(This web page is a collection/summary of the works of several authors, referenced at the bottom of the page.)
BACKGROUND & METABOLISM
Selenium was discovered as an essential nutrient in 1957, though it was not discovered what role it played in the body until 1973. The discovery of Se in glutathione peroxidase was the key to understanding its importance in nutrition and health. Glutathione peroxidase, or GSH-Px, is essential for protecting cellular membranes from being destroyed.
Compounds called free radicals are highly reactive molecules, and if left unchecked will destroy cellular membranes. Vitamin E and GSH-Px are two molecules that help prevent this damage. Vitamin E prevents the dangerous molecules (peroxides) from being formed, but even with adequate vitamin E, some peroxides evade destruction. GSH-Px destroys the peroxides before they have a chance to cause membrane damage. GSH-Px concentration and activity is directly related to the selenium status of the animal. Selenium and vitamin E are both antioxidants because they both protect the membranes from oxidative damage. Due to this shared duty, there is a relationship between the compounds, in which one can substitute for the other in a very small way. For instance, more Se is needed when an animal's vitamin E concentrations are low. The sparing effect is an extension of this idea of substitution. Selenium spares vitamin E by:
- preserving pancreas integrity for normal fat digestion, thus normal vitamin E absorption
- reducing the amount of vitamin E needed to maintain lipid membranes via GSH-Px
- aiding in the retention of vitamin E in the blood
Vitamin E spares Se by:
- maintaining body Se in an active form and prevents loss from the body
- preventing destruction of membrane lipids from within the membrane, which inhibits the production of hydroperoxides and decreases the amount of GSH-Px needed
Selenium has also been recently found in another enzyme, 5'-deiodinase. 5'-deiodinase is an enzyme that catalyzes the reaction of the inactive form of thyroxine to the active form. Thyroxine is a very important hormone from the thyroid that helps in regulating body temperature, metabolism, reproduction, circulation, and muscle function. It is known that Se protects the body from heavy metals such as cadmium, mercury, and silver by forming unreactive complexes with them. There are theories that Se may be involved in many other functions in the body, such as-
- a selenoprotein in sperm
- in RNA
- role in prostaglandin synthesis
- role in essential fatty acid metabolism
- required for normal immune response
Elemental selenium (Se (0))can be reacted upon in several ways: it can be reduced to a Se(-2), called selenide, or it can be oxidized to a (+4) state, selenite, or a (+6) state, selenate. Selenium is very similar to sulfur in its chemical properties; it is therefore, not surprising that the main form of organic Se in the body is as selenomethionine and selenocystine. Methionine and cystine are sulfur-containing amino acids, the Se can replace the sulfur because of its chemical similarities to sulfur.
There is not a lot of information on the absorption and pathway of Se from the gastrointestinal tract. It is known that it is absorbed mostly from the upper small intestine; there is no absorption from the stomach, rumen, or abomasum. The amount absorbed depends on the chemical form in which it is ingested. There does not seem to be any feedback loop to reduce the amount of Se absorbed; it has been shown, in rats, that 95% of dietary Se was absorbed regardless if fed deficient or toxic amounts. Absorbed Se travels in the plasma on a protein to its destination tissue. Tissue concentrations vary, the kidneys retain a large amount of Se, along with cardiac and skeletal muscle, and the liver. It is deposited more readily when it is in an organic form. Selenium is readily transferable through the placenta, the mammary barrier, and from hen to egg, so the animal's status will affect offspring and milk concentrations. The primary routes of excretion are through the urine and the feces, exhalation of Se only occurs in cases of toxicity. It has been found that the microorganisms in the rumen may convert Se into insoluble compounds, causing the ruminant animal to absorb less than its monogastric counterpart. It has also been suggested that more Se is absorbed when administered with a high-protein diet, the reasons have not been confirmed.