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What is selenium?

Selenium is a trace mineral that appears to have functions similar to vitamin E. All human tissues contain minute amounts of selenium, with the highest concentrations in the kidney, liver, spleen, pancreas, and testes.

Why is selenium important to nutrition?

The precise role of selenium in human nutrition is unknown. However, nutrition researchers do know that selenium can fulfill some of the antioxidant roles of vitamin E. (Antioxidants are substances that protect tissues from damage due to peroxides and other high concentrations of oxygen.) More specifically, selenium is a major component of an enzyme called glutathione peroxidase, which is thought to deactivate lipid (fat) peroxides.

In recent years, as yet unproved reports that selenium may prevent cancer have gained considerable media attention. Population (epidemiological) studies suggest that people living in areas in which the soil is deficient in selenium have an increased incidence of pancreatic, lung, and certain other cancers. In addition, researchers at the University of California in San Diego compared blood samples from 111 cancer patients with a matched group of cancer-free volunteers, and they found that the cancer patients had significantly lower selenium levels than their healthy counterparts.

Selenium is believed to play a role in maintaining muscles, especially the heart muscle (the myocardium). It may also protect against some of the liver damage caused by alcoholism. Other possible roles of selenium include ensuring proper growth and normal tooth development and preventing cataracts.

How much selenium do I need?

The revised 1989 Recommended Dietary Allowances (RDAs) for selenium, stated in micrograms, are as follows:

• 10 for infants under 6 months.
• 15 for infants 6 to 12 months.
• 20 for children 1 to 6 years.
• 30 for children 7 to 10 years.
• 40 for males 11 to 14.
• 45 for females 11 to 14.
• 50 for both sexes 15 to 18.
• 70 for males over 19.
• 55 for females over 19.
• 65 for pregnant women.
• 75 during lactation.

What happens if I don't get enough selenium?
The effects of selenium deficiency in humans are poorly documented and based mostly on epidemiological studies. For example, epidemiologists note that there is a high incidence of Keshan's disease, a highly lethal degeneration of the heart muscle (cardiomyopathy), among children in parts of China and the Soviet Union where the soil is especially deficient in selenium. Fatalities from Keshan's disease decline dramatically when children in these areas are given selenium supplements. Selenium deficiency has been linked to growth abnormalities and generalized muscle diseases, including muscular dystrophy, but further study is needed to document its role.

Animal studies, however, have attributed a variety of growth disorders to selenium deficiency. Laboratory studies have also found that selenium supplements help prevent pancreatic cancer in rats, and other animal studies indicate that selenium may prevent cataracts.

What happens if I get too much selenium?
Overdoses of selenium can produce nausea, abdominal cramps, diarrhea, fatigue, irritability, balding, and abnormal nail development. In extreme cases, it can result in death. It is

not known how much selenium produces these adverse effects, but the Food and Nutrition Board of the National Academy of Sciences recommends that daily intake should not exceed 0.2 milligrams, which is considerably more than the RDAs.

In animals, excessive selenium produces alkali disease. This disorder, which occurs mostly in cattle that graze on lands with extra-ordinarily high selenium content, is characterized by stiffness, blindness, deformed hooves, and hair loss.

Should I take selenium supplements?
No; selenium supplements should be taken only under a doctor's supervision. In the United States, a balanced diet that includes a variety of foods from the four basic groups (milk and milk products; meat, fish, poultry, and meat substitutes; fruits and vegetables; cereals, breads, pasta, and other grain products) provides ample selenium.

Selenium deficiency is most common in people who have intestinal disorders that hinder absorption. Alcoholics and patients with AIDS or certain cancers may also develop selenium deficiency. Experimental studies have found some beneficial effects of selenium supplements for patients with AIDS, certain cancers, and liver disorders due to alcoholism. But supplements are not recommended for healthy persons.

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