Selenium is an essential micronutrient that can be found in soil, water, and plant and animal foods. It was discovered in 1817 by the Swedish chemist Jons Jacob Berzelius. It was named after the Greek word selene which means the moon, thanks to its light gray color when melting. The biological role of selenium began to be explored in the 1950s with a focus on the toxic effects of this element. 17 years later, scientists began to stress its metabolic function and the consequences of selenium deficiency, whereby a new era of research started that continues even today.
It appears in organic (selenomethionine and selenocysteine) and inorganic (selenite and selenate) forms in the nature.
With a good and balanced diet, we take 70-100 µg of selenium. Absorption mainly takes place in the duodenum (50-80%) and changes depending on the form.
The absorption of inorganic selenium varies from 50 to 100%. Organic selenium is absorbed quickly and almost completely (90%). The best absorption is in the form of selenomethionine from food of animal origin.
The recommended daily doses of selenium for a healthy adult vary by geographical location. The World Health Organization (WHO) recommends 55 µg/day.
Selenium is excreted in the urine (60%) and faeces (35%), and in the case of elevated intake also by exhaling the gaseous form. The reserves are in the form of selenoproteins, with the largest accumulation in erythrocytes, spleen, liver and teeth. It is also proven in testes and sperm.
Selenium is essential for mammals and higher plant species.
It is required for the biosynthesis of 25 various selenoproteins. It is incorporated into selenoproteins as part of the amino acid of selenocysteine.
Some of the most well-known selenoproteins are the enzymes glutathione peroxidase (the most important and prevalent antioxidant enzyme in the human body), thioredoxin reductase and iodothyronine deiodinase.
These selenoenzymes are involved in thyroid hormone metabolism, regulation of redox states, and protection against oxidative damage. Although the exact mechanism of action has not yet been fully explained, selenoenzymes, due to their antioxidant role, appear to be an important factor in carcinogenesis and reduced tumor progression.
Selenium stimulates the immune system: stimulates the production of antibodies, increases the activity of Th cells, cytotoxic T cells and NK cells.
Selenium is involved in cell growth, programmed cell death, and cell signaling.
It changes the metabolism of some toxic metals (cadmium, lead, arsenic, mercury), thereby reducing their toxicity.
It is required for testosterone biosynthesis, as well as for sperm production and development. The testicular tissue contains high concentrations of selenium and is responsible for sperm quality and male fertility.
Selenium, as a component of glutathione peroxidase, protects cell membranes from oxidative damage and, together with vitamin E, helps preserve their integrity. Their simultaneous application results in enhanced antioxidant activity and thus affects fertility.
Vitamins A, D and E promote selenium absorption.
Selenium deficiency in the human body can lead to serious diseases of various systems: cardiovascular, endocrine, immune, musculoskeletal, nervous and reproductive systems.
The importance of selenium for the human body was observed in 1979 when Keshan disease was linked to a deficiency of selenium in the soil in the region of China. Keshan and Kashin-Beck diseases have been observed primarily in women and children in that region. Keshan disease is manifested in decreased function of the heart muscle, and the occurrence of Keshan-Beck disease is characterized by osteoarthritis with cartilage damage in the joints of the hands and feet.
The effect of selenium deficiency on the thyroid gland has been observed when endemic cretinism occurred in some regions of Africa. It is characterized by iodine and selenium deficiency, hypothyroidism, developmental difficulties and decreased intellectual capacity.
Selenium deficiency leads to an increased activity of some viruses (Keshan disease), and selenium in people with hepatitis B and C and infected with HIV has been shown to support the defense of the body and thus slows down the progression of the disease.
Studies have shown that selenium deficiency can lead to bad mood and irritability, and is even linked to the development of Alzheimer’s disease.
The thyroid gland is the organ with the highest content of selenium (0.2-2 µg/g tissue).
Selenium is crucial in the synthesis, activation and metabolism of the thyroid hormone.
It is an integral part of the thyroxine deodinase enzyme responsible for the translation of thyroid hormone thyroxine (T4) into its active (triiodothyronine, T3) and inactive (rT3) form. Selenium deficiency can lead to disturbances in the balance of this process.
High priority is given to selenium in autoimmune thyroid diseases. Hashimoto’s thyroiditis is an autoimmune thyroid disease characterized by the presence of antibodies.
Antibodies can be targeted to the protein thyroglobulin or to the enzyme thyroid peroxidase, two components necessary for hormone production.
Various studies conducted on subjects who were prescribed thyroid medication (levothyroxine) showed that selenium supplementation at a dose of 200 µg for several months (three or more) resulted in a significant decrease in antibody levels (30-50%). Patients noted a better general condition of the organism relative to the placebo group.
Selenium supplementation during pregnancy can be beneficial, especially for pregnant women with antibodies present. Studies show a decrease in antibody levels and a decreased incidence of thyroid abnormalities in the postpartum period. Some countries, such as Germany, have cited in their guidelines selenium supplements at a dose of 200 µg in the morning during pregnancy and 12 months after birth in women with a positive TPO antibody finding as postpartum hypothyroidism prevention.
Low doses of selenium are also associated with the appearance of an enlarged thyroid gland and the formation of thyroid nodules.
In Graves’ disease, a form of hyperthyroidism, selenium supplementation in patients undergoing thyroid medication has been shown to help restore its normal functioning more quickly.
The amount of selenium in food depends on the location of the soil on which the plants grew and the animals lived. It is determined by the amount of selenium in the soil in a given area, soil type, agro-climatic conditions and geographical location.
The richest source of selenium is meat, dairy products, fish, seafood, cereals and nuts. Brazil nut contains 0.2-512 µg/g, therefore caution should be exercised when consuming mixtures of nuts containing it to prevent exceeding the recommended daily intake of selenium.
Eggs, yeast, bread, mushrooms and garlic are an abundant source of selenium.
Zinc is a trace element involved in a large number of enzymatic systems. It is involved in cell growth, sexual maturation, reproduction, adaptation to darkness, and wound healing. It participates in the metabolism of some vitamins and sensory functions (taste and smell). It affects the brain function.
Zinc and selenium play a role in more than 300 metabolic reactions in the body. Like selenium, it is repeatedly involved in the immune system.
The impact of reduced selenium and zinc level:
Besides iodine, the thyroid gland requires several other elements, including selenium, iron, zinc, copper, and calcium for its proper function. Zinc, along with selenium, is thought to play a key role in the pathogenesis of Kashin-Beck disease.
Selenium is an essential micronutrient. Although it has been known to scientists for 200 years, it is still the subject of much research.
It occurs in nature in inorganic and organic form. Absorption takes place in the duodenum and is best when taking the organic form of selenomethionine. The richest sources of selenium in food are meat, fish, seafood, Brazil nut and other nuts and cereals.
Selenium is an integral part of selenoproteins that are incorporated into enzymes required for numerous physiological processes in the body. The best known are glutathione peroxidase, thioredoxin reductase, and iodothyronine deodinase.
Selenium affects the function of the cardiovascular, immune, musculoskeletal, nervous and reproductive and endocrine systems. Emphasis is placed on its effect on thyroid function. It is involved in the regulation of thyroid hormone synthesis and is associated with decreased antibody titre in Hashimoto’s thyroiditis.
Synergistic antioxidant action with vitamin E protects cells from damage and is often combined with zinc, the mineral that is said to have antioxidant properties and a positive effect on the immune system.