Selenium (Se) is an essential trace element in human nutrition involved in the defense against the toxicity of reactive oxygen species, also in the regulation of thyroid hormone metabolism, and the regulation of the oxidation state of cells. Selenium has also been shown to have an anticarcinogenic activity, and to inhibit the effects of a number of toxic metals, including cadmium and arsenic. Proteins in the high-Selenium form of yeast are enzymatically digested in the small intestine to yield amino acids, oligopeptides, and L-Selenomethionine. The L-Selenomethionine is then efficiently absorbed from the small intestine where it is transported via the portal circulation to the liver, where a fraction is extracted by the hepatocytes, and the remaining amount is transported by the circulation to the various tissues of the body.
Importance
A major antioxidant nutrient, protects cell membranes and prevents free radical generation thereby decreasing the risk of cancer and disease of the heart and blood vessels. Medical surveys show that increased selenium intake decreases the risk of breast, colon, lung and prostate cancer. Selenium also preserves tissue elasticity; slows down the aging and hardening of tissues through oxidation; helps in the treatment and prevention of dandruff.
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. May result in premature aging, heart disease, dandruff, loose skin.
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.