Selenium activates an antioxidant enzyme called glutathione peroxidase, which may help protect the body from cancer. 2 Selenium is also essential for healthy immune functioning. As a result, selenium supplementation has reduced the incidence of hepatitis in deficient populations. 4 Selenium is also needed to activate thyroid hormones. Although details are not understood, selenium supplementation has partially normalized the actions of sperm cells in infertile men in double blind research.
Brazil nuts are the best source of selenium.
Yeast, whole grains, and seafood are also good sources. While most people probably don't take in enough selenium, gross deficiencies are rare in Western countries. Soils in some areas are selenium deficient, and people who eat foods grown primarily on selenium-poor soils are at risk for deficiency. People with AIDS have been reported to be depleted in selenium. Similarly, limited research has reported an association between heart disease and depleted levels of selenium.