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 together with vitamin E (tocopherol) as an antioxidant. Antioxidants are organic substances that are able to counteract the damage done to human tissue by oxidation (the breakdown of fatty acids). Selenium's antioxidant properties have been studied with respect to several diseases and disorders. In addition to its antioxidant properties, selenium also appears to work as an anti-inflammatory agent in certain disorders.
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 the product to damaged or broken skin. In addition to irritating skin, selenium can enter the body through broken skin. This process is known as percutaneous absorption and can cause selenium toxicity if the preparation is used for a long period of time. Patients should wash their hands carefully after applying the selenium product to affected areas. Doing so will minimize absorption through small breaks in the skin of the hands.
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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Berger, Timothy G., M.D. "Skin, Hair, and Nails." In Current Medical Diagnosis and Treatment, edited by Lawrence M. Tierney, M.D., et al. 39th ed. New York: McGraw-Hill, 2000.
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Dong, Y., C. Ip, and H. Ganther. "Evidence of a Field Effect Associated with Mammary Cancer Chemoprevention by Methylseleninic Acid." Anticancer Research 22 (1A)(January-February 2002): 27-32.
Kurz, B., B. Jost, and M. Schunke. "Dietary Vitamins and Selenium Diminish the Development of Mechanically Induced Osteoarthritis and Increase the Expression of Antioxidative Enzymes in the Knee Joint of STR/1N Mice." Osteoarthritis and Cartilage 10 (February 2002): 119-126.
Nelson, M. A., M. Reid, A. J. Duffield-Lillico, and J. R. Marshall. "Prostate Cancer and Selenium." Urology Clinics of North America 29 (February 2002): 67-70.
Tan, J., W. Zhu, W. Wang, et al. "Selenium in Soil and Endemic Diseases in China." The Science of the Total Environment 4 (February 2002): 227-235.
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Rebecca J. Frey, PhD
Selenium is a trace element considered a micro-nutrient, meaning a nutrient needed in very small amounts, that is required as an essential cofactor for the antioxidant enzymes of the body to counteract the damaging effects of reactive oxygen in tissues.
The body requires selenium for the function of a special class of enzymes, called selenoproteins. Proteins are long folded chains of amino acids and selenoproteins are made by the body (selenoprotein synthesis), by incorporating dietary selenium in the form of an unusual amino acid, called selenocysteine, into a very specific location in their amino acid sequence. Animals and humans both require selenium, but not plants. Plants can however, incorporate selenium present in the soil into compounds that usually also contain sulfur.
The major function of selenoproteins is to prevent or reduce the damage (oxidative stress) caused by reactive oxygen species (ROS) or reactive nitrogen species (RNS). These can occur in the body mostly in the form of free radicals, such as peroxides. There are many types of selenoprotein enzymes that protect cells from these damaging molecules. For instance, some convert peroxides into non-toxic alcohols, thus protecting cells from membrane damage while others protect against other types of free radicals. Selenoproteins are also required to:
- Participate in the roduction the white blood cells of the immune system.
- Maintain balanced thyroid gland function.
- Promote healthy vision.
- Maintain healthy skin and hair.
- Protect cells against toxic minerals, such as mercury, lead, and cadmium.
- Help liver function.
- Help break down dietary fats.
- Maintain elasticity of tissues.
