Q.
What is selenium?
Selenium is a trace element. Trace (or minor) elements
are rare in nature, occurring in only 'trace' quantities.
Other examples are antimony, arsenic, cadmium, chromium, molybdenum,
thallium, uranium, and vanadium. While many are essential or beneficial
for human health, in too high of a concentration they can become
toxic. Selenium is a trace element of particular concern.
Selenium has an atomic
number of 34 and atomic weight of 78. Because of its similar electrostatic
balance and ionic radius, selenium behaves like sulfur and can substitute
for it in many molecules. It was discovered in 1817 by Jons Berzelius:
its name refers to the Greek 'Selene', moon.
In human terms, selenium
is found in compounds that are essential for human nutrition
and growth. Too much selenium, however, may lead to selenosis, the
body's response to toxic doses.
Q.
Is too much (or too little) selenium unhealthy?
Indications of selenium
deficiency have not been fully described in humans, but very low
levels of selenium are associated with Keshan Disease, a form of
juvenile cardiomyopathy (a disease of the heart muscle), and Kaschin-Beck Disease, a form of chondrodystrophy (a type of abnormal bone growth).
Both diseases have been recognized in rural China where levels of
selenium in the soil are abnormally low.
Chronic selenium toxicity
(too much selenium) is recognized by skin lesions, hair loss, or abnormalities of the
skin and nails. Certain rural areas of China also have abnormally
high levels of selenium in soils. Mostly inorganic forms of selenium
are found in soils, but some plants such as Astragalus (milk vetch) convert
these compounds to organic forms that can enter the food chain.
Selenomethionine is the principal selenium compound found in foods.
Selenium is relatively
rare in low-protein foods, but it is found in grains, fish, meat,
Brazil nuts, and brewer's yeast. People who eat a balanced diet
of food grown in the western United States probably get enough selenium
from food.
Selenium compounds can
be harmful at daily dietary levels 5-10 times higher than the daily
requirement. Accidentally swallowing a large amount of selenium
(for example, a very large quantity of selenium supplement pills)
could be life-threatening without immediate medical treatment.
People exposed to very
high levels of selenium in the atmosphere have reported dizziness,
fatigue, irritation, collection of fluid in the lungs, and severe
bronchitis. The exact levels at which these effects occur are not
known. Upon contact with skin, selenium compounds have caused rashes,
swelling, and pain.
Q.
What is the effect of too much selenium on wildlife/animals?
In animals toxic levels
may lead to death or birth deformities in offspring. In fish and birds, in addition to overt deformities, too much selenium can lead to reproductive failure and Cachexia -- starvation and wasting even though there is enough food. Note that because of bioaccumulation, the levels for protection of wildlife (5 ppb) are ten times lower than levels to protect humans (50 ppb).
Q.
Why does selenium cause deformities?
Selenium may substitute for sulfur in the amino acids that make up proteins, and because selenium has a smaller
ionic radius than sulfur, the proteins essential for transmitting
information for growth and development become malformed. This is different from genetic mutation.
Q.
Does
selenium have therapeutic uses?
Medical research studies
have claimed benefits including boosted immune function, male fertility,
anti-cancer effects (prostate, colorectal, and lung only), anti-oxidant,
treatment of psoriasis, and, when combined with other ingredients,
Alzheimer's Disease.
Selenium sulfide is an
ingredient in anti-dandruff shampoo. Selenium
sulfide has not caused cancer in animals when it is placed on the
skin, and the use of anti-dandruff shampoos containing selenium
sulfide is considered safe. The
Department of Health and Human Services (DHHS), however, has determined
that selenium sulfide is reasonably anticipated to be a carcinogen.
The compound has produced liver tumors in rats and mice and lung
tumors in mice fed daily at very high levels. The EPA believes that
other selenium compounds are not classifiable with regard to their
carcinogenicity. Studies of laboratory animals and people show that
most selenium compounds probably do not cause cancer.
Q.
Are there other uses of selenium?
Other uses include glass
manufacturing, chemicals and pigments, agriculture, metallurgy,
and electronics.
Q.
What are our sources of information?
The Selenium Forum
U.S. Department of Agriculture
U.S. Department of Health
and Human Services, Public Health Service, Agency for Toxic Substances
and Disease Registry
U.S. Environmental
Protection Agency
See the selenium links
page for additional sources information.
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