There is an optimal range of selenium concentration in animals (or broadly speaking, in almost every selenium-dependent lifeform). Too much results in toxicity, too little results in deficiency.
Toxicity Although selenium is an essential
trace element, it is toxic if taken in excess. Exceeding the
Tolerable Upper Intake Level of 400 micrograms per day can lead to selenosis. This 400 microgram (
μg) Tolerable Upper Intake Level is based primarily on a 1986 study of five Chinese patients who exhibited overt signs of selenosis and a follow-up study on the same five people in 1992. The 1992 study actually found the maximum safe dietary Se intake to be approximately 800 micrograms per day (15 micrograms per kilogram body weight), but suggested 400 micrograms per day to not only avoid
toxicity, but also to avoid creating an imbalance of nutrients in the diet and to account for data from other countries. In China, people who ingested corn grown in extremely selenium-rich stony coal (carbonaceous
shale) have suffered from selenium toxicity. This coal was shown to have selenium content as high as 9.1%, the highest concentration in coal ever recorded in literature. Symptoms of selenosis include a garlic odor on the breath, gastrointestinal disorders, hair loss, sloughing of nails, fatigue, irritability, and neurological damage. Extreme cases of selenosis can result in
cirrhosis of the liver,
pulmonary edema, and death. Elemental selenium and most metallic
selenides have relatively low toxicities because of their low
bioavailability. By contrast,
selenates and
selenites are very toxic, having an oxidant mode of action similar to that of arsenic trioxide. The chronic toxic dose of selenite for humans is about 2400 to 3000 micrograms of selenium per day for a long time.
Hydrogen selenide is an extremely toxic, corrosive gas. Selenium also occurs in organic compounds, such as dimethyl selenide,
selenomethionine,
selenocysteine and
methylselenocysteine, all of which have high
bioavailability and are toxic in large doses. The
Occupational Safety and Health Administration (OSHA) has set the legal limit (
permissible exposure limit) for selenium in the workplace at 0.2 mg/m3 over an 8-hour workday. The
National Institute for Occupational Safety and Health (NIOSH) has set a
Recommended exposure limit (REL) of 0.2 mg/m3 over an 8-hour workday. At levels of 1 mg/m3, selenium is
immediately dangerous to life and health.
Selenium pollution Selenium pollution of water systems may result whenever new agricultural runoff courses through normally dry, undeveloped lands. This process leaches natural soluble selenium compounds (such as selenates) into the water, which may then be concentrated in new "wetlands" as the water evaporates. High selenium levels produced in this fashion have been found to have caused certain congenital disorders in wetland birds.
Notable cases of toxicity On 19 April 2009, 21
polo ponies died shortly before a match in the United States Polo Open. Three days later, a pharmacy released a statement explaining that the horses had received an incorrect dose of one of the ingredients used in a vitamin/mineral supplement compound that had been incorrectly prepared by a
compounding pharmacy. Analysis of blood levels of
inorganic compounds in the supplement indicated the selenium concentrations were 10 to 15 times higher than normal in the
blood samples and 15 to 20 times higher than normal in the liver samples. Selenium was later confirmed to be the toxic factor.
Deficiency Selenium is essential for human reproduction and growth. Moderate selenium deficiency is linked to muscle weakness and
muscle diseases. Mental health effects include low mood, confusion, and anxiety. Selenium interacts with other nutrients, such as
iodide and
vitamin E. The interaction is observed in the
etiology of many deficiency diseases in animals, and pure selenium deficiency is rare. The effect of selenium deficiency on health remains uncertain, particularly in relation to
Kashin-Beck disease. In the regions (e.g., regions within North America) where low selenium soil levels lead to low concentrations in the plants, some animal species may be deficient unless selenium is supplemented with diet or injection.
Ruminants are particularly susceptible. In general, absorption of dietary selenium is lower in ruminants than in other animals and is lower in forages than in grain. Ruminants grazing certain forages, e.g., some
white clover varieties containing
cyanogenic glycosides, may have higher selenium requirements, and glutathione peroxidases are deactivated by the cyanide acting on the glutathione
moiety. Neonate ruminants at risk of
white muscle disease may be administered both selenium and vitamin E by injection; some of the WMD
myopathies respond only to selenium, some only to vitamin E, and some to either.
In wild animals In fish and other wildlife, selenium is necessary for life but toxic in high doses. For
salmon, the optimal selenium concentration is about 1 microgram selenium per gram of whole body weight. Much below that level, young salmon die from deficiency; much above, they die from toxic excess.
In model organisms The
E. coli bacterium and
S. cerevisiae yeast are two model organisms commonly used for biological study. They are also used as
expression systems for producing selenium-substituted proteins, which are used in a form of
X-ray crystallography called
single- or
multi-wavelength anomalous diffraction to determine the
tertiary structure of a protein. For these organisms to produce substituted proteins, they are grown in high-selenium environments, which also results in signs of toxicity. Part of this toxicity is due to the random substitution of methionine and cysteine in proteins with the selenium-containing version. Although this is the precise goal of this kind of protein production, alternation of the organism's own proteins in this way can render them nonfunctional. == Interaction with other nutrients ==