Selenium

Selenium is a trace element that is naturally present in many foods, added to others, and available as a dietary supplement. Selenium, which is nutritionally essential for humans, is a constituent of more than two dozen selenoproteins that play critical roles in reproduction, thyroid hormone metabolism, DNA synthesis, and protection from oxidative damage and infection [1].

Selenium exists in two forms: inorganic (selenate and selenite) and organic (selenomethionine and selenocysteine) [2]. Both forms can be good dietary sources of selenium [3]. Soils contain inorganic selenites and selenates that plants accumulate and convert to organic forms, mostly selenocysteine and selenomethionine and their methylated derivatives.

Most selenium is in the form of selenomethionine in animal and human tissues, where it can be incorporated nonspecifically with the amino acid methionine in body proteins. Skeletal muscle is the major site of selenium storage, accounting for approximately 28% to 46% of the total selenium pool [3]. Both selenocysteine and selenite are reduced to generate hydrogen selenide, which in turn is converted to selenophosphate for selenoprotein biosynthesis [4].

The most commonly used measures of selenium status are plasma and serum selenium concentrations [1]. Concentrations in blood and urine reflect recent selenium intake. Analyses of hair or nail selenium content can be used to monitor longer-term intakes over months or years. Quantification of one or more selenoproteins (such as glutathione peroxidase and selenoprotein P) is also used as a functional measure of selenium status [3]. Plasma or serum selenium concentrations of 8 micrograms (mcg)/dL or higher in healthy people typically meet needs for selenoprotein synthesis [5].