Selenium is an important trace mineral incorporated into a number of important selenoproteins. These selenoproteins have important biological functions and include glutathione peroxidase, thioredoxin reductase, iodothyronine deiodinase and selenoprotein-P. In order for selenium compounds to be of use to the body it must first be absorbed and distributed to the tissues. Evidence from scientific research suggests that generally, dietary selenium is highly bioavailable (> 90 % selenomethionine absorption, ~ 100 % selenate absorption, > 50 % of selenite absorption). The selenium content of both plant and animal foodstuff varies depending on the extent of the selenium to which the species is exposed. Plants grown in high selenium soil tend to have high selenium contents and animals fed plants containing high selenium contents tend to have high selenium content to their meat.
Cereals are a good source of selenium if grown in rich soil and tend to contain selenomethionine in concentrations that vary from 0.01 to 0.55 µg/g fresh weight. Brazil nuts and some allium and brassica vegetables are able to accumulate selenium and are therefore also a rich source. However, there will still be variation between plants based on geographical location and environmental conditions. Brazil nuts contain high levels of selenium because the proteins in the nuts are high in the sulphur containing amino acids and selenomethionine can non-specifically replace methionine. Non-selenium accumulating plants tend to contain mainly selenate, selenomethionine and selenocysteine, but selenium accumulating plants tend to contain mainly γ-glutamyl methylselenocysteine. In animals, the major forms of selenium are selenomethionine and selenocysteine which are found mainly in the muscle tissue.
Selenium shows differences between the absorption of organic and inorganic forms in supplements. Generally selenium as selenite or selenate is very well absorbed but appears to be less well retained, compared with selenomethionine and selenocysteine. In animals, supplementation with organic forms of selenium results in meat with higher selenium content compared to inorganic selenium. Once absorbed, selenate, selenite, selenomethionine and selenocysteine enter the selenide pool and from here the selenium is used to make the various selenoproteins. If the selenium is not needed it is excreted as a selenosugar. Selenomethionine can be incorporated non-specifically into proteins in place of methionine. The γ-glutamyl methylselenocysteine found in brassica and allium vegetables is metabolised to se-methylselenocysteine and then converted into methylselenol by the enzyme β-lyase. Methylselenol is them excreted in the breath or urine or may enter the selenide pool for incorporation into selenoproteins.
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