The important role of the trace mineral selenium (Se) in the function of certain enzymes wasn’t established until the early 1970s. Currently, about two dozen selenium-containing human proteins have been identified. Selenium functions as a co-factor for several antioxidant enzymes and with other selenium-dependent enzymes in the activation of thyroid hormone, making it essential for normal growth, development, and metabolism.

A selenium deficiency by itself seldom causes obvious symptoms or illness. However, additional stresses, such as viral infections, chemical exposure, or low intakes of other antioxidant nutrients, together with low selenium intake can cause clinical illness. For example, a selenium deficiency increases the likelihood of developing a vitamin E deficiency when intake of the vitamin are low. This occurs because selenium functions as a cofactor for an important antioxidant enzyme system and a deficiency of selenium increases oxidative stress and places a greater demand on the antioxidant function of vitamin E. Low-selenium status increases the risk of a particular form of heart disease and may increase the risk of some cancers. Inadequate selenium may also decrease immune function.

The RDA for selenium is 55 micrograms per day for adults. The highest concentrations of selenium are present in organ meats and seafood. The most common sources in the diet are meat and cereals. The selenium content of meats is fairly consistent, but the selenium content of plant foods varies widely depending on the content of the mineral in the soil where the plant was grown. Dietary selenium is generally well-absorbed, and unlike most other minerals, its absorption is not regulated in relation to nutritional status. (INFOGRAPHIC 14.11)

The UL for selenium is 400 micrograms per day in adults. Although a single high dose of selenium may be fatal, selenium toxicity generally occurs with long-term exposure to moderately high levels of intake either through supplementation or, in rare instances, through frequent consumption of selenium-rich foods. For example, a single one-ounce serving of about six Brazil nuts contains 544 micrograms of selenium, which is about 25% above the UL. The most common toxicity symptoms are hair and nail loss, and brittleness.