Selenium is an essential trace mineral. The body needs small amounts of selenium to make special proteins called selenoproteins. These proteins are also antioxidants, which prevent cell damage from free radicals. Free radicals are natural by-products of the breakdown of oxygen. Selenoproteins also regulate immune system and thyroid function. It has been theorized that selenium may have a potential role in the prevention of cancer and heart disease. Selenium can be found in foods such as tuna, beef, turkey, brazil nuts, chicken, and some whole grains. The recommended daily allowance of selenium is 55 micrograms per day. Most people in the United States consume up to 220 micrograms per day.
One study evaluated the selenium intake of approximately 13,000 adults over a 12 year period. Those with higher blood levels of selenium had a significantly lower risk of death as long as the blood levels were less than 130ng/mL. Once levels rose above that level, the risk of death increased. The study evaluated selenium dietary intake. However, the effect of selenium supplementation on longevity was not evaluated.