Dietary sources of selenium may contain organic or inorganic selenium. Organic forms include selenomethionine and selenocysteine, while inorganic forms include selenates and selenites. In plant food, organic selenium arises from the inorganic selenium compounds in soil. Plants which accumulate selenium include alfalfa, cruciferous species such as cabbage, broccoli and cauliflower, as well as yeasts.
Most selenium in animal tissue is in the form selenomethionine, mostly stored in the skeletal muscle, which has anywhere from 28 to 46% of the body’s selenium. Seafoods and organ meats are the foods that contain the highest concentrations of selenium, while other good sources include:
Plant foods show a wider variation in selenium content than animal foods because of:
Elemental selenium is found in the form of metallic crystals, which are gray to black, it is mostly insoluble, and does not enter the food chain readily. However, natural selenium is often combined with sulfides, copper, silver, nickel and lead. In addition, selenium forms white or colorless crystals in combination with oxygen.
Selenium in water may result from the weathering of rocks, Selenium absorption by plants is highest when the soil is alkaline and when the selenium is in the form of inorganic compounds which dissolve readily in water and do not adsorb to soil particles.
Selenium may also enter the water cycle through irrigation runoff, and enter the food chain from aquatic organisms, becoming concentrated from level to level of the food web. Elemental selenium is found deep in the soil, but it can be oxidized at the soil surface to form selenium dioxide.
Rock weathering also leads to the release of selenium dust into the air. The latter may also accompany volcanic eruptions, or fossil fuel combustion. Gaseous selenium compounds include hydrogen selenide gas.