Selenium

Selenium (Se) is a chemical element—number 34 on the periodic table, with properties similar to sulfur, another element in its group. It is a micronutrient that is necessary for proper functioning of several enzymes (selenoproteins) that catalyze chemical reactions involving oxygen. In particular, selenoproteins reduce or prevent the damaging effects of free radicals; they also help regulate thyroid function and play a role in the immune system. Like all essential elements, it is toxic at high levels.

How Selenium Works

Selenium functions at the core of at least 30 selenoproteins, many of which are well-understood. Selenium works in this context by donating or receiving electrons from adjacent molecules in the midst of a highly-structured protein complex. Examples include the glutathione peroxidases (GPXs), thioredoxin reductases (TRs) and iodothyronine deiodinases (DIs) that are capable of modifying cell function by acting as antioxidants and modifying the oxidation/reduction status of other metabolites. ^[1]

Sources

The selenium content of food varies widely according to the local soil content of selenium. In Nebraska and the Dakotas, for example, Se concentrations are relatively high in the soil, the plants, and (unsurprisingly) the animals and people. In contrast, soils in some parts of China and Russia have very low Se levels, and Se deficiency may be a public health concern if the population's food is mostly or entirely locally-derived. Because Se is an element, it is not created or destroyed by living organisms; the amount of Se in the ecosystem is relatively constant. Plants, unlike animals, do not appear to require selenium for survival; they incorporate selenium nonspecifically into compounds that would otherwise contain sulfur. ^[2]

Benefits

Small amounts of selenium are necessary for life. Se supplementation beyond what is found in a usual diet may offer additional benefits; some benefits of dietary Se supplementation are well-documented; others are more speculative.

Selenium and cancer

Observational studies indicate that death from cancer, including lung, colorectal, and prostate cancers, is lower among people with higher blood levels or intake of selenium. In addition, the incidence of nonmelanoma skin cancer is significantly higher in areas of the United States with low soil selenium content. The effect of selenium supplementation on the recurrence of different types of skin cancers was studied in seven dermatology clinics in the U.S. from 1983 through the early 1990s. Taking a daily supplement containing 200 µg of selenium did not affect recurrence of skin cancer, but significantly reduced the occurrence and death from total cancers. The incidence of prostate cancer, colorectal cancer, and lung cancer was notably lower in the group given selenium supplements. ^[3]

Research suggests that selenium affects cancer risk in two ways. As an antioxidant, selenium can help protect the body from damaging effects of free radicals, and Selenium may also prevent or slow tumor growth. Certain breakdown products of selenium are believed to prevent tumor growth by enhancing immune cell activity and suppressing development of blood vessels to the tumor

Selenium and heart disease

Some population surveys have suggested an association between lower antioxidant intake and a greater incidence of heart disease. Evidence also suggests that oxidative stress from free radicals, which are natural by-products of oxygen metabolism, may promote heart disease. For example, it is the oxidized form of low-density lipoproteins (LDL, often called "bad" cholesterol) that promotes plaque build-up in coronary arteries. Selenium is one of a group of antioxidants that may help limit the oxidation of LDL cholesterol and thereby help to prevent coronary artery disease. ^[4]

Selenium and arthritis

Surveys indicate that individuals with rheumatoid arthritis, a chronic disease that causes pain, stiffness, swelling, and loss of function in joints, have reduced selenium levels in their blood . In addition, some individuals with arthritis have a low selenium intake. The body's immune system naturally makes free radicals that can help destroy invading organisms and damaged tissue, but that can also harm healthy tissue . Selenium, as an antioxidant, may help to relieve symptoms of arthritis by controlling levels of free radicals. Current findings are considered preliminary, and further research is needed before selenium supplements can be recommended for individuals with arthritis. ^[5]

Selenium and HIV

HIV/AIDS malabsorption can deplete levels of many nutrients, including selenium. Selenium deficiency is associated with decreased immune cell counts, increased disease progression, and high risk of death in the HIV/AIDS population. HIV/AIDS gradually destroys the immune system, and oxidative stress may contribute to further damage of immune cells. Antioxidant nutrients such as selenium help protect cells from oxidative stress, thus potentially slowing progression of the disease. Selenium also may be needed for the replication of the HIV virus, which could further deplete levels of selenium. ^[6]

Recommended Daily Intake

The recommended daily intake of selenium is 70 micrograms; a "safe and adequate" intake ranges from about 50 to 200 micrograms per day, and this level is probably achieved throughout the developed world. ^[7] As a result, diseases attributable to too much or too little selenium are now relatively rare, but populations on restricted or unusual diets may be at increased risk for inappropriate dietary levels of selenium.