Recent research is also indicative of a physiological role for selenium, such as maintaining the blood brain barrier that protects the brain against harmful substances, with several other studies suggesting other protective roles. Although many findings are still uncertain, it is thought that an adequate selenium intake may have an anti-cancer effect, while protecting against lipid intake disorders (hyperlipidaemia), hypertension and other heart diseases. There are also reports suggesting
|Age||Recommended Dietary Allowance (mcg)|
|Children 1–3 yrs.||20|
|Children 4–8 yrs.||30|
|Children 9–13 yrs.||40|
|Brazil nuts (from Brazil), 1oz.||544|
|Egg, 1 whole||14|
|Fish (cod, shellfish, tuna) 1 oz.||10-20|
|Enriched noodles or macaroni, 1 cup||10|
|Rice, white, 1 cup||10|
|Organ and muscle meat, 1 oz.||8-12|
|Turkey or chicken, 1 oz.||7-11|
|Rice, brown, long-grain, 1||7|
|Cheddar cheese, 1 oz.||5|
|Walnuts, 1 oz.||5|
|Oatmeal or bran, 1 cup||3-4|
|Garlic, 1 oz.||0.25|
|mcg = microgram|
(Illustration by GGS Information Services/Thomson Gale.)
that oral selenium supplements that increase the plasma selenium concentration result in higher sperm motility.
At least 11 selenoproteins have been discovered to date, with evidence suggesting that many more exist. They are found in cell membranes, blood, organs, prostate gland and testicles. They include:
- Glutathione peroxidases (GPx). Four have been identified: classical or cellular GPx, plasma or extracellular GPx, phospholipid hydroperoxide GPx, and gastrointestinal GPx. They are all antioxidant enzymes that reduce ROS, such as hydrogen peroxide and lipid hydroperoxides, to harmless products like water and alcohols.
- Thioredoxin reductase. This enzyme maintains thio-redoxins, proteins that act as antioxidants, in the form required to properly regulate cell growth and viability.
- Iodothyronine deiodinases. Three of these have been identified. These selenoproteins are essential enzymes for normal development, growth, and the regulation of thyroid hormones. The thyroid gland releases very small amounts of biologically active thyroid hormone (T3) and larger amounts of an inactive form of thyroid hormone (T4) into the circulation. Most T3 is created by the removal of one iodine atom from T4 in a reaction made possible by iodothyronine deiodinases.
- Selenoprotein P (SeP). This one is found in plasma and in the cells that line the inner walls of blood vessels. It is believed to function as a transport protein, as well as an antioxidant capable of protecting cells from damage by RNS.
- Selenoprotein W (SeW). SeW is found in muscle. Its function is presently unknown, but it is believed to play a role in muscle metabolism.
- Selenophosphate synthetase. This enzyme is required to incorporate the special selenocysteine amino acid when selenoproteins are made (protein biosynthesis).
The richest food sources of selenium are Brazil nuts, organ meats and fish, followed next by muscle meats. As for plants and grains, there is a wide variation in their selenium content because it depends on the selenium content of the soil in which they grow. For example, Brazil nuts grown in areas of Brazil with selenium-rich soil provide more selenium than those grown in a selenium-poor soil. In the United States, grains are a good source of selenium, but not fruits and vegetables. However, people living in areas with low soil selenium avoid deficiency because they eat foods produced in areas with higher soil selenium. It appears that selenium from different sources is absorbed by the body with varying efficiency. For example, a recent study showed that the mean absorption of selenium from fish was 85-90%, compared with 50% from yeast. Some good food sources of selenium include (per 1oz-serving or as indicated):
- Brazil nuts from Brazil(~544 µg)
- Fish (cod, shellfish, flounder, tuna)(~10-20 µg)
- Eggs (~14 µg per egg, whole)
- Organ and muscle meats (~8-12 µg)
- Turkey, chicken (~7–11 µg)
- Long-grained brown rice (~7 µg per cup)
- Walnuts (~5 µg)
- Cheddar cheese (~5 µg)
- Oatmeal, bran (~3–4 µg per cup)
- Garlic (0.25 (µg/g)
- Enriched noodles, macaroni, white rice (~10 µg per cup)
The Recommended Dietary Allowance (RDA) for selenium is:
- Infants: There is insufficient information on selenium to establish a RDA for infants.
- Children (1-3 y): 20 µg
- Children (4-8 y): 30 µ>g
- Children (9-13 y): 40 µg
- Adolescents (14-18): 55 µg
- Adults: 55 µg
Amino acid— Organic (carbon–containing) molecules that serve as the building blocks of proteins.
