ACS Chemistry for Life

Selenium is one of the micronutrients that humans require in their diets to maintain good health.  It is possible to get too much of a good thing, however, and selenium is toxic in large doses.

Because selenium is under sulfur in the same group on the periodic table, the two elements share many properties. Two amino acids, selenocysteine and selenomethionine, contain selenium substituted for the usual sulfur, and these amino acids are used in the formation of a small class of selenoproteins that have functions such as antioxidant enzymes.  Although many plants do not specifically need selenium, if the mineral is present in the soil, the plants will take up small amounts and somewhat randomly substitute it for sulfur. 

Deficiency of selenium is unusual in the United States, even though not all regions have soil that is rich in the element. Large regional and national food distribution networks result in people eating produce from outside their local areas, which allows people to get the necessary amount of this important nutrient. People living in areas of China and Russia that have low selenium content in the soil run a much higher risk of selenium deficiency since their food tends to be grown and consumed locally. An inadequate supply of selenium rarely results in a specific physiological condition, but it has the overall impact of weakening an individual’s further resistance to other diseases. Deficiencies in selenium also exacerbate the impact of iodine deficiencies since both of these nutrients are required for a healthy thyroid.

The best sources of selenium are seafood, organ meats, and muscle meats. Brazil nuts are actually even better sources of the micronutrient, but their selenium content varies widely based on the soil composition where they are grown. 

More information on this nutrient is available at http://lpi.oregonstate.edu/infocenter/minerals/selenium/ and http://ods.od.nih.gov/factsheets/selenium/