The journal Science has a report from Dr. George Graham at University of Saskatchewan in Saskatoon, Canada, showing that mercury in fish may not be as dangerous as some scientists have been telling us.
There are two major recorded studies on mercury poisoning from food: one from highly contaminated fish in Japan in the 1960's, and the other from highly contaminated grain in Iraq in the 1970's. In Minimata, Japan, 111 people died or became very ill from eating fish daily over extended periods from waters that were severely polluted with mercury from local industrial discharge. The Iraq study was massive mercury poisoning from insecticides. No studies have ever shown damage in humans who consume moderate amounts of fish from a variety sources.
The U.S. Food and Drug Administration recommends that pregnant women and young children avoid large predatory fish because they may accumulate large amounts of mercury that could possibly cause brain damage in young children. However, there are no reports of North American children actually being harmed by mercury in fish.
The FDA's concern is about carbon mercury, called methyl mercury. Fish absorb methyl mercury from water as it passes over their gills and as they feed on aquatic organisms. Larger predatory fish are exposed to higher levels of methyl mercury from their prey. Methyl mercury binds tightly to the proteins in fish tissue, including muscle, and it accumulates over the years, so the older the fish, the more mercury it may contain.
This new study shows that the mercury in fish may not be carbon mercury, but is really carbon-sulfur mercury that is far less toxic and therefore of far less concern. Mercury binds to a carbon molecule by a process called methylation, and it also is bound to a sulfur molecule that makes the mercury far less absorbable into human tissue. Mercury is toxic to nerves only when it is methylated, meaning that it is attached to carbon ion. Dr. Graham used X-ray absorption spectroscopy to demonstrate the physical structure of the mercury compounds in fish muscle tissue. He found that the mercury in fish is actually attached to both a carbon atom and a sulfur atom, and sulfur attaches more tightly to mercury than carbon. The sulfur prevents most of the mercury from being absorbed and taken up by human tissue.
We await further reports on the safety of deep water fish. Pregnant women and young children should still restrict large predatory fish until further studies are done to confirm or refute this report.
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