The best science indicates that trace amounts of mercury in the fish Americans eat simply aren't high enough to pose a health risk. But measuring only mercury further exaggerates this hypothetical risk. There's another scientific wrinkle that few environmental groups are talking about -- largely because it doesn't help to promote their scare campaigns. An accurate picture of the health consequences of eating fish must include other substances that affect the way mercury interacts with the human body.

Selenium is plentiful in fish, but the public hasn't heard much about its role in the mercury puzzle. As biochemists, pharmacologists, and neurologists study this nutrient, we're gaining a better understanding of its importance.

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:

Think of dietary selenium as if it were your income and dietary mercury as if it were a bill that you need to pay. Just as we all need a certain amount of money to cover living expenses such as food and rent, we all need a certain amount of selenium ... Only one major study has shown negative effects from exposure to mercury from seafood, and that seafood was pilot whale meat. Pilot whale meat is unusual in that it contains more mercury than selenium. When you eat pilot whale meat, it's like getting a bill for $400 and a check for less than $100. If that happens too much, you go bankrupt. On the other hand, if you eat ocean fish, it's like getting a check in the mail for $500 and getting a bill for $25. The more that happens, the happier you are.

Dr. Ralston is right. Researchers at the National Oceanic and Atmospheric Administration found that most of the fish we eat contains significantly more selenium than mercury. Seafood that contains more mercury (Hg) than selenium (Se) includes pilot whale, tarpon, marlin, and some shark. Fish we most commonly consume, including all forms of tuna and salmon, are rich in selenium.



On the other end of the scale, pilot whale is by far the worst offender. This may help explain why researchers in the Faroe Islands insist that dietary mercury is harmful to island residents. (Unlike the vast majority of people, the Faroese eat lots of pilot whale meat.) By contrast, a similar study in the Seychelles Islands -- where people eat lots of selenium-rich fish but no whale meat -- found no negative health effects from the tiny amounts of mercury in fish.



Selected Sources:

L.Cuvin-Aralar and R. Furness. "Mercury and Selenium Interaction: A Review." Ecotoxicology and Environmental Safety. 1990 Oct 10;21:348-364.

L. Raymond and N. Ralston. "Mercury: Selenium interactions and health implications." SMDJ Seychelles Medical and Dental Journal. 2004 Nov.; Special Issue, Vol 7, No 1.

P. Beyrouty and H. Chan. "Co-consumption of selenium and vitamin E altered the reproductive and developmental toxicity of methylmercury in rats." Neurotoxicology and Teratology. 2006 Jan;28:49-58.

K. Beijer and A. Jernelov. "Ecological Aspects of Mercury-Selenium Interaction in the Marine Environment." Environmental Health Perspectives. 1978 Aug;25:43-5.

G. Ohi et al. "The Protective Potency of Marine Animal Meat Against the Neurotoxicity of Methylmercury: Its Relationship with the Organ Distribution of Mercury and Selenium in the Rat." Food and Cosmetics Toxicology. 1980 Apr;18:139-145.

Y Sugiura et al. "Selenium Protection Against Mercury Toxicity: High Binding Affinity of Methylmercury by Selenium Containing Ligands in Comparison with Sulfur Containing Ligands." Bioinorganic Chemistry. 1978 Aug; 9(2):167-180.