Selenium is an important trace mineral in human nutrition. An outright deficiency of selenium leads to the formation of Kashin-Beck syndrome and Keshan disease, but such deficiency diseases are rare worldwide. However, chronic low intakes of selenium, above the threshold for outright deficiency, but below the optimal intake, leads to long term metabolic changes that can include a downregulation of cellular antioxidant defences. This relates to the role of selenium as a cofactor in the glutathione peroxidase enzyme, an enzyme that is pivotal to antioxidant protection of cells. Low intakes of selenium have been shown to increase free radical generation, and this may increase the risk of cancer significantly. Obtaining adequate selenium is therefore important in order to maintain the function of the glutathione peroxidase enzyme, and the health of the individual, and a daily intake of selenium must be obtained. However, the exact amount of selenium required will depend on the form of the mineral that is ingested.
Selenium is much more readily absorbed as organic compounds such as selenomethionine and selenocysteine, when compared to inorganic forms such as selenate and selenite. Organic forms may also be retained by the body for longer.
Selenium bioavailability from both supplements and food depends on the chemical form of the selenium bound within the tablet or food matrix. Some forms of selenium are more absorbable than others. A number of different organic and inorganic selenium compounds have been identified in plants including selenate, selenite, selenocystine, selenomethionine, selenohomocysteine, seleno-methylselenocysteine, γ-glutamil-selenocystathionine, selenomethionine selenoxide, γ-glutamyl selenomethylselenocysteine, selenocysteineselenic acid, seproponylselenocysteine selenoxide, selenomethylselenomethionine, selenocystathionine, dimethyl diselenide, selenosinigrin, selenopeptide and selenowax. In animals, selenium compounds identified include selenocysteine, selenotrisulfides of cystine, selenate and selenite. Selenate and selenite are inorganic forms of selenium, and these may not be as bioavailable as the forms of selenium that are bound to organic molecules.
Selenium interacts with a number of other metal ions and heavy metals in human nutrition. In particular, selenium may decrease the bioavailability and absorption of mercury. Such antagonism between metals is common, and in many cases low intakes of essential minerals increases the risk of heavy metal toxicity.
About 80 % of dietary selenium has been estimated to be absorbed, although many factors such as the exact makeup of the diet can affect this figure. Selenium shows a number of interactions with other minerals. Certain antagonistic, synergistic and additive interactions between selenium and other metals are known, and these can include interactions with heavy metals. For example, in vitro experimentation suggests that selenium can antagonise the absorption of mercury from fish, and in this way the health benefits of selenium may extend to an inhibitory effect on mercury toxicity. However, reciprocally, although fish such as tuna is a rich source of selenium, the high mercury content can limit the bioavailability of the selenium contained within the flesh. Both wheat and meat are therefore considered the most important sources of selenium worldwide, with selenium in plants coming originally from the soil and the selenium in meat coming originally from the animal feed, which is often supplemented with selenium.
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