Antioxidant enzyme— An enzyme that can counteract the damaging effects of oxygen in tissues.
Cofactor— A compound that is essential for the activity of an enzyme.
Blood brain barrier— A physiological mechanism that alters the permeability of brain capillaries, so that some substances, such as certain drugs, are prevented from entering brain tissue, while other substances are allowed to enter freely.
Enzyme— A biological catalyst, meaning a substance that increases the speed of a chemical reaction without being changed in the overall process. Enzymes are proteins and vitally important to the regulation of the chemistry of cells and organisms.
Free radicals— Highly reactive chemicals that damage components of cell membranes, proteins or genetic material by ‘oxidizing‘ them, the same chemical reaction that causes iron to rust.
Hormone— A chemical substance produced in the body that controls and regulates the activity of certain cells or organs.
Immune system— Defense system of the body responsible for protecting it against infections and foreign substances.
Kashin–Beck disease— A disorder of the bones and joints of the hands and fingers, elbows, knees, and ankles of children and adolescents who slowly develop stiff deformed joints, shortened limb length and short stature. The disorder is endemic in some areas of eastern Siberia, Korea, China and Tibet.
Keshan’s disease— A potentially fatal form of cardi-omyopathy (disease of the heart muscle).
Macro minerals— Minerals that are needed by the body in relatively large amounts. They include
sodium, potassium, chlorine, calcium, phosphorus, magnesium.
Oxidative stress— Accumulation in the body of destructive molecules such as free radicals that can lead to cell death.
Peroxides— Peroxides are highly reactive free radical molecules, used as powerful bleaching agents and as disinfectant. In the body, they form as intermediate compounds, for example during the oxidation of lipids, and may damage tissues.
Plasma— The liquid part of the blood and lymphatic fluid, which makes up about half of its volume. It is 92% water, 7% protein and 1% minerals.
Reactive oxygen species (ROS)— Damaging molecules, including oxygen radicals such as superoxide radical and other highly reactive forms of oxygen that can harm biomolecules and contribute to disease states.
Reactive nitrogen species (RNS)— Highly reactive chemicals, containing nitrogen, that react easily with other molecules, resulting in potentially damaging modifications.
Selenocysteine— Unusual amino acid consisting of cysteine bound to selenium. The process of inserting selenocysteine into proteins is unique to cysteine, and occurs in organisms ranging from bacteria to man.
Selenoprotein— Enzyme that requires selenium to function. At least eleven have been identified.
Thyroid— A gland located beneath the voice box that produces thyroid hormone, a hormone that regulates growth and metabolism.
Trace minerals— Minerals needed by the body in small amounts. They include: selenium, iron, zinc, copper, manganese, molybdenum, chromium, arsenic, germanium, lithium, rubidium, tin.
- Pregnancy: 60 µg
- Lactation: 70 µg
Selenium in nutritional supplements is available mostly in the form of sodium selenite and sodium selenate, two inorganic forms of selenium or as selenomethionine in ‘high selenium yeasts‘ generally considered to be the best absorbed and utilized form of selenium.
Selenium is trace element that is essential in small amounts, but is toxic in larger amounts. Excessive intake can result in symptoms that may include fatigue and irritability, with increased toxicity leading to loss of hair and nails, white blotchy nails, and garlic breath odor. If not corrected, it leads to a condition called chronic selenium toxicity (selenosis), with symptoms of vomiting, nausea, nerve damage, skin rashes, and brittle bones. Selenium toxicity is rare in the Unites States with a few reported cases associated with industrial accidents or manufacturing errors leading to an excessively high dose of selenium in a supplement.
On the other hand, diabetes and arthritis have been extensively shown to be associated with selenium deficiency. Gastrointestinal problems, such as Crohn’s disease, or surgical removal of part of the stomach can lead to selenium defieciency.
