Selenium is a nonmetallic element with an atomic number of 34 and an atomic weight of 78.96. Its chemical symbol is Se. Selenium is most commonly found in nature in its inorganic form, sodium selenite. An organic form of selenium, selenomethionine, is found in foods.
The role of selenium in human nutrition and other therapeutic applications has provoked intense controversy over the past two decades. In contrast to such major minerals as magnesium and calcium, neither selenium's benefits nor its toxic aspects are yet fully understood. Until very recently, selenium was considered a toxic element that was not necessary to human health. In 1989, selenium was reclassified as an essential micronutrient in a balanced human diet when the National Research Council established the first recommended daily allowance (RDA) for it. It is considered a minor mineral, or a trace element, as distinct from a major mineral such as calcium or phosphorus, or an electrolyte such as sodium or chloride. There is less than 1 mg of selenium in the average human body. The selenium is concentrated in the liver, kidneys, and pancreas. In males, selenium is also found in the testes and seminal vesicles. Selenium currently has a variety of applications, ranging from standard external preparations for skin problems to experimental and theoretical applications in nutrition and internal medicine.
Naturopaths use selenium supplements to treat asthma, acne, tendinitis, infertility problems in men, and postmenopausal disorders in women. Selenium is also considered an important component in naturopathic life extension (longevity) diets, because of its role in tissue repair and maintaining the youthful elasticity of skin.
Selenium has been used since the 1960s in dandruff shampoos and topical medications for such skin disorders as folliculitis ("hot tub" syndrome) and tinea versicolor, a mild infection of the skin caused by the yeast-like fungus Pityrosporum orbiculare. When selenium is compounded with sulfur to form a sulfide, it has antibiotic and antifungal properties. Selenium sulfide is absorbed by the outermost layer of skin cells, the epithelium. Inside the cells, the compound splits into selenium and sulfide ions. The selenium ions counteract the enzymes that are responsible for producing new epithelial cells, thus lowering the turnover of surface skin cells. As a result, itching and flaking of the skin associated with dandruff and tinea versicolor is reduced.
Prior to 1989, there were no established RDA values for selenium. In 1989, the National Research Council of the National Academy of Sciences defined the RDAs for selenium as follows: Males aged 15–18 years, 50 g; 19–24 years, 70 g; 25–50 years, 70 g; 51 years and older, 70 g. Females: aged 15–18 years, 50 g; 19–24 years, 55 g; 25–50 years, 55 g; 51 years and older, 55 g; pregnant, 65 g; lactating, 75 g. The generally higher levels for males are related to the importance of selenium in producing vigorous sperm.
The amount of selenium in the diet is influenced by its level in the soil. Most selenium is absorbed from food products, whether plants grown in the soil or animals that have eaten the plants. Much of the selenium in foods is lost during processing. About 60% of dietary selenium is absorbed as food passes through the intestines. Selenium leaves the body in the urine and feces; males also lose some selenium through ejaculation of sperm. Selenium levels in soil vary widely, not only in different countries but also across different regions. For example, in the United States the western states have higher levels of selenium in the soil than the eastern states. South Dakota has the highest rates of soil selenium in the United States, while Ohio has the lowest.
Foods that are high in selenium contain the element in an organic form, selenomethionine. This form of selenium is considerably less toxic than inorganic sodium selenite or elemental selenium. Good sources of selenium include brewer's yeast, wheat germ, wheat bran, kelp (seaweed), shellfish, Brazil nuts, barley, and oats. Onions, garlic, mushrooms, broccoli, and Swiss chard may contain high amounts of selenium if they are grown in selenium-rich soil. Selenium is also present in drinking water in some parts of the world and can be added to drinking water as a health measure. Nursing mothers should note that human milk is much richer in selenium than cow's milk.
There is no widely recognized deficiency syndrome for selenium, unlike the syndromes associated with calcium or magnesium (hypocalcemia and hypomagnesemia, respectively). However, many researchers who
have investigated Keshan disease, a form of heart disease in children, believe that it is caused by selenium deficiency. The disease can be prevented but not cured with supplemental selenium; it responds to treatment with 50 g per day. The symptoms of Keshan disease, which is named for the region of China where it was discovered, include enlargement of the heart and congestive heart failure. The soil in the Keshan region is low in selenium. The researchers observed that the local Chinese treat Keshan disease with astragalus (Astragalus membranaceus), a plant that absorbs selenium from the soil.
Selenium toxicity is still a matter of controversy. It is a known fact that humans can tolerate higher levels of selenium in its organic form (selenomethionine) than in its inorganic forms. Humans can show symptoms of selenium toxicity after doses as low as 1 mg of sodium selenite. On the other hand, some researchers speculate that the organic forms of selenium may accumulate in the body and interfere with the functioning of sulfur molecules in the body, or that they may cause genetic mutations. These long-term questions await further research. In addition, researchers disagree about how much selenium will produce symptoms of toxicity. It has been suggested that toxicity can result from a daily intake of 2 mg in people who already have body stores of 2.5 mg of selenium or higher. Another measurement suggests that selenium toxicity may occur wherever the food or water regularly contains more than 5 or 10 parts per million of selenium. Patients with symptoms of selenium toxicity usually have blood plasma levels of 100 g/dl or higher, which is about four times the upper limit of normal levels.
The symptoms of selenium toxicity are not always clearly defined. People living in areas of selenium-rich soil sometimes develop heart, eye, or muscular problems. Eating foods containing high amounts of selenium over a long period of time increases the risk of tooth decay. It is thought that the selenium may compete with the fluoride in teeth, thus weakening their structure. Other symptoms associated with high levels of selenium include a metallic taste in the mouth, garlic-like breath odor, dizziness, nausea, skin inflammation, fatigue, and the loss of hair or nails. The symptoms of acute selenium poisoning include fever, kidney and liver damage, and eventual death.
