Introduction

Selenium is a trace mineral that is essential to good health but required only in small amounts. Selenium is incorporated into proteins to make selenoproteins, which are important antioxidant enzymes. The antioxidant properties of selenoproteins help prevent cellular damage from free radicals. Free radicals are natural by-products of oxygen metabolism that may contribute to the development of chronic diseases such as cancer and heart disease. Other selenoproteins help regulate thyroid function and play a role in the immune system.

Food Sources

Plant foods are the major dietary sources of selenium in most countries throughout the world. The content of selenium in food depends on the selenium content of the soil where plants are grown or animals are raised. In Europe the soil is relatively poor in selenium. Levels of selenium are lowest in Spain, Greece and Eastern Europe and are generally lower in Europe than in the US . In the United States, the soils in the high plains of northern Nebraska and the Dakotas have very high levels of selenium. Soils in some parts of China and Russia have very low amounts of selenium. Selenium deficiency is often reported in those regions because most food in those areas is grown and eaten locally.

Selenium also can be found in some meats and seafood. Animals that eat grains or plants that were grown in selenium-rich soil have higher levels of selenium in their muscle.

Some important food sources of Selenium

Paranuts	Tuna steak	Shrimp	Cod	Sunflower seeds
Cashew nuts	Rice	Walnuts	Eggs	Chicken

Recommended Dietary Allowance (RDA)

The European Union RDA for the general population is set at 55 µg/day.

Inhibitors/stimulators:

The following food components have been found to stimulate the absorption of selenium

Iodine – Selenium deficiency may increase the effects of iodine deficiency. Iodine is essential for the synthesis of thyroid hormone, but the selenoenzymes, iodothyronine deiodinases, are also required for the conversion of thyroxine (T4) to the biologically active thyroid hormone triiodothyronine (T3).

Antioxidant nutrients – As an integral part of the glutathione peroxidases and thioredoxin reductase, selenium probably interacts with every nutrient that affects the antioxidant balance of the cell. Other minerals that are critical components of antioxidant enzymes include copper, zinc (as superoxide dismutase), and iron (as catalase). Selenium as gluthathione peroxidase also appears to support the activity of vitamin E (a-tocopherol) in limiting the oxidation of lipids. Animal studies indicate that selenium and vitamin E tend to spare one another and that selenium can prevent some of the damage resulting from vitamin E deficiency in models of oxidative stress. Thioredoxin reductase also maintains the antioxidant function of vitamin C by catalyzing its regeneration.

Functions in the Body

During selenoprotein synthesis, selenocysteine is incorporated into a very specific location in the amino acid sequence in order to form a functional protein. At least eleven selenoproteins have been characterized, and there is evidence that additional selenoproteins exist.

Glutathione peroxidases

Four selenium-containing glutathione peroxidases (GPx) have been identified: 1) cellular or classical GPx, 2) plasma or extracellular GPx, 3) phospholipid hydroperoxide GPx, and 4) gastrointestinal GPx. Although each GPx is a distinct selenoprotein, they are all antioxidant enzymes that reduce potentially damaging reactive oxygen species (ROS), such as hydrogen peroxide and lipid hydroperoxides, to harmless products like water and alcohols by coupling their reduction with the oxidation of glutathione.

Thioredoxin reductase

In conjunction with the compound thioredoxin, thioredoxin reductase participates in the regeneration of several antioxidant systems, possibly including vitamin C. Maintenance of thioredoxin in a reduced form by thioredoxin reductase is important for regulating cell growth and viability.

Iodothyronine deiodinases (thyroid hormone deiodinases)

The thyroid gland releases very small amounts of biologically active thyroid hormone (triiodothyronine or T3) and large amounts of an inactive form of thyroid hormone (thyroxine or T4) into the circulation. Most of the biologically active T3 in the circulation and inside cells is created by the removal of one iodine atom from T4 in a reaction catalyzed by selenium-dependent iodothyronine deiodinase enzymes. Through their actions on T3, T4, and other thyroid hormone metabolites, three different selenium-dependent iodothyronine deiodinases (types I, II, and III) can both activate and inactivate thyroid hormone, making selenium an essential element for normal development, growth, and metabolism through the regulation of thyroid hormones.

Selenoprotein P

Selenoprotein P is found in plasma and is also associated with vascular endothelial cells (cells that line the inner walls of blood vessels). Although the function of selenoprotein P has not been clearly elucidated, it has been suggested to function as a transport protein, as well as an antioxidant capable of protecting endothelial cells from damage by a reactive nitrogen species (RNS) called peroxynitrite.

Selenoprotein W

Selenoprotein W is found in muscle. Although its function is presently unknown, it is thought to play a role in muscle metabolism.

Selenophosphate synthetase

Incorporation of selenocysteine into selenoproteins is directed by the genetic code and requires the enzyme selenophosphate synthetase. A selenoprotein itself, selenophosphate synthetase catalyzes the synthesis of monoselenium phosphate, a precursor of selenocysteine, which is required for the synthesis of selenoproteins.

Deficiency

Insufficient selenium intake results in decreased activity of the glutathione peroxidases. Even when severe, isolated selenium deficiency does not usually result in obvious clinical illness. However, selenium deficient individuals appear to be more susceptible to additional physiological stresses.

Human selenium deficiency is most notable in 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.

Toxicity

Although selenium is required for health, high doses can be toxic. Acute and fatal toxicities have occurred with accidental or suicidal ingestion of gram quantities of selenium. Clinically significant selenium toxicity was reported in 13 individuals after taking supplements that contained 27.3 milligrams (27,300 µg) per tablet due to a manufacturing error. Chronic selenium toxicity (selenosis) may occur with smaller doses of selenium over long periods of time.

The most frequently reported symptoms of selenosis are hair and nail brittleness and loss. Other symptoms may include gastrointestinal disturbances, skin rashes, a garlic breath odour, fatigue, irritability, and nervous system abnormalities. In an area of China with a high prevalence of selenosis, toxic effects occurred with increasing frequency when blood selenium concentrations reached a level corresponding to an intake of 850 µg/day (approximately 20 times the required dose (RDA)).

Regulation

Absorption of selenium is dependent upon the solubility of the ingested compound and upon the dietary ratio of selenium to sulphur. The availability of selenium for absorption is dependent upon many different factors, among which are the nature of the food source (i.e., seafood is high in selenium, but the selenium is poorly absorbed) and the method of food processing.

Once absorbed, selenium interacts with the sulphur-containing amino acids (e.g., cysteine and methionine) to form the enzyme glutathione peroxidase, and for incorporation into various proteins, such as hemoglobin and myoglobin. Excesses of selenium are secreted in the urine, and the selenium-containing molecule dimethyl selenide is excreted during respiration.