Since selenium is part of the selenoproteins enzymes of the body, it is believed to interact with every nutrient that affects the antioxidant balance of cells. Selenium as gluthathione peroxidase also appears to work in conjunction with vitamin E in limiting the oxidation of lipids. Animal oxidative stress studies indicate that selenium can prevent some of the damage resulting from vitamin E deficiency. Thioredoxin reductase is also believed to maintain the antioxidant function of vitamin C. A selenium deficiency may also worsen the effects of iodine deficiency in the thyroid. At present, few interactions between selenium and medications are known. The anticonvulsant medication, valproic acid, has been found to decrease plasma selenium levels. Supplemental sodium selenite has been found to decrease toxicity from the antibiotic nitrofurantoin and the herbicide paraquat in animals.
When the diet is corrected for selenium imbalance, most symptoms tend to disappear on intake to recommended RDA levels. People at risk of selenium deficiency, due to gastrointestinal disease or severe infection, are evaluated by physicians for depleted selenium blood levels to determine the need for supplementation.
Acute and fatal complications have occurred with accidental ingestion of gram quantities of selenium. Significant selenium toxicity was reported in 13 individuals who took supplements that contained 27,300 mg per tablet due to a manufacturing error. Selenosis may occur with smaller doses of selenium over long periods of time. Overall, selenium deficiency is rare in the United States. When it occurs, it results in a decrease of the activity of the selenium-dependent enzymes, especially if the vitamin E is also missing. A lack of antioxidants in the heart, liver and muscles can lead to tissue death and organ failure. Selenium deficiency has also been suggested as a probable cause of Keshan’s disease and Kashin-Beck disease and is currently associated with anemia, cataracts, increased risk of cancer, heart disease, stroke , diabetes, arthritis, decreased immune function, early aging, infertility, miscarriages, and birth defects in women.
Maintaining good nutrition in the home includes keeping informed about the food sources of various minerals such as selenium. A first source of information is the Nutrition Fact labels that list them in milligrams or micrograms and as a percentage of the RDA. Parents should also be aware of the risks associated with both selenium deficiency and over-consumption.
Bogden, J., ed Clinical Nutrition of the Essential Trace Elements and Minerals (Nutrition and Health). Totowa, NJ: Humana Press, 2000.
Challem, J., Brown, L. User’s Guide to Vitamins & Minerals. Laguna Beach, CA: Basic Health Publications, 2002.
Garrison, R., Somer, E. The Nutrition Desk Reference. New York, NY: McGraw–Hill, 1998.
Griffith, H. W. Minerals, Supplements & Vitamins: The Essential Guide. New York, NY: Perseus Books Group, 2000.
Larson Duyff, R. ADA Complete Food and Nutrition Guide, 3rd ed. Chicago, IL: American Dietetic Association, 2006.
Newstrom, H. Nutrients Catalog: Vitamins, Minerals, Amino Acids, Macronutrients—Beneficials Use, Helpers, Inhibitors, Food Sources, Intake Recommendations. Jefferson, NC: McFarland & Company, 1993.
Quesnell, W. R. Minerals : The Essential Link to Health. Long Island, NY: Skills Unlimited Press, 2000.
Wapnir, R. A. Protein Nutrition and Mineral Absorption. Boca Raton, FL: CRC Press, 1990.
American Dietetic Association (ADA). 120 South Riverside Plaza, Suite 2000, Chicago, IL 60606-6995. 1-800/877-1600. <www.eatright.org>.
American Society for Nutrition (ASN). 9650 Rockville Pike, Bethesda, MD 20814. (301) 634-7050. <www.nutrition.org>.
U.S. Department of Agriculture, Food and Nutrition Information Center. National Agricultural Library, 10301 Baltimore Avenue, Room 105, Beltsville, MD 20705. (301) 504-5414. <www.nal.usda.gov>.
Monique Laberge, Ph.D.
Note: This article, originally published in 1998, was updated in 2006 for the eBook edition.