Selenium is most widely recognized as a substance that speeds up the metabolism of fatty acids and works
CARDIOVASCULAR DISEASES. Low levels of selenium have been associated with high risk of heart attacks and strokes. It is thought that the antioxidant properties of selenium can help prevent atherosclerosis (narrowing and hardening of the arteries) by decreasing the formation of fatty deposits in the arteries. It does so by soothing the inflamed arterial walls and binding the free radicals that damage the tissues lining the arteries. Other studies indicate that selenium reduces the symptoms of angina pectoris.
CATARACTS. Cataracts in the eye contain only one-sixth as much selenium as normal lens tissue. The healthy lens requires adequate levels of three antioxidant enzymes: superoxide dismutase, catalase, and glutathione peroxidase. Glutathione peroxidase in the human eye is dependent on selenium, which suggests that a selenium deficiency speeds up the progression of cataracts.
CANCER. Low dietary levels of selenium have been associated with an increased incidence of cancer. Cancers of the respiratory system and the gastrointestinal tract seem to be especially sensitive to the level of selenium in the body. In a recent study, patients with histories of skin cancer were given 200 g of selenium per day. Results indicated that the patients had a reduced incidence of rectal, prostate, and lung cancers as well as a lower rate of mortality from all cancers. In addition, cervical dysplasias (abnormal growths of tissue) in women are associated with low levels of selenium in the patient's diet. In animal studies, as little as 1–4 parts per million of selenium added to the water or food supply is associated with a decreased incidence of cancer. It is not yet known, however, exactly how selenium protects against cancer. Some researchers believe that it may prevent mutations or decrease the rate of cell division, particularly on the outer surfaces of the body. A recent study of the effects of a selenium compound on mammary tissue indicates that selenium may inhibit the growth of tumors in deeper layers of tissue, not just cancers arising from the epithelium.
As of 2002, selenium is being studied as a possible chemopreventive for prostate cancer. The researchers hope to learn more about the mechanisms by which selenium slows the progress of an established cancer as well as discover a preventive strategy that makes use of selenium.
PERIODONTAL DISEASE. Selenium appears to speed up the healing of fragile gum tissue as well as opposing the actions of free radicals, which are extremely damaging to gum tissue.
RHEUMATOID ARTHRITIS. Selenium may be useful for treating several autoimmune diseases, especially lupus and rheumatoid arthritis (RA). It has been discovered that patients suffering from RA have low selenium levels. Selenium is necessary for production of the enzyme glutathione peroxidase, which reduces the production of inflammatory substances in the body (prostaglandins and leukotrienes) as well as opposing free radicals. Although supplemental selenium by itself has not been shown to cause improvement in RA, selenium taken together with vitamin E appears to have measurable positive results.
OSTEOARTHRITIS. Recent research in Germany indicates that selenium is beneficial in the prevention and treatment of osteoarthritis (OA), particularly OA resulting from physical wear and tear or structural problems in the patient's joints. Selenium supplements are even more effective when given together with vitamins in treating OA.
Selenium is available in topical preparations and as a dietary supplement.
Selenium sulfide for the treatment of dandruff is available as over-the-counter (OTC) scalp preparations or shampoo containing 1% or 2.5% solutions of the drug. A topical 2.5% solution of selenium sulfide is available for the treatment of tinea versicolor. Common trade names include Exsel™, Selsun™, and Selsun Blue™.
Selenium is widely available in vitamin/mineral dietary supplements and in nutritional antioxidant formulas. Although the average diet supplies enough selenium, some naturopaths recommend daily supplements of 100–200 g for adults and 30–150 g for children. Sexually active males are advised to take higher doses. Some naturopaths recommend taking selenium together with vitamin E on the grounds that their combined effect is greater than the sum of their individual effects. There are at present no definitive studies on the positive effects on health of selenium taken as a dietary supplement.
Persons using selenium compounds to control dandruff or tinea versicolor should be careful to avoid applying
It is difficult to assess the effectiveness of dietary supplements containing selenium because there is little agreement on standards for interpreting selenium levels in human blood. Depending on their intake, healthy adults may have blood plasma levels of selenium in the range of 8–25 g/dl. In addition, most of the selenium in the body is not carried in the blood but is stored in tissue. Analysis of hair has not been useful in measuring selenium. In the absence of a useful test, people who wish to take supplemental selenium should first find out whether they live in an area that already has high levels of selenium in the drinking water and soil. Most people will probably not need more selenium than is in standard vitamin/mineral supplements. In addition, the body seems to utilize selenium more efficiently when it is taken together with vitamin E.
The side effects of contact with compounds containing selenium sulfide include stinging of the skin; irritation of the lining of the eyes; hair discoloration or loss; and oily scalp. Both topical products and megadoses of selenium taken by mouth can cause selenium toxicity. The symptoms of selenium toxicity include nausea, vomiting, tiredness, abdominal pain, a garlicky breath odor, and the loss of hair and fingernails. These symptoms usually last 10–12 days after the selenium preparation is discontinued.
Topical preparations containing selenium may interact with the metals in costume jewelry. Patients should remove all their jewelry before applying the shampoo or lotion.
With regard to dietary supplements, there is some evidence that vitamin C inactivates selenium within the digestive tract. Persons who are concerned about their selenium intake may prefer to take supplemental selenium in the absence of vitamin C.
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Rebecca J. Frey, PhD