Selenium is a mineral your body needs in small amounts to make certain enzymes. You can get selenium from some of the foods you eat, and it's also available as a dietary supplement. Selenium can become toxic at high levels. Talk to your doctor before taking selenium supplements.
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Selenium is needed for the production of selenoproteins, which are enzymes that work as antioxidants that protect the cells in your body from free radical damage. Free radicals are by-products of oxidation and can occur during normal metabolism, but you can also be exposed to free radicals in smoke, pollution or other toxic exposures. Seleneoproteins also help regulate thyroid gland function and aid the immune system.
Selenium and Cancer
According to the Office of Dietary Supplements, death from certain types of cancer, including lung, colorectal and prostate cancers, is lower among people who have a higher intake of selenium. Rates of a type of skin cancer called melanoma are lower in parts of the United States where soil levels of selenium are highest. Selenium's potential anti-cancer properties may be due to it's antioxidant capabilities or because it may be able to slow down the growth of tumors.
Sources of Selenium
The Institute of Medicine sets the dietary reference intakes for selenium at 55 mcg per day for adults. Plant-based food sources contain the most selenium, but that amount depends on the amount of selenium in the soil where the plants were grown. Brazil nuts contain more selenium than any other food, with 544 mcg per 1 oz. serving. Tuna, beef, wheat products, oatmeal, rice and nuts are also good sources of selenium.
Selenium deficiency is rare in the United States, but when it does occur, it's usually as a result of severe gastrointestinal disease. Selenium deficiency can lead to heart disease, hypothyroidism and a weakened immune system. Three diseases are due to selenium deficiency: Keshan disease causes heart problems in children, Kashin-Beck disease causes a type of arthritis, and myxedematous endemic cretinism results in mental retardation.