Selenium is a member of the chalcogen family. The chalcogens are elements in Group 16 (VIA) of the periodic table. The periodic table is a chart that shows how chemical elements are related to one another. Other chalcogens are oxygen, sulfur, tellurium, and polonium. The name chalcogen comes from the Greek word chatkos, meaning "ore." The first two members of the family, oxygen and sulfur, are found in most ores.
Selenium is a metalloid. A metalloid is an element that has some characteristics of a metal and some of a non-metal.
Selenium and tellurium are often associated with each other. They tend to occur together in the Earth and have somewhat similar properties. They have many uses in common. In recent years, some important new uses have been found for selenium. It is now used in the manufacture of plain paper photocopiers and laser printers, in photovoltaic cells that convert sunlight into electricity, and in X-ray systems for medical applications.
Group 16 (VIA)
Discovery and naming
Selenium was discovered in 1818 by Swedish chemists Jons Jakob Berzelius (1779-1848) and J. G. Gahn (1745-1818). The men were studying the chemicals used in making sulfuric acid at a plant where they had just become part-owners. Among these chemicals they found a material that they thought was the element tellurium. Tellurium had been discovered some 30 years earlier, mixed with some gold deposits in Hungary.
Tellurium is a rare element. Berzelius decided to study the sample more carefully. He took it back to his laboratory in Stockholm. There, he found that he and Gahn had been mistaken. The substance was similar to tellurium, but it also had different properties. They realized they had found a new element. Berzelius suggested naming the element selenium, from the Greek word selene, for "moon." The name seemed a good choice because the element tellurium is named after the Latin word tellus for "Earth." Just as the Earth and the Moon go together, so do tellurium and selenium.
Selenium exists in a number of allotropic forms. Allotropes are forms of an element with different physical and chemical properties. One allotrope of selenium is an amorphous red powder. Amorphous means "without crystalline shape." A lump of clay is an example of an amorphous material. A second allotrope of selenium has a bluish, metallic appearance. A number of other allotropes have properties somewhere between these two forms.
The amorphous forms of selenium do not have specific melting points. Instead, they gradually become softer as they are heated. They may also change from one color and texture to another.
Selenium from the Greek word for moon, selene.
Some of the most important physical characteristics of selenium are its electrical properties. For example, selenium is a semiconductor. A semiconductor is a substance that conducts an electric current better than non-conductors, but not as well as conductors. Semiconductors have many very important applications today in the electronics industry. Selenium is often used in the manufacture of transistors for computers, cellular phones, and hand-held electronic games.
Selenium is also a photoconductor, a material that changes light energy into electrical energy. Furthermore, it becomes better at making this conversion as the light intensity or brightness increases.
Selenium is a fairly reactive element. It combines easily with hydrogen, fluorine, chlorine, and bromine. It reacts with nitric and sulfuric acids. It also combines with a number of metals to form compounds called selenides. An example is magnesium selenide (MgSe). One of its interesting reactions is with oxygen. It burns in oxygen with a bright blue flame to form selenium dioxide (SeO2). Selenium dioxide has a characteristic odor of rotten horseradish.
Selenium and tellurium are often associated with each other. They tend to occur together in the Earth and have somewhat similar properties.
Occurrence in nature
Selenium is a very rare element. Scientists estimate its abundance at about 0.05 to 0.09 parts per million. It ranks among the 25 least common elements in the Earth's crust. It is widely distributed throughout the crust. There is no ore from which it can be mined with profit. Instead, it is obtained as a by-product of mining other metals. It is now produced primarily from copper, iron , and lead ores. The major producers of selenium in the world are Japan, Canada, Belgium, the United States, and Germany.
There are six naturally occurring isotopes of selenium, selenium-74, selenium-76, selenium-77, selenium-78, selenium-80, and selenium-82. Isotopes are two or more forms of an element. Isotopes differ from each other according to their mass number. The number written to the right of the element's name is the mass number. The mass number represents the number of protons plus neutrons in the nucleus of an atom of the element. The number of protons determines the element, but the number of neutrons in the atom of any one element can vary. Each variation is an isotope.
