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Seafood FAQ: Is selenium a mercury antidote?

Scientists say seafood may contain a natural neutralizer of this toxin

Light tuna is a rich source of selenium, which may
    counter mercury's harmful effects on human health. - Photo courtesy of the Center for Consumer
By James Wright
October 01, 2006

Few consumers, or even seafood professionals, know that seafood is a rich source of selenium, a mineral that some scientists say counteracts the harmful neurotoxin methylmercury found in some species of fish.

Other experts disagree that selenium is the antidote to mercury, and the two viewpoints clashed during the Eighth International Conference on Mercury as a Global Pollutant (Mercury 2006), held in Madison, Wisc., from Aug. 6 through 11. The consensus: Further studies on selenium and mercury are necessary for a full understanding of how the two interact.

There is little doubt that exposure to mercury is harmful to human health. The issue took center stage in 2004 when the federal government warned consumers that pregnant women, nursing mothers and young children - those most susceptible to the harmful effects of mercury - should limit their consumption of fish that are low in mercury to two servings (12 ounces) per week, and to avoid four species it deemed to be high in mercury: shark, tilefish, swordfish and king mackerel.

But often lost in the fray of NGO campaigns and media coverage about mercury is the message of seafood's overwhelming health benefits. The Center for Consumer Freedom in Washington, D.C., recently conducted a poll of 1,011 consumers in which 61 percent believed that at least 1,000 "childhood cases of mercury poisoning from eating fish" are reported each year. The real number is zero, CCF says, citing volumes of scientific evidence. Removing or reducing seafood from our diets, many scientists say, causes more problems than it prevents.

Selenium is a recent addition to the mercury debate, and one that your customers may be asking about. The information below can help you answer their questions.


Q. What is selenium and what foods are a good source?

Selenium is a trace mineral considered essential to good health, but only in small amounts. It is incorporated into proteins to make selenoproteins, which are important antioxidant enzymes that help prevent cellular damage from natural byproducts that contribute to chronic diseases. Selenoproteins also help regulate thyroid function and boost the immune system.

Plants are abundant sources of selenium, as are nuts, some meats and seafood. According to the National Institutes of Health, 3 ounces of canned light tuna contain about 63 micrograms of selenium, or about 95 percent of the daily value.

Of the 1,100 foods the U.S. De­partment of Agriculture ranked for selenium content, 16 of the top 25 were marine fishes, most notably swordfish (65.4 micrograms per 4-ounce serving), a species the U.S. Food and Drug Administration says to avoid due to high mercury content.


Q. Does the selenium in seafood counteract the effects of methylmercury?

Most commercially caught fish contain five to 20 moles (the number of atoms in an amount of matter) of selenium for every one mole of methylmercury, and one mole of selenium can protect against 75 moles of methylmercury.

Studies have shown that selenium has a high "binding affinity" for mercury. When the two elements coexist, they tend to connect and form a new substance. If the selenium content is higher, it renders mercury harmless to brain cells, says Nicholas Ralston, Ph.D., at the University of North Dakota's Energy and Environmental Re­search Center.

Conversely, excess mercury may effectively sequester selenium, Ralston adds. Numerous studies indicate that selenium protects against methylmercury exposure, but when too much methylmercury is in the body, it prevents selenium from performing its neurological functions.

Dr. Laura Raymond, a biochemist and colleague of Ralston's at UND, says studies about mercury in seafood need to take the amount of selenium into account as well.

"The question is, 'Is the selenium in the fish the reason we're seeing different findings about mercury in recent major studies?' Now we have to reassess it," says Raymond.

"Our research shows that, considering the amount of selenium in ocean fish compared to the amount of mercury, selenium will protect against the toxic risk of mercury."

How and why the two elements are attracted to each other is still being explored. Whether selenium acts as a "magnet" for mercury or whether any mercury-related health impacts coincide with brain cells' depleted levels of selenium is a focus of scientific debate. Further, some question whether mercury from seafood consumption is actually harmful to human health.

At the Mercury 2006 conference, Dr. Walter Willett, a nutrition professor at Harvard University, said, "[Seafood's health benefits] are likely to be at least 100-fold greater than the estimates of harm, which may not exist at all."

Dr. Gina Solomon, senior scientist for the Natural Resources Defense Council, said, "Even the higher-mercury-containing fish, if they are not eaten frequently, are not a big concern."

However, the official panel of 37 scientists at Mercury 2006 signed the Conference Declaration, which stated, "There is no evidence that selenium in the diet protects people from the neurological and developmental effects of methylmercury."


Q. What species have 
the most selenium?

A number of freshwater species, especially walleye pike, are high in selenium, but the levels vary greatly from lake to lake; so do the levels of methylmercury. Raymond says that all marine species contain selenium; for example, halibut has 74.4 micrograms per 6 ounces, and sockeye salmon has 58.6 micrograms.

The USDA also touts shellfish, such as oysters (56 micrograms per 3 ounces), clams (41 micrograms), lobsters (36 micrograms), shrimp (35 micrograms), crabs (34 micrograms) and scallops (25 micrograms), as rich sources of selenium.


Q. What are selenium's 
other health benefits?

A recent study by researchers at the University of North Carolina at Chapel Hill found selenium may help fight colorectal cancer. In a test of 803 colonoscopy patients, those with highest levels of selenium in their blood had a 40 percent lower risk of colorectal adenomas, which are polyps that may turn into cancer.

Selenium intake is also linked to lowering the risk of heart disease and relieving the symptoms of rheumatoid arthritis.


Q . Can you get too 
much selenium?

According to the National Institute of Health, high blood levels of selenium (greater than 100 micrograms per deciliter) can result in selenosis, although rarely. Symptoms include hair loss, gastrointestinal upset, fatigue, 
irritability and mild nerve damage.


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