Selenium is an essential constituent of a number of enzymes, some of which have antioxidant functions. Deficiency of the element in animals makes them susceptible to injury by certain types of oxidative stress. At least 1 human disease occurs only in selenium-deficient individuals. Therefore, it seems prudent to avoid selenium deficiency. The plasma (or serum) selenium concentration is often used to assess selenium nutritional status. A plasma selenium concentration of 8 micrograms/dL or greater in a healthy subject indicates that plasma selenoproteins are optimized and the subject is selenium replete. The Third National Health and Nutrition Examination Survey determined plasma selenium in 17,630 subjects in the United States. Its results indicate that more than 99% of the subjects studied were selenium replete. The Institute of Medicine has set the Recommended Dietary Allowance for selenium at 55 micrograms per day for adults. Since most estimates of selenium intake in the United States are 80 micrograms per day or greater, routine selenium supplementation is not recommended in the United States.