Selenium deficiency

Human selenium deficiency is rare in the U.S. but is seen in other countries, most notably China, where soil concentration of selenium is low . There is evidence that selenium deficiency may contribute to development of a form of heart disease, hypothyroidism, and a weakened immune system . There is also evidence that selenium deficiency does not usually cause illness by itself. Rather, it can make the body more susceptible to illnesses caused by other nutritional, biochemical or infectious stresses. Three specific diseases have been associated with selenium deficiency:

• Keshan Disease, which results in an enlarged heart and poor heart function, occurs in selenium deficient children.
• Kashin-Beck Disease, which results in osteoarthropathy
• Myxedematous Endemic Cretinism, which results in mental retardation

Selenium deficiency has also been seen in people who rely on total parenteral nutrition (TPN) as their sole source of nutrition. TPN is a method of feeding nutrients through an intravenous (IV) line to people whose digestive systems do not function. Forms of nutrients that do not require digestion are dissolved in liquid and infused through the IV line. It is important for TPN solutions to provide selenium in order to prevent a deficiency. Physicians can monitor the selenium status of individuals receiving TPN to make sure they are receiving adequate amounts.

Severe gastrointestinal disorders may decrease the absorption of selenium, resulting in selenium depletion or deficiency. Gastrointestinal problems that impair selenium absorption usually affect absorption of other nutrients as well, and require routine monitoring of nutritional status so that appropriate medical and nutritional treatment can be provided.

Selenium toxicity

Selenium provides another example of the adage "The dose makes the poison" often attributed to Paracelsus. Examples of chronic overexposure to Se come from the upper Great Plains (Nebraska, South Dakota, Wyoming) in the 1930s and Hubei Province in China in the early 1960s. Mild cases exhibited discoloration and decay of teeth, changes in the nails, and gastrointestinal disturbances. Severe cases (mostly in China, from inhaling Se-laden smoke) resulted in hair and nail loss, loss of sensation in the fingers and toes, and increased dental cavities.

High blood levels of selenium (greater than 100 µg/dL) can result in a condition called selenosis . Symptoms of selenosis include gastrointestinal upsets, hair loss, white blotchy nails, garlic breath odor, fatigue, irritability, and mild nerve damage

Selenium toxicity is rare in the U.S. The few reported cases have been associated with industrial accidents and a manufacturing error that led to an excessively high dose of selenium in a supplement. The Institute of Medicine of the National Academy of Sciences has set a tolerable upper intake level (UL) for selenium at 400 micrograms per day for adults to prevent the risk of developing selenosis.

Clinical Trials

Open clinical trials: ClinicalTrials.gov

History

Selenium was discovered by Jöns Jacob Berzelius in 1818, a Swede who was also involved in the discovery and isolation of silicon, thorium, cerium, lithium, and vanadium. ^[8] The name is derived from Selene, the Greek goddess of the moon. For many years it was thought to be biologically inert, until it was identified as the toxicant in animal feeds in South Dakota the early 20th century. ^[9]

Interesting Facts

Selenium poisoning may have played a role in the late arrival of troops to the battle of Little Big Horn. General George Custer was hoping for reinforcements, but a "peculiar sickness" developed in the horses traveling across the plains, through what are now known to be high selenium regions.

References

1. ↑ Beckett GJ, Arthur JR. Selenium and endocrine systems. J Endocrinol. 2005 Mar;184(3):455-65. Abstract Full Text PDF
2. ↑ Rayman MP. The importance of selenium to human health. Lancet. 2000;356(9225):233-241. Abstract
3. ↑ Greenwald P, Anderson D, Nelson SA, Taylor PR. Clinical trials of vitamin and mineral supplements for cancer prevention. Am J Clin Nutr. 2007 Jan;85(1):314S-317S. Abstract Full Text PDF
4. ↑ Flores-Mateo G, Navas-Acien A, Pastor-Barriuso R, Guallar E. Selenium and coronary heart disease: a meta-analysis. Am J Clin Nutr. 2006 Oct;84(4):762-73. Abstract Full Text PDF
5. ↑ Ryan-Harshman M, Aldoori W. The relevance of selenium to immunity, cancer, and infectious/inflammatory diseases. Can J Diet Pract Res. 2005 Summer;66(2):98-102. Abstract
6. ↑ Taylor EW, Cox AG, Zhao L, et al. Nutrition, HIV, and drug abuse: the molecular basis of a unique role for selenium. J Acquir Immune Defic Syndr. 2000 Oct 1;25 Suppl 1:S53-61. Abstract
7. ↑ Levander OA. The global selenium agenda. In: Hurley LS, Keen CL, Lonnerdal B, Rucker RB (eds.) Trace elements in man and animals, Vol 6. New York: Plenum 1988, 1-6
8. ↑ Oldfield JE. The two faces of selenium. J Nutr. 1987 Dec;117(12):2002-8. Abstract PDF
9. ↑ Frankek W. A new toxicant occurring naturally in certain samples of plant foodstuffs. I. Results obtained in preliminary feeding trials. J. Nutr. 1934 8: 597-608