As an essential cofactor of glutathione peroxidase, selenium is an important antioxidant. Selenium is incorporated into proteins to make selenoproteins, which are important antioxidant enzymes. The antioxidant properties of selenoproteins help block cellular damage from free radicals. High doses of vitamin C (over one gram) may enhance the absorption of selenium. This mineral is best taken in conjunction with antioxidant supplements, such as vitamins C or E.

Se-Methyl L-Selenocysteine (SeMC) is a naturally occurring seleno-amino acid that is synthesized by plants such as garlic and broccoli. SeMC is an organoselenium compound and also more bioabsorbable than other forms. Unlike selenomethionine which is indiscriminately incorporated into proteins in place of methionine, SeMC is selectively incorporated into seleno-containing enzymes, i.e. selenoproteins, such as glutathione peroxidase.