Selenium is best known antioxidant & catalytic for the production of active thyroid hormone, antibody production, sperm motility & may reduce the risk of miscarriage. An elevated selenium intake may be associated with reduce cancer risk.

Fats, carbohydrates & protein are macronutrients, Vitamin & minerals are micronutrients, selenium is one of the most important mineral required for the various important functions of the vital organs of the body
First report of selenium deficiency was appeared in 1961, & similar report in 1967, selenium administration to children with Kwashiorkor resulted is significant weight increase. Selenium deficiency may occur in Protein Energy malnutrition. Selenium is required for the proper functioning of the immune system & appears to be key nutrients in counteracting the development of virulence & inhibiting HIV progress to AIDS.  Its deficiency especially when combined with Vit-E deficiency reduces antibody production.
Sources of selenium are seafood, muscle meat, & cereals. Selenium levels of drinking water vary greatly in different geographical area and are usually much less than the guide line value of 0.01 mg. per liter. Food stuffs are the principal source and level depends according to geographical area of production. The selenium content of cereals is determined by the soil concentration, countries with low soil concentration include some part of China & New- Zealand. North American soil has higher selenium content than European and Asian soil & decreasing reliance of Europe on imported American food in recent decades has increased the prevalence of selenium deficiency.
  Selenium is an essential element for human and forms an integral part of enzyme glutathione peroxides which helps to protect free radicals damage of cell and another important hemeprotein T4-Monodeiodinase which catalyzes the oxidation of the iodide radical to iodine with the subsequent iodination of many organic compounds, particularly proteins and convert thyroxine to tri iodothyroxine. Deficiency of the selenium can cause hypothyroidism, cardiomyopathy in children & myopathy in adults. Most selenium compounds are water soluble. The toxicity of long term exposure is manifested in nails, hair & liver.
      Selenosysteine this rare amino acid residue is introduced during protein synthesis rather than created through post synthetic modification it contains selenium rather than sulphur of cysteine.
Recommended dose of selenium in children is 25 microgram/day, in male & female 55 microgram/day, in pregnancy 60 microgram/day & in lactation 70 microgram/day.
         Thus selenium is best known antioxidant & catalytic for the production of active thyroid hormone, antibody production, sperm motility & may reduce the risk of miscarriage. An elevated selenium intake may be associated with reduce cancer risk.