Table of Contents
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
(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
- 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.
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>.
Office of Dietary Supplements, National Institutes of Health. National Institutes of Health, Bethesda, Maryland 20892 USA. <ods.od.nih.gov>.
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.