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.
