Selenium is a trace element that is essential in small amounts but can be toxic in larger amounts. Humans and animals require selenium for the function of a number of selenium-dependent enzymes, also known as selenoproteins. During selenoprotein synthesis, selenocysteine is incorporated into a very specific location in the amino acid sequence in order to form a functional protein. Unlike animals, plants do not appear to require selenium for survival. However, when selenium is present in the soil, plants incorporate it non-specifically into compounds that usually contain sulfur.
It is involved in the defense against the toxicity of reactive oxygen species, in the regulation of thyroid hormone metabolism and the regulation of the redox state of cells. Recognition of the vital importance of selenium in human and animal nutrition was long impeded by its very real toxic potential and by fears that selenium might be carcinogenic, fears that have now been largely displaced by some evidence suggesting just the opposite--that selenium may provide protection against some cancers.
The amount of selenium in food is a function of the selenium content of the soil. Selenium enters the food chain through incorporation into plant proteins as the amino acids L-selenocysteine and L-selenomethionine. Selenium, like most trace elements and minerals, is not evenly distributed in the world's soil.
Selenium is an essential trace mineral that works closely with vitamin E to provide the body with powerful antioxidant protection from the free radicals that may contribute to the development of cancer, heart disease and arthritis.
Dietary selenium comes from cereals, meat, fish, and eggs. The recommended dietary allowance for adults is 55 micrograms per day. Liver and Brazil nuts are particularly rich sources of Selenium.