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Selenium is a chemical element in the periodic table that has the symbol Se and atomic number 34. It is a toxic nonmetal that is chemically related to sulfur and tellurium. It occurs in several different forms but one of these is a stable gray metallike form that conducts electricity better in the light than in the dark and is used in photocells. This element is found in sulfide ores such as pyrite.
Selenium and health
Although it is toxic in large doses, selenium is an essential micronutrient in all known forms of life. 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 thioredoxin reductase. It also plays a role in the functioning of the thyroid gland by participating as a cofactor for thyroid hormone deiodinases . Dietary selenium comes from cereals, meat, fish, and eggs. The recommended dietary allowance for adults is 55 micrograms per day. Liver and Brazil nuts are particularly rich sources of selenium. A list of selenium rich foods can be found on The Office of Dietary Supplements Selenium Fact Sheet.
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 metalic selenides have relatively low toxicities due to their low bioavailability. By contrast, selenate and selenite are very toxic, and have modes 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 and selenocysteine, all of which have high bioavailability and are toxic in large doses.
Selenium poisoning as a result of agricultural runoff was also found to have caused birth defects in birds.
Deficiency
Selenium deficiency is relatively rare in healthy, well-nourished individuals. It can occur in patients with severely compromised intestinal function, or those undergoing total parenteral nutrition. Alternatively, people dependent on food grown from selenium-deficient soil are also at risk. The recommended dietary allowance for adults is 55 micrograms per day.
Selenium defficiency can lead to Keshan disease and Kashin-Beck disease. The primary symptom of Keshan disease is myocardial necrosis, leading to weakening of the heart. Kashin-Beck disease results in atrophy, degeneration and necrosis of cartilage tissue. These diseases are most common in certain parts of China where the soil is extremely deficient in selenium. Studies in Jiangsu Province of China have indicated a reduction in the prevalence of these diseases by taking selenium supplements. Selenium defficiency has also been associated with goitre, cretinism and recurrent miscarriage in humans.
Controversial Health Effects
Cancer
Several studies have suggested a link between cancer and selenium deficiency. A study conducted on the effect of selenium suplementation on the recurrence of skin cancers did not demonstrate a reduced rate of reccurence of skin cancers, but did show a significantly reduced occurrence of total cancers. 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 long-term study of selenium levels in over 60,000 participants did not show any correlation between selenium levels and risk of 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 31% 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 . The SELECT study is currently investigating the effect of selenium and vitamin E supplementation on incidence of prostate cancer.
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 the HIV virus 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 corelate with the progression of the disease and the risk of death. Selenium supplementation may help mitigate the symptoms of AIDS and reduce the risk of mortality. It should be emphasized that the evidence to date does not suggest that selenium can reduce the risk of infection or the rate of spread of AIDS, but rather treat the symptoms of those who are already infected.
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