Selenium

Selenium is a non-metal, chemically related to sulphur and tellurium, and rarely occurs in its elemental state in nature. It is toxic in large amounts, but trace amounts of it are necessary for cellular function in most, if not all, animals, forming the active centre of the enzymes glutathione peroxidase and thioredoxin reductase (which indirectly reduce certain oxidized molecules in animals and some plants) and three known deiodinase enzymes (which convert one thyroid hormone to another). Selenium requirements in plants differ by species, with some plants apparently requiring none.

Isolated selenium occurs in several different forms, the most stable of which is a dense purplish-grey semi-metal (semiconductor) form that is structurally a trigonal polymer chain. It conducts electricity better in the light than in the dark, and is used in photocells (see allotropic section below). Selenium also exists in many non-conductive forms: a black glass-like allotrope, as well as several red crystalline forms built of eight-membered ring molecules, like its lighter chemical cousin sulphur.

Selenium is found in economic quantities in sulphide ores such as pyrite, partially replacing the sulphur in the ore matrix. Minerals that are selenide or selenate compounds are also known, but all are rare.

Health Effects

Although it is toxic in large doses, selenium is an essential micronutrient for animals. In plants, it occurs as a bystander mineral, sometimes in toxic proportions in forage (some plants may accumulate selenium as a defence against being eaten by animals, but other plants such as locoweed require selenium, and their growth indicates the presence of selenium in soil). It is a component of the unusual amino acids: selenocysteine and selenomethionine. In humans, selenium is a trace element nutrient which functions as cofactor for reduction of antioxidant enzymes such as glutathione peroxidases and certain forms of thioredoxin reductase found in animals and some plants (this enzyme occurs in all living organisms, but not all forms of it in plants require selenium).

Selenium also plays a role in the functioning of the thyroid gland by participating as a cofactor for the three known thyroid hormone deiodinases.

Dietary selenium comes from nuts, cereals, meat, fish and eggs. Brazil nuts are the richest ordinary dietary source (though this is soil-dependent, since the Brazil nut does not require high levels of the element for its own needs). High levels are found in kidney, tuna, crab and lobster, in that order.

Toxicity

Although selenium is an essential trace element, it is toxic if taken in excess. Exceeding the Tolerable Upper Intake Level of 400 micrograms per day can lead to selenosis. Symptoms of selenosis include a garlic odour on the breath, gastrointestinal disorders, hair loss, sloughing of nails, fatigue, irritability and neurological damage. Extreme cases of selenosis can result in cirrhosis of the liver, pulmonary edema and death.

Elemental selenium and most metallic selenides have relatively low toxicities because of their low bioavailability. By contrast, selenate and selenite are very toxic, the acute toxicity differs from the chronic toxicity which for selenite the chronic toxic dose for human beings is about 2400 to 3000 micrograms of selenium per day for a long time, and have an oxidant mode of action similar to that of arsenic. Hydrogen selenide is an extremely toxic, corrosive gas. Selenium also occurs in organic compounds such as dimethyl selenide, selenomethionine, selenocysteine and methylselenocysteine, all of which have high bioavailability and are toxic in large doses. Nano-size selenium has equal efficacy, but much lower toxicity.

Selenium poisoning of water systems may result whenever new agricultural runoff courses through normally dry undeveloped lands. This process leaches natural soluble selenium compounds (such as selenates) into the water, which may then be concentrated in new "wetlands" as the water evaporates. High selenium levels produced in this fashion have been found to have caused certain congenital disorders in wetland birds.

Deficiency

Selenium deficiency is relatively rare in healthy well-nourished individuals. It can occur in patients with severely compromised intestinal function, those undergoing total parenteral nutrition, and also on advanced aged people (over 90). Alternatively, people dependent on food grown from selenium-deficient soil are also at risk.

Controversial health effects

Cancer: Although several studies have suggested a possible link between cancer and selenium deficiency, the recent SELECT study, comprising a 5 1/2 year study of 35,000 men, did not show significant benefits for selenium supplementation, either alone or in concert with Vitamin E, on incidence of prostate cancer, putting these earlier suggestions of a link in question. An earlier study (of a much smaller group) conducted on the effect of selenium supplementation on the recurrence of skin cancers did not demonstrate a reduced rate of recurrence of skin cancers, but did show a reduced occurrence of total cancers, although without a statistically significant change in overall mortality.

Dietary selenium prevents chemically induced carcinogenesis in many rodent studies. I It has been proposed that Selenium may help prevent cancer by acting as an antioxidant or by enhancing immune activity. Not all studies agree on the cancer-fighting effects of selenium. One study of naturally occurring levels of selenium in over 60,000 participants did not show a significant correlation between those levels and cancer. The SU.VI.MAX study concluded that low-dose supplementation (with 120 mg of ascorbic acid, 30 mg of vitamin E, 6 mg of beta carotene, 100 µg of selenium, and 20 mg of zinc) resulted in a 30% reduction in the incidence of cancer and a 37% reduction in all cause mortality in males, but did not get a significant result for females. However, there is evidence that selenium can help chemotherapy treatment by enhancing the efficacy of the treatment, reducing the toxicity of chemotherapeutic drugs, and preventing the body's resistance to the drugs. Studies of cancer cells in vitro showed that chemotherapeutic drugs, such as Taxol and Adriamycin, were more toxic to strains of cancer cells grown in culture when selenium was added.

HIV/AIDS: Some research has indicated a geographical link between regions of selenium deficient soils and peak incidences of HIV/AIDS infection. For example, much of sub-Saharan Africa is low in selenium. However, Senegal is not, and also has a significantly lower level of AIDS infection than the rest of the continent. AIDS appears to involve a slow and progressive decline in levels of selenium in the body. Whether this decline in selenium levels is a direct result of the replication of HIV or related more generally to the overall malabsorption of nutrients by AIDS patients remains debated. Low selenium levels in AIDS patients have been directly correlated with decreased immune cell count and increased disease progression and risk of death. Selenium normally acts as an antioxidant, so low levels of it may increase oxidative stress on the immune system leading to more rapid decline of the immune system. Others have argued that HIV encodes for the human selenoenzyme glutathione peroxidase, which depletes the victim's selenium levels. Depleted selenium levels in turn lead to a decline in CD4 helper T-cells, further weakening the immune system.

Regardless of the cause of depleted selenium levels in AIDS patients, studies have shown that selenium deficiency does strongly correlate with the progression of the disease and the risk of death.

Tuberculosis: Some research has suggested that selenium supplementation, along with other nutrients, can help prevent the recurrence of tuberculosis.

Diabetes: A well-controlled study showed that selenium intake is positively correlated with the risk of developing type II diabetes. Because high serum selenium levels are positively associated with the prevalence of diabetes, and because selenium deficiency is rare, supplementation is not recommended in well-nourished populations such as the U.S.

Mercury: Experimental findings have demonstrated an interaction between selenium and methylmercury, but epidemiological studies have found little evidence that selenium helps to protect against the adverse effects of methylmercury.

Biological Applications

Medical use: The substance loosely called selenium sulfide, SeS₂, actually selenium disulfide or selenium (IV) sulfide, is the active ingredient in some dandruff shampoos. The effect of the active ingredient is to kill the scalp fungus Malassezia which causes shedding of dry skin fragments. The ingredient is also used in body lotions to treat Tinea versicolor due to infection by a different species of Malassezia fungus.

Nutrition: Selenium is used widely in vitamin preparations and other dietary supplements, in small doses (typically 50 to 200 micrograms per day for adult humans). Some livestock feeds are fortified with selenium as well.

Selenium is found in the following products:

ANDRO-VITO

APHRO-VITA