Mercury in Fish May Not Be Toxic

Scientists Identify Chemical Compound of Mercury in Fish

From the WebMD Archives

Aug. 28, 2003 -- It is one of those mixed health messages that drives people nuts. Experts say eating fish is good for you, but they also warn that levels of mercury in certain types of fish may be dangerous. Now a new study suggests there may be no reason to fear that tuna sandwich or grilled swordfish.

Stanford University researchers report that the chemical form of mercury in the fish we eat is different, and potentially less harmful, than previously thought. They used newly available X-ray technology to identify the specific type of mercury that is present in the muscle tissue of fish. They found that this mercury in fish contains both a carbon atom and a sulfur atom -- potentially making it less dangerous.

Their findings are published in the Aug. 29 issue of the journal Science.

Much of the mercury in fish comes from the burning of coal and mercury-tainted trash. Mercury has been linked to a wide range of nerve problems.

Cautious Optimism

Now that scientists know what is in the mercury, they are in a much better position to accurately study it's health effects, lead researcher Graham George, DPhil, tells WebMD. "Right now, there is reason for cautious optimism that the fish we thought were potentially harmful may not be, but we don't actually know this."

There are 26 different known mercury compounds, each with its own toxic profile. A compound known as methylmercury chloride has traditionally been used to study the toxic properties of mercury in fish. But George and his research team from Stanford's Synchrotron Radiation Laboratory report that the chemical structure in the commonly eaten fish they examined more closely resembled a compound known as methylmercury cysteine.

In as yet unpublished studies, the researchers found that methylmercury cysteine was much less toxic to day-old zebrafish larvae than methylmercury chloride. The next step, George says, is to determine what happens to mercury in the stomachs of mammals that have eaten fish. It is now possible to track mercury as it enters body tissue.

"We are getting ready to do these experiments, and should probably have some answers within a year," says George, who is now with the University of Saskatchewan in Saskatoon, Canada.

No Swordfish, Mackerel, or Shark

The FDA advises women of childbearing years and young children not to eat swordfish, king mackerel, tilefish, and shark -- the fish with the highest levels of mercury. There is also some concern that eating a lot of canned tuna may be harmful, but the FDA has been relatively silent on this issue. An FDA spokesperson tells WebMD that a new advisory designed to clarify the agency's guidelines on mercury in fish and how much fish is OK to eat should be made public by the end of the year.

Results from a nine-year-long human study, published in the spring of this year, should reassure those who worry about levels of mercury in fish. Researchers found no evidence of developmental damage in the offspring of mothers living off the coast of Africa who ate an average of 12 fish meals a week.

Lead researcher Gary J. Myers, MD, of the University of Rochester, says the newly published research may help explain his findings.

"We looked at people who ate about 10 times as much fish as the average American eats, and the fish they ate had mercury levels that were similar to those that Americans eat," he tells WebMD. "We found no evidence that women who eat a lot of fish during pregnancy put their unborn children at risk."

Show Sources

SOURCES: Science, Aug. 29, 2003. Graham George, DPhil, Canada Research Chair in X-Ray Absorption Spectroscopy, University of Saskatchewan, Saskatoon, Canada. Gary J. Myers, MD, University of Rochester, New York. FDA. WebMD Medical News: "Mercury in Fish No Problem in Pregnancy."
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