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. 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
The richest food sources of selenium are organ meats and seafood, followed by muscle meats. In general, there is wide variation in the selenium content of plants and grains because plants do not appear to require selenium. Thus, the incorporation of selenium into plant proteins is dependent only on soil selenium content. Brazil nuts grown in areas of Brazil with selenium-rich soil may provide more than 100 mcg of selenium in one nut, while those grown in selenium-poor soil may provide ten times less .
In the U.S., grains are a good source of selenium, but fruits and vegetables tend to be relatively poor sources of selenium. In general, drinking water is not a significant source of selenium in North America. The average dietary intake of adults in the U.S. has been found to range from about 80 to 110 mcg/day. Because of food distribution patterns in the U.S., people living in areas with low soil selenium avoid deficiency because they eat foods produced in areas with higher soil selenium . The table below lists some good food sources of selenium and their selenium content in micrograms (mcg). For more information on the nutrient content of specific foods, search the USDA food composition database.