Selenium is a trace element. It is required in minute quantities, and is found in seafood, poultry and eggs. It is also found in other meats, and in cereals. In plant foods the selenium level is dependent on the soil content, which can vary from 0.01 to 1000 microgram/g.
Selenium functions in the body as an antioxidant, taking up electrons in various redox reactions. Antioxidants reduce the damage done to enzymes, proteins and DNA in the body, by potentially toxic molecules such as reactive oxygen species.
The antioxidant properties of selenium are important in cell growth and transformation. It is vital in the regeneration of ascorbic acid from its oxidized form, to play an antioxidant role in various cell processes. Selenium may also be important in muscle function, fertility and preventing prostate cancer. Prolonged oxidative stress is a cause of many chronic illnesses, including cancers of various types, and rheumatoid arthritis.
Selenium is also involved in thyroid metabolism. In addition, it is found as an essential constituent of many proteins, called selenoproteins, which number about 30 in mammals. These include the glutathione peroxidases, selenoprotein P, thioredoxin reductases, and in some deiodinases involved in the conversion of thyroid hormone T4 to the active form T3.
55-70% of food selenium is absorbed by the gut, mainly in the form of selenomethionine and selenocysteine. Absorption is chiefly in the duodenum. Selenium transport in the body is in association with plasma albumin and hemoglobin in the red cell. It is broken down to selenide, and then used for various purposes.
Excessive selenium intake is excreted mostly by the kidneys, while some is removed through the skin, the hair and the feces.
Severe selenium deficiency causes endemic cardiomyopathy, which is seen in some parts of China. It affects mostly women and children. It is called Keshan disease, and manifests as heart enlargement and weak pumping of blood, associated with abnormal heart rhythm, culminating in early death due to heart failure. It can be prevented by supplementing sodium selenite.
Kashin-Beck disease is a cartilage disease common in selenium-deficient areas, affecting adolescents and preadolescents. Its incidence cannot be, however, reduced by selenium supplementation. Combined iodine and selenium deficiency increases cretinism rates. The thinking today is that isolated selenium deficiency does not cause disease, except in interaction with other viral infections, other deficiencies or stress.
Selenium intakes are assessed using selenium in blood, nails, hair, or by measuring the presence of selenoproteins in blood. The required intake of selenium ranges from 12-25 micrograms/day in children to 50-70 micrograms/ day above the age of 18 years. The intake should be increased by about 10 micrograms/day during pregnancy and lactation.
Selenium overdoses are currently described as being above 400 micrograms/day. The common manifestations of chronic selenium toxicity, or selenosis, are brittle hair and nails, with subsequent loss. Other symptoms include nausea, vomiting, diarrhea, a garlicky odor to the breath due to the presence of methylated selenium derivatives, and sometimes mild nerve damage. There is a reported increase in the risk of certain skin cancers, in high-risk individuals who are put on selenium supplements. However, this cannot be extrapolated to the general population.