Selenium - Mineral

• Method of Action
• Consequences of Deficiency
• Deficiency Symptoms
• Recommended Dietary Allowances

• Properties and Uses
• Toxicity Levels
• Food Sources

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 anti-oxidizing 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 peroxide acts to destroy.

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

Absorption of selenium is dependent upon the solubility of the ingested compound and upon the 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 garlic odor characteristic of selenium toxicity.

Properties and 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.

Consequences 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 conclusively established, but there is strong evidence indicating the same deficiencies in humans.

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 to everyone’s satisfaction.

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

Some of the recent literature indicates all the items in the following section as selenium deficiency problems in humans.

Toxicity Levels

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 as a result of high industrial exposure. The salt form of selenium (sodium selenite) is toxic at relatively low levels, while the amino acid form (selenomethionine or selenocysteine) may be safely ingested at levels up to 1,000 mcg for an adult. There have been cases of vitamin companies putting large doses of the salt form of selenium into their product causing grave health problems among those who have taken the product. Reported symptoms of toxicity include hair loss, depigmentation of skin, abnormal nails, and weariness. A garlic odor on the breath, without garlic ingestion, may be an indication of selenium toxicity.

Recommended Dietary Allowances

Food Sources

Summary of Deficiency Symptoms

• Inadequate detoxification of heavy metal poisoning

• Increased vulnerability to cancer

• Decreased production of the selenium containing enzyme glutathione peroxidase,resulting in:

• Poor antibody production by white blood cells

• Poor blood clotting mechanism

• Lowered ability to destroy "free radicals"