About a dozen radioactive isotopes of selenium are known also. A radioactive isotope is one that breaks apart and gives off some form of radiation. Radioactive isotopes are produced when very small particles are fired at atoms. These particles stick in the atoms and make them radioactive.
Only one radioactive isotope of selenium is used commercially, selenium-75. This isotope is used to study the function of two organs in the body, the pancreas and the parathyroid gland. (The pancreas helps with digestion and the parathyroid gland releases hormones.) The radioactive selenium is injected into the blood stream. It then goes primarily to one or both of these two organs. The isotope gives off radiation when it reaches these organs. A technician can tell whether the organs are functioning properly by the amount and location of radiation given off.
Selenium is obtained as a by-product from other industrial processes. For example, when copper is refined, small amounts of selenium are produced as by-products. This selenium can be removed from the copper-refining process and purified. Selenium is also obtained as a secondary product during the manufacture of sulfuric acid.
The miracle of copying
W e sometimes forget what an amazing step forward the invention of the copy machine was. Hundreds of years ago, making a copy of a document was a long, difficult process. Some people spent their whole lives making copies of important documents. Each copy was written out by hand. The process was not only dull and monotonous, but it also resulted in many errors.
Even thirty years ago, copying was slow and difficult. For example, carbon paper allowed a person to make one or more copies while writing or typing. But every error had to be corrected on every copy. The copies were often messy and difficult to read.
Mimeograph machines made it possible to reproduce dozens of copies in a few minutes, but required handwritten or typed originals. The final product was printed in purple.
Then came the photocopy machine. Copies could be made by simply placing the original on a glass cover and pushing a button. What goes on inside a copy machine to make this happen?
An essential part of a photocopier is a drum-shaped unit or a wide moving belt. Fine selenium powder is spread on the surface of the drum or the belt. An electric charge is then applied to the selenium.
Another part of the photocopy machine consists of a set of mirrors. When the machine's "Copy" button is pushed, a bright light shines on the page being copied. The light reflects off the white parts of the page. But it is not reflected off the dark parts, such as text or images. The light reflects off the mirrors to the drum or belt.
Selenium is important because when light strikes the charged selenium, the charge disappears. The sections on the drum or belt struck by light have no charge. The sections not struck by light continue to have a charge.
Next, a toner is spread out over the surface of the drum or belt. A toner is usually finely-divided carbon. It sticks to the areas that still carry an electric charge. But it does not stick to the selection without a charge.
Finally, a piece of paper is pressed against the drum or belt. The toner sticks to the paper. A blast of heat causes the carbon to melt and stick tightly to the paper. A copy of the original document is produced by the machine.
The two most important uses of selenium are in glass-making and in electronics. Each accounts for about 30 to 35 percent of all the selenium produced each year. The addition of selenium to glass can have one of two opposite effects. First, it will cancel out the green color that iron compounds usually add to glass. If a colorless glass is desired, a little selenium is added to neutralize the effects of iron. Second, selenium will add its own color—a beautiful ruby red—if that is wanted in a glass product.
Selenium is also added to glass used in architecture. The selenium reduces the amount of sunlight that gets through the glass.
A growing use of selenium is in electronic products. One of the most important uses is in plain-paper photocopiers and laser printers. The element is also used to make photovoltaic ("solar") cells. When light strikes selenium, it is changed into electricity. A solar cell is a device for capturing the energy of sunlight on tiny pieces of selenium. The sunlight is then changed into electrical energy.
Currently, that process is not very efficient. Too much sunlight is lost without being converted into electricity. More efficient solar cells will be able to make use of all the free sunlight that strikes the planet every day.
