Selenium

Description

Selenium is an essential nutrient for many species, including humans. Deficiencies of this element are very apparent in animals, with symptoms ranging from muscular dystrophy in lambs, to the destruction of liver tissue in pigs.

Selenium is an important constituent of the enzyme glutathione peroxidase, which is responsible for destroying lipid-damaging peroxides. In this role, selenium complements the antioxidizing function of vitamin E.

The enzyme glutathione peroxidase is contained within white blood cells and blood platelets, and has importance in the immune system and blood clotting mechanisms of the body.

Selenium may also be important in the regulation of serum cholesterol levels, although this effect has not been conclusively proven experimentally.

Selenium serves to prevent the incidence of mercury poisoning by affecting the body's metabolism of the potential toxin.

Method of Action

Selenium is an important constituent of the enzyme glutathione peroxidase. This enzyme acts to destroy peroxides, thereby protecting cells and membranes against oxidative damage. Vitamin E and selenium tend to enhance the effect of one another in that vitamin E works to prevent the formation of peroxides, and glutathione peroxidase destroy what is already formed.

Glutathione peroxidase is also a constituent of blood platelets and white blood cells, making it an important part of the body's immune response system and blood clotting mechanism.

Absorption of selenium is dependent upon the solubility of the ingested compound and upon dietary ratio of selenium to sulfur. The availability of selenium for absorption is dependent upon many different factors, among which are the nature of the food source (i.e., seafood is high in selenium, but the selenium is poorly absorbed) and the method of food processing.

Once absorbed, selenium interacts with the sulfur-containing amino acids (e.g., cysteine and methionine) to form the enzyme glutathione peroxidase, and for incorporation into various proteins, such as hemoglobin and myoglobin.

Excesses of selenium are secreted in the urine, and the selenium-containing molecule dimethyl selenide is excreted during respiration. This molecule gives breath a garlicky odor characteristic of selenium toxicity.

Properties & Uses

Selenium supplementation has been useful in the treatment of a certain congestive heart disease found primarily in Chinese children. Selenium has also been used to eliminate skeletal muscle pain in some people. Selenium, in conjunction with vitamin E, may be useful in the prevention of heart disease.

Selenium supplements have been used in the treatment of anemia and growth problems which would not respond to other kinds of treatment.

Selenium moderates the symptoms of mercury toxicity and may be useful in instances of subacute mercury poisoning.

Adequate selenium intake has been statistically linked to a lower incidence of cancer mortality, but the conclusive evidence regarding selenium's relation to cancer development is not available.

Consequence of Deficiency

Selenium deficiency has not been conclusively linked to any specific set of symptoms in humans, although monkeys and other animals have serious physiological manifestations of deficiency. Symptoms in animals include muscle pain, red blood cell fragility, pancreas degeneration, growth retardation, and cataract formation. The link between animal and human symptoms of deficiency have not been concretely established.

As a side note, children with kwashiorkor have unusually low selenium stores and, therefore, selenium deficiency may occur concomitantly with abnormal protein metabolism.

Selenium deficiencies have been statistically linked to a higher incidence of cancer mortality, but the conclusivity of this relationship has not been settled.

Deficiencies of selenium may contribute to the excessive buildup of fats and other lipids in the liver, producing fatty liver syndrome.

Toxicity Factors

Selenium toxicity is most frequently seen in livestock grazing in regions of the Midwest where plants contain unusually high amounts of the element.

Human toxicity has not been conclusively determined but is considered to occur only as a result of high industrial exposure. Reported symptoms of toxicity include hair loss, depigmentation of skin, abnormal nails, and weariness. A garlicky odor on the breath, without garlic ingestion, may be an indication of selenium toxicity.

Recommended Dietary Allowance

age	RDA (mcg)	RNI (mcg)
infants/children
0-3 months	10	10
4-6 months	10	13
6-12 months	15	10
1-3 years	20	15
4-6 years	20	20
7-10 years	30	30
males
11-14 years	45	45
15-18 years	50	70
19+ years	70	75
females
11-14 years	45	45
15-18 years	50	60
19+ years	55	60
pregnancy	65	-
lactation	75	75

For over thirty years, Recommended Daily Amounts has existed in the United Kingdom. It has been used to measure the adequacy of an individual's diet. However, in 1991 the Committee on Medical Aspects of Food Policy (COMA) gave forth a whole new set of figures upon the request of the Department of Health's Chief Medical Officer. Reference Nutrient Intake (RNI) is one of these sets collectively known as "Dietary Reference Values." RNI is an amount of a nutrient that is enough for almost every individuals, even someone who has high needs for the nutrient. This level of intake is, therefore, considerably higher than what most people would need. If individuals are consuming the RNI of a nutrient they are most unlikely to be deficient in that nutrient.

Food Sources

Beef	Beef kidney
Beef liver	Brewer's yeast
Brown rice	Cereals
Chicken liver	Egg
Fish	Garlic
Lamb liver	Milk
Nuts	Poultry
Pumpkin seed	Turkey liver
Veal liver	Whole wheat bread

Abstracts

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