SELENIUM

Selenium (Se) is an essential trace element that functions as a component of enzymes involved in antioxidant protection and thyroid hormone metabolism. In several intra- and extra-cellular glutathione peroxidases, iodothyronine 5í-deiodinases, and in thioredoxin reductase, selenium is located at the active centers as the selenoamino acid, selenocysteine (SeCYS). At least two other proteins of unknown function also contain SeCYS. Although SeCYS is an important dietary form, it is not directly incorporated into these specific selenium-proteins; instead, a co-translational process yields tRNA-bound SeCYS. In contrast, selenium as seleno-methionine is incorporated non-specifically into many proteins, as it competes with methionine in general protein synthesis. Therefore, tissues often contain both specific, as well as the nonspecific, selenium-containing proteins when both SeCYS and selenomethionine are consumed, as found in many foods.

Deficiencies: Selenium deprivation reduces activities of the selenium-dependent enzymes. The signs in animals depend upon vitamin E status and appear only when both nutrients are limiting. They vary according to species. For example, selenium- and vitamin E-deficient animals show myopathies of skeletal (e.g., sheep, cow, horse), cardiac (pig) or smooth (dog, cow) muscle; hepatic necrosis (rat, pig); increased capillary permeability (chicken); or pancreatic acinar degeneration (chicken). Characteristic signs of selenium deficiency have not been described in humans, but very low selenium status is a factor in the etiologies of a juvenile cardiomyopathy (Keshan Disease) and a chondrodystrophy (Kashin-Beck Disease) that occur in selenium-deficient regions of China.

Diet recommendations: The Recommended Dietary Allowances are in µg/day: 0-0.5 yrs, 10; 0.5-1.0 yrs, 15; 1-6 yrs, 20; 7-10 yrs, 30; males 11-14 yrs, 40; females 11-14 yrs, 45; males and females 15-18 yrs, 50; adult males, 70; adult females, 55; pregnancy, 65; and lactation, 75.

Food sources: The most important sources in American diets are meats, fish and grains. Brazil nuts can have relatively high selenium concentrations. Foods of low protein content, including most fruits and vegetables, provide little selenium. Food selenium is absorbed with efficiencies of 60-80%; the greatest factor affecting the utilization of food selenium is its chemical form.

Toxicity: Selenium toxicity is characterized by dermatologic lesions; selenotic animals and humans develop brittle hair and nails/hooves. Sporadic cases of selenium-poisoning have been reported involving industrial or accidental exposures to selenium-compounds. In certain rural Chinese communities chronic intakes of very high amounts (several milligrams per day) of selenium were linked to skin, hair and nail abnormalities which disappeared upon resuming regular selenium intakes. Selenium has been identified as the cause of birth deformities in migratory wildfowl in a wetland area (Kesterson Reservoir, CA) which receives selenium-enriched irrigation wastewater. This case involved the biological amplification of selenium by aquatic plants which were important in the diet of affected animals. The Reference Dose (RfD) set by the Environmental Protection Agency is 5 µg/kg body weight/day or 350 µg/day for a 70 kg individual. This intake is regarded as having no significant risk of a deleterious effect over a lifetime of exposure.

Recent research: Preliminary findings suggest that selenium may have an anticancer effect in humans. Animal studies indicate that selenium deficiency may decrease the resistance of the host to infection with certain viruses.

For further information:

Beck, M. A. & Levander, O. A. (1998) Dietary oxidative stress and the potentiation of viral infection. Ann. Rev. Nutr. 18: 93-116.

Combs, G. F., Jr. & Gray, W. P. (1998) Chemopreventive agents: selenium. Pharmacol. Expt. Therapeut. 79: 179-192.

Xia, Y. M., ed. (1997) Proceedings of Selected Papers of the Sixth International Symposium on Selenium in Biology and Medicine. Biomed. Environ. Sci. 10: 113-368.

Prepared By:

Gerald F. Combs, Jr., Ph.D.
Director of Graduate Studies - Nutrition
Division of Nutritional Sciences
Cornell University
122 Savage Hall
Ithaca, NY 14853-6301
Phone: 607-255-2140
FAX: 607-255-1033
Email: gfc2@cornell.edu

Orville A. Levander, Ph.D.
Research Leader
Nutrition Institute
USDA, ARS
Beltsville, MD 20705
Phone: 301-504-8504
FAX: 301-504-9062
Email: levander@307.bhnrc.usda.gov