Selenium is an essential micronutrient although human body only need a small amount. The essentiality of selenium is due to the fact that selenium has to be obtained externally for synthesis of certain proteins  containing selenium. Selenium is covalently attached to protein via amino acid cysteine. In the human genome, approximately 30 genes for selenoprotein have been identified. Not all of them are well characterized for their biological functions. The most well characterized and studied selenoproteins are the antioxidant enzymes: five glutathione peroxidase, three thioredoxin reductases and enzymes important in the activation and deactivation of thyroid hormones – three forms of iodothyronine deiodinases. The long term health implications in selenium supplementation have not yet been thoroughly examined. The implicit importance of selenium to human health is recognized unambiguously. Another form of selenium that is beneficial to the body is selenomethionine where it can be incorporated nonspecifically with the amino acid methionine in protein. Non-specificity suggests that Se-Met are not always the active site for enzyme or protein activity as Se-Cys does in the selenoproteins.

Antioxidant enzymes glutathione peroxidases, thioredoxin reductases and iodothyronine deiodinases are involved in redox reactions and Se-Cys (selenium-cysteine) is an active-site amino acid residue required for the catalytic activity of these enzymes. Both enzymes (glutathione peroxidases, thioredoxin reductases) are important in clean up free radicals in the human body and are part of the natural antioxidant defense system. Free radicals are widespread threats to the human health, because the free radicals can damage a range of bio-macromolecules including various proteins , DNA (genes), cellular membrane. Free radicals also cause unwanted inflammation. All antioxidants are also anti-inflammatory but not vice versa. (see post “free radical theory of aging”).

Selenium deficiency is uncommon. Selenium deficiency is generally caused by the deficiency of selenium in the soil where plants and foods are grown. But if the body experiences selenium deficiency, a variety of health problems can develop including decreased immune function, infection, thyroid disorder (hypothyroidism), keshan disease, cognitive decline, increased cancer and cardiovascular risk, skin/hair problem. Selenoproteins are believed to be closely linked with cancer and carcinogenesis because numerous research have revealed the inverse correlation between selenium supplementation and cancer risk. Other secondary symptoms associated with selenium deficiency are general fatigue, mental fatigue among other findings.

The selenium deficiency can be measured from blood samples. Concentration in blood reflect short-term selenium supplementation. Longer-term selenium status in the body can be analyzed from hair. Quantification of selenoproteins is also used for measure of selenium sufficiency. Generally speaking, in healthy people 8 mcg/dL is typically marked as sufficient for selenoprotein synthesis. The recommended RDA (recommended dietary allowances) is 55 mcg for adults 20-50+. And the safety threshold amount is not to exceed 400 mcg. Excess selenium is toxic. The condition result from selenium toxicity is known as selenosis. Clinical signs of chronic selenosis include hair/skin problems, nausea, fatigue, irritability, lesion of nervous system. Acute selenosis can cause severe gastrointestinal and neurological symptoms, acute respiratory distress syndrome, myocardial infarction and etc. Food sources of selenium are those grown in the soil abundant in selenium such as brazil nuts. Other dietary source of selenium include whole grains and dairy products, lean meats, poultry, beans, nuts, seafood.