• Description
• Functions
• Dietary sources
• Required Intakes

Description

Selenium is a trace mineral that is essential to good health but required only in small amounts. Selenium is incorporated into proteins to make selenoproteins, which are important antioxidant enzymes. The antioxidant properties of selenoproteins help prevent cellular damage from free radicals. Free radicals are natural by-products of oxygen metabolism that may contribute to the development of chronic diseases such as cancer and heart disease. Other selenoproteins help regulate thyroid function and play a role in the immune system.

Functions

Selenium has a variety of functions, and research is revealing new information. Its main role is as an antioxidant in the enzyme selenium-glutathione peroxidase. Selenium is part of a nutritional antioxidant system that protects cell membranes and intracellular structural membranes from lipid peroxidation. It is actually the selenocysteine complex that is incorporated into glutathione peroxidase (GP), an enzyme that helps prevent cellular degeneration from the common peroxidase free radicals, such as hydrogen peroxide. (Selenomethionine can be supplemented to generate the organically complexed and active selenocysteine.) GP also aids red blood cell metabolism and has been shown to prevent chromosome damage in tissue cultures. Solidification of tissue membranes may occur through the oxidation of fatty acids. As an antioxidant, then, selenium in the form of selenocysteine prevents or slows the biochemical aging process of tissue degeneration and hardening-that is, loss of youthful elasticity. This protection of the tissues and cell membranes is enhanced by vitamin E. The antioxidant effect may also benefit the cardiovascular system and protect against cancer. We need adequate daily amounts of selenium for the maintenance of these antioxidant functions and for selenium's other cellular functions as well.

Selenium also appears to help stimulate antibody formation in response to vaccines. This immunostimulating effect is also enhanced by vitamin E; the presence of these two nutrients can increase antibody formation by 20-30 times, as shown by research.

Selenium is thought to offer protection against cardiovascular disease, possibly by its antioxidant function but possibly by another, as yet, unknown mechanism. Epidemiological studies show an increased incidence of strokes and other cardiovascular problems in many low-selenium areas.

Selenium is also being found to have an anticarcinogenic effect; its blood or tissue levels may correlate more closely with cancer risk than those of any other substance. Public health research shows this relationship in many cases; good selenium levels correlate with low cancer rates and low levels with increased cancer rates. I do not yet know exactly how this works other than possibly through the antioxidant function. Perhaps selenium decreases cell division or helps cell repair, or perhaps it protects against mutagenic changes in the first place.

Selenium also seems to protect us from the toxic effects of heavy metals and other substances. People with adequate selenium intake have fewer adverse effects from cigarette smoking, alcohol, oxidized fats, and mercury and cadmium toxicity. Aside from the likely antioxidant influence, the specific mechanism by which selenium affords this protection is not known, though the effect is confirmed by some research.

Selenium may also aid in protein synthesis, growth and development, and fertility, especially in the male. It has been shown to improve sperm production and motility. Thus, selenium may prevent male infertility; however, we do not know whether selenium deficiency will actually cause male infertility. These are only some of the conjectures about other selenium functions.

Dietary sources

Plant foods are the major dietary sources of selenium in most countries throughout the world. The content of selenium in food depends on the selenium content of the soil where plants are grown or animals are raised. For example, researchers know that soils in the high plains of northern Nebraska and the Dakotas have very high levels of selenium. People living in those regions generally have the highest selenium intakes in the United States (U.S.). In the U.S., food distribution patterns across the country help prevent people living in low-selenium geographic areas from having low dietary selenium intakes. Soils in some parts of China and Russia have very low amounts of selenium. Selenium deficiency is often reported in those regions because most food in those areas is grown and eaten locally. Selenium also can be found in some meats and seafood. Animals that eat grains or plants that were grown in selenium-rich soil have higher levels of selenium in their muscle. In the U.S., meats and bread are common sources of dietary selenium. Some nuts are also sources of selenium. Selenium content of foods can vary. For example, Brazil nuts may contain as much as 544 micrograms of selenium per ounce. They also may contain far less selenium. It is wise to eat Brazil nuts only occasionally because of their unusually high intake of selenium. Selected food sources of selenium are provided in Table 1.

*DV = Daily Value.

Required Intakes

Selenium occurs in staple foods such as corn, wheat, and soybean as selenomethionine, the organic selenium analogue of the amino acid methionine. Selenomethionine can be incorporated into body proteins in place of methionine, and serves as a vehicle for selenium storage in organs and tissues. Selenium supplements may also contain sodium selenite and sodium selenate, two inorganic forms of selenium. Selenomethionine is generally considered to be the best absorbed and utilized form of selenium.

Human selenium deficiency is rare in the U.S. but is seen in other countries, most notably China, where soil concentration of selenium is low. There is evidence that selenium deficiency may contribute to development of a form of heart disease, hypothyroidism, and a weakened immune system. There is also evidence that selenium deficiency does not usually cause illness by itself. Rather, it can make the body more susceptible to illnesses caused by other nutritional, biochemical or infectious stresses.

Most cases of selenium depletion or deficiency are associated with severe gastrointestinal problems, such as Crohn's disease, or with surgical removal of part of the stomach. These and other gastrointestinal disorders can impair selenium absorption. People with acute severe illness who develop inflammation and widespread infection often have decreased levels of selenium in their blood. Physicians will evaluate individuals who have gastrointestinal disease or severe infection for depleted blood levels of selenium to determine the need for supplementation.