Selenium in the Human Body
represented by the chemical symbol Se, is a non-metal, chemically related to sulfur 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 center of the enzymes that plays a role in the functioning of the thyroid gland by participating as a cofactor for the three known thyroid hormone named deiodinases.
Se it is the antioxidant element that works with vitamin E to protect cell membranes from oxidative damage.
Although Se 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 odor on the breath, gastrointestinal disorders, and 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 Se and most metallic selenides have relatively low toxicities be/bcause 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 Se per day for a long time, and has an oxidant mode of action similar to that of arsenic.
Some research has indicated a geographical link between regions of Se deficient soils and peak incidences of HIV/AIDS infection. For example, much of sub-Saharan Africa is low in Se. 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 the Se element in the body.
Low Se levels in AIDS patients have been directly correlated with decreased immune cell count and increased disease progression and risk of death. Element Se 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.
Regardless of the cause of depleted Se element levels in AIDS patients, studies have shown that the deficiency does strongly correlate with the progression of the disease and the risk of death.
Some research has suggested that Se supplementation, along with other nutrients, can help prevent the recurrence of tuberculosis.
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.
Experimental findings have demonstrated an interaction between Se and methylmercury, but epidemiological studies have found little evidence that Se helps to protect against the adverse effects of methylmercury.
The substance loosely called Se sulfide, SeS2, actually Se disulfide or Se(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.
Se 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 as well.
Dietary Se comes from Brazil nuts, poultry, seafood, whole-grain product, cereals, onions, garlic, mushrooms, nuts, brown rice, organ meats, 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.
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