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.
Most selenium is in the form of selenomethionine in 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.
Selenium exists in two forms: inorganic (selenate and selenite) and organic (selenomethionine and selenocysteine). Both forms can be good dietary sources of selenium. Soils contain inorganic selenites and selenates that plants accumulate and convert to organic forms, mostly selenocysteine and selenomethionine and their methylated derivatives.
Most Americans consume adequate amounts of selenium. According to an analysis of data from the 2009–2010 National Health and Nutrition Examination Survey (NHANES), the average daily selenium intake in Americans aged 2 years and older from foods is 108.5 mcg and from both foods and supplements is 120.8 mcg. Adult men have higher daily intakes (134 mcg from foods and 151 mcg from foods and supplements) than adult women (93 mcg from foods and 108 mcg from foods and supplements). In the United States, 18% to 19% of adults and children use a dietary supplement containing selenium.
According to an analysis of NHANES data from 2003–2004, the mean serum selenium concentration in U.S. adults aged 40 years or older is 13.67 mcg/dL. Men have slightly higher serum selenium levels than women and whites have higher levels than African Americans.
Selenium intakes and serum concentrations in the United States and Canada vary somewhat by region because of differences in the amounts of selenium in soil and in local foods consumed. For example, concentrations are higher in residents of the Midwestern and Western United States than in the South and Northeast. The extensive transport of food typically allows people living in low-selenium areas to obtain sufficient amounts of selenium.