Selenium is an essential trace element that is required in human and animal nutrition. Selenium plays an important role in defending against toxicity of oxygen, in regulation of hormone metabolism in the thyroid, and in the regulation of the redox state of cells. Due to its potential toxicity and fears that it might be a carcinogen, selenium's role in human and animal nutrition was not discovered for many years. Many of these fears have now been replaced by evidence suggesting that selenium may protect against some forms of cancer by activating an antioxidant enzyme called glutathione peroxidase.

Selenium is considered to have an important role in promoting healthy immune function and is believed to reduce the probability of viral hepatitis in selenium-deficient populations. Selenium supplementation has been found to stimulate the activity of white blood cells, primary components of the immune system, even in selenium non-deficient populations and especially in the elderly.

The amount of selenium in the food you eat is directly related to the amount of selenium in the soil where the food was grown. Selenium enters the food chain through plants as the amino acids L-selenocysteine and L-selenomethionine from the soil where the plant is grown. Selenium, like most trace elements and minerals found in the soil, is not found in the same concentrations in all regions of the world. Because of the uneven concentrations of selenium, disorders of both selenium deficiency and selenium excess are not uncommon. China has the highest variance of selenium distribution, having both the highest and lowest levels soil concentrations in the world.

Selenium Supplementation Dosage Recommendations

Available forms of selenium supplements are high-selenium yeast, L-selenomethionine, sodium selenate, and sodium selenite. Recommended Dietary Allowance for most adults is 55 mcg per day, but supplementation with 100–200 mcg of selenium per day has been recommended by some doctors.