About a third of all selenium produced is used as pigments (coloring agents) for paints, plastics, ceramics, and glazes. Depending on the form of selenium used, the color ranges from deep red to light orange.
Selenium is also used to make alloys. An alloy is made by melting and mixing two or more metals. The mixture has properties different from those of the individual metals. The addition of selenium to a metal makes it more machinable. Machinability means working with a metal: bending, cutting, shaping, turning, and finishing the metal, for example.
About 5 percent of all selenium produced is used in agriculture. It is added to soil or animal feed to provide the low levels of selenium needed by plants and animals.
Very few compounds of selenium have any important practical applications. One exception is selenium sulfide (SeS2). This compound is used to treat seborrhea, or "oily skin." It is sometimes added to shampoos for people with especially oily hair. Another compound, selenium diethyldithiocarbonate (Se[SC(S)N(C2H5)2]4), is used as a vulcanizing ("toughening") agent for rubber products.
Selenium has some rather interesting nutritional roles. It is essential in very small amounts for the health of both plants and animals. Animals that do not have enough selenium in their diets may develop weak muscles. But large doses of selenium are dangerous. In some parts of California, for example, selenium has been dissolved out of the soil by irrigation systems. Lakes accumulate unusually high levels of selenium and birds and fish in the area develop health problems.
A serious selenium problem occurred at the Kesterson Reservoir in Northern California. In the Late 1970s, scientists found that birds nesting in the reservoir were developing genetic deformities. They traced the problem to high levels of selenium in the water. A large artificial lake was built and the birds were moved to the artificial lake. They were no longer allowed to nest in the dangerous waters of the reservoir.
Selenium (from the Greek word selēnē —the Moon), discovered by Swedish chemist Jöns Jakob Berzelius in 1817, ranks thirty-fourth among elements in Earth's crust. It has six naturally occurring isotopes , a large number of allotropes (elemental forms), and in compounds has oxidation states −2, +4, and +6. The gray elemental form has the unique photoelectric property of exhibiting lowered electrical resistance when struck by light, and it is used in photovoltaic cells and photocells (e.g., light meters) and in xerography. It conducts electricity in a "unipolar" manner, hence it is commonly used in electrical rectifiers. It is also used to tint glass red and to decolorize green glass.
Selenium substitutes for sulfur in amino acids to form seleno-cysteine, cystine, and methionine. The selenium-containing antioxidant glutathione peroxidase is biologically important, and selenium is a necessary trace nutrient in warm-blooded animals. Grazing animals develop a form of muscular dystrophy and other disorders when grazing in areas in which the selenium has been depleted; with selenium-depleted diets, people develop
Keshan disease, a form of cardiomyopathy. When its intake is too high, selenium disrupts enzyme function, causing poor health in mammals and birth defects and reproductive failure in birds and fish. Good sources of selenium in human diets include wheat, garlic, Brazil nuts, and walnuts.
see also Chalcogens.
Barceloux, Donald G. (1999). "Selenium." Journal of Toxicology: Clinical Toxicology 37(2): 145–172.
Frankenberger, William T., and Engberg, Richard A., eds. (1998). Environmental Chemistry of Selenium. New York: Marcel Dekker.
Requirements are of the order of 50 μg/day; in parts of New Zealand, Finland, and China soils are especially poor in selenium and deficiency occurs. In China selenium deficiency is associated with Keshan disease. Rich sources include: fish and shellfish, mung (dahl) and red kidney beans, Brazil nuts, bread, kidney, lentils, liver, pork, rabbit, veal.
Selenium is toxic in excess; mild selenium intoxication results in production of foul‐smelling hydrogen selenide, which is excreted on the breath and through the skin. Intakes above 450 μg/day are considered hazardous.
se·le·ni·um / səˈlēnēəm/ • n. the chemical element of atomic number 34, a gray crystalline nonmetal with semiconducting properties. (Symbol: Se) DERIVATIVES: sel·e·nide / ˈseləˌnīd; -nid/ n.