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Seafood FAQ: Is selenium a mercury antidote?
Scientists say seafood may contain a natural neutralizer of this toxin
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.