another example.