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.

Selenium's primary function is that it inhibits the oxidation of fats. Combined with vitamin E, selenium is a powerful antioxidant. Selenium and vitamin E have been shown to act synergistically in producing antibodies and in helping to maintain a healthy heart and liver.
    As an antioxidant, it protects the immune system by inhibiting the formation of free radicals, which can cause damage to the body. It also has been shown to confer a protective effect against certain forms of cancer.

Adults should have 55 micrograms every day, pregnant women should have 65 micrograms.

Intake:

Recommended Dietary Allowance (RDA) for Selenium
Babies 0-6 months	15 mcg/day
Babies 7-12 months	20 mcg/day
Children 1 to 3 years	20 mcg/day
Children 4 to 8 years	30 mcg/day
Children 9 to 13 years	40 mcg/day
Teenagers 14 to 18	50 mcg/day
Adults	55 mcg/day
Pregnant women	60 mcg/day
Breastfeeding	70 mcg/day

Selenium appears to help preserve tissue elasticity by delaying the oxidation of polyunsaturated fatty acids.
    It aids in the production of prostaglandins, substances that affect blood pressure. A prostaglandin deficiency can result in a deficiency in other substances, which helps to keep the arteries free from platelet aggregation.
    Combined with vitamin E and zinc, selenium can help reduce an enlarged prostate. In addition, selenium supplementation has been shown to be helpful in treating people with alcoholic cirrhosis of the liver, cancer, heart disease, sterility, aging and high cholesterol levels.

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 10 times less.