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Mercury in Fish May be Less Toxic Than Previously Thought

What is not at dispute is that nearly all of the commercial fish in the world that are consumed are contaminated with mercury. I see this nearly every day in my office where we correlate hair mercury levels with level of fish consumption. Now new research involving University of Saskatchewan scientists raises the possibility that the mercury compounds that build up in fish may not be so bad after all. That's because the mercury that collects in fish may be in a form that is less harmful than the form that had previously been thought to build up in fish tissue.

The exact chemical identity of the mercury in fish is important because different methylmercury compounds have quite different toxicities. It has been known for many years that the mercury in fish is a methylmercury-containing species--but exactly which methylmercury species has remained a mystery. The safe limits for human consumption of fish have been to some extent based on studies using methylmercury chloride as a model.

The researchers studied swordfish and orange roughy bought at a local seafood store. With conventional chemical analysis fish tissues always need to be chemically pulled apart before an analysis can be done, which partly destroys the molecules. But with the synchrotron instrument that these researchers used the intact sample can be measured and no chemical pre-treatment is required, which allows the molecules to be observed directly.

The researchers collected information on the atoms bound to mercury in the fish muscle tissue. They found that the methylmercury in both fish is bound to a sulfur atom, and is most likely methylmercury cysteine. The cysteine form of methylmercury was found to be less toxic to day-old zebrafish larvae. So what do we do? I think many of us can breathe easier with respect to mercury, but this research may not pan out after further investigation. However, let us not forget that mercury is not the only toxin in fish. It is typically also loaded with PCBs. So I will continue to avoid nearly all seafood except those that I am convinced are low in toxins, such as wild Alaskan salmon.

University of Saskatchewan September 2, 2003

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