Selenium is a trace element that is essential in small amounts; but, like all essential elements, it is toxic at high levels. Humans and animals require selenium for the function of a number of selenium-dependent enzymes, also known as selenoproteins.1 These include glutathione peroxidase (GPx), an antioxidant enzyme that contains the powerful antioxidant peptide glutathione (GSH);2,3 thioredoxin reductase, which helps regenerate several antioxidants, and regulates cell growth and viability;4 iodothyronine deiodinases, which help produce biologically active thyroid hormone;5 and a number of other selenoproteins.6
Selenium has many benefits to the body, and supplementation has been shown to particularly promote immune response. Small studies showed healthy individuals7,8 and those with suboptimal immune function9 who supplemented with 200 mcg/d of selenium as sodium selenite for eight weeks showed an enhanced immune cell response to foreign antigens compared with those taking a placebo. A considerable amount of basic research also indicates selenium plays a role in regulating the expression of cell-signaling molecules called cytokines, which orchestrate the immune response.10
The antioxidant and immune-bolstering effects may also help fight mutagenicity. A case-control study within a prospective study of more than 50,000 male health professionals in the United States found a significant inverse relationship between toenail selenium content (a measure of selenium status in the body) and the incidence of mutated prostate cell development.11 Individuals whose toenail selenium content was consistent with an average dietary intake of 159 mcg/d of selenium had a 65-percent lower incidence of risk of mutated prostate cell development, compared to those with toenail selenium content consistent with an average intake of 86 mcg/d. Similar results have been seen in studies examining selenium intake and the incidence of mutated cells from the lung12,13 and liver.14