In scientific jargon, selenium has an unusually high "binding affinity" for mercury. In layman's terms, this means that when the two elements are found together, they tend to connect, forming a new substance. This makes it difficult for the human body to absorb the mercury separately. So when mercury "binds" to selenium, it's no longer free to "bind" to anything else -- like brain tissue.

The research world is still developing explanations for exactly how selenium cancels out mercury's potentially toxic effects, but most scientists accept one of two competing theories.

The conventional idea describes selenium as a sort of "mercury magnet." Under this theory, once selenium is digested it can locate and neutralize mercury molecules. In one study, Japanese researchers found that adding selenium to the diets of birds "gave complete protection" from large amounts of mercury. Research carried out by scientists in Scotland and the Philippines has concluded that the relationship between mercury and selenium is one of "toxicological antagonism." And in the United States, the Environmental Protection Agency describes selenium as an element that is "antagonistic to the toxic effects of mercury."

The more recent selenium hypothesis holds that mercury takes a more active role in the relationship. Under this theory, when mercury enters the body it seeks out selenium and takes it out of circulation, preventing the body from creating enzymes that depend on selenium to perform their functions. Enzymes are special proteins that control the various steps in chemical reactions that make life possible. Without enough selenium-based enzymes, the functions of the brain and other organs can be affected.

While this might sound scary, problems can only occur if we don't get enough selenium to counteract the trace amounts of mercury in the fish we eat. And fish are so rich in selenium that this is not likely to happen. The U.S. Department of Agriculture has measured selenium levels in more than 1,000 commonly consumed foods, and 16 of the 25 best sources of dietary selenium are ocean fish. University of North Dakota environmental scientist Dr. Nicholas Ralston is an expert on the relationship between selenium and mercury. Here's how he describes it: