Selenium: Its Benefits, How to Take It, and What Happens When It’s Deficient

Selenium is an essential micronutrient found in soil, water, and foods of both plant and animal origin. It was discovered in 1817 by the Swedish chemist Jöns Jacob Berzelius. Its name comes from the Greek word selene, meaning “moon,” due to its light gray color when melted. The biological role of selenium began to be studied in the 1950s, initially focusing on its toxic effects. Seventeen years later, scientists highlighted its metabolic functions and the consequences of selenium deficiency, marking a new era of research that continues today. In nature, selenium exists in organic forms (selenometionine and selenocysteine) and inorganic forms (selenite and selenate). A balanced diet provides about 70–100 µg of selenium daily. Most absorption occurs in the duodenum (50–80%) and varies depending on the form. Inorganic selenium absorption ranges from 50–100%, while organic selenium is absorbed quickly and almost completely (90%). The best absorption is from selenomethionine in foods of animal origin. Recommended daily doses for healthy adults vary by geographic location. The World Health Organization (WHO) recommends 55 µg/day. Selenium is excreted via urine (60%) and feces (35%), and at high intake levels, also through exhaled gaseous forms. Selenium reserves exist in the form of selenoproteins, accumulating mostly in red blood cells, spleen, liver, and teeth. It is also found in the testes and sperm.

Beneficial Effects of Selenium

Selenium is essential for mammals and several plant species. It is required for the biosynthesis of 25 different selenoproteins. Selenium is incorporated into selenoproteins as the amino acid selenocysteine. The most well-known selenoproteins include the enzymes glutathione peroxidase (the most important and abundant antioxidant enzyme in the human body), thioredoxin reductase, and iodothyronine deiodinase. These selenoenzymes are involved in thyroid hormone metabolism, redox regulation, and protection against oxidative damage. Although the exact mechanism is not fully understood, selenoenzymes, due to their antioxidant role, are considered important in cancer prevention and slowing tumor progression.

Selenium also supports the immune system by stimulating antibody production and increasing the activity of Th cells, cytotoxic T cells, and NK cells. It is involved in cell growth, programmed cell death, and cellular signaling. Selenium modifies the metabolism of some toxic metals (cadmium, lead, arsenic, mercury), reducing their toxicity. It is essential for testosterone biosynthesis and the development of sperm. Testicular tissue contains high concentrations of selenium, which is crucial for sperm quality and male fertility. As part of glutathione peroxidase, selenium protects cell membranes from oxidative damage and, together with vitamin E, helps maintain their integrity. Combined supplementation enhances antioxidant effects and supports fertility. Vitamins A, D, and E promote selenium absorption.

Selenium Deficiency

Selenium deficiency can lead to serious disorders affecting various systems:

• Cardiovascular

• Endocrine

• Immune

• Musculoskeletal

• Nervous

• Reproductive

The importance of selenium was first recognized in 1979 when Keshan disease was linked to selenium-deficient soil in China. Keshan and Kashin-Beck diseases primarily affected women and children in the region. Keshan disease manifests as impaired heart muscle function, while Kashin-Beck disease is characterized by osteoarthritis with cartilage damage in hand and foot joints. Selenium deficiency also affects the thyroid, seen in endemic cretinism in parts of Africa, characterized by iodine and selenium deficiency, hypothyroidism, developmental delays, and reduced intellectual ability. Low selenium levels can increase the activity of certain viruses (Keshan disease), and studies show selenium supports defense against hepatitis B and C and HIV, slowing disease progression. Deficiency has also been associated with poor mood, irritability, and possibly an increased risk of Alzheimer’s disease.

Selenium and the Thyroid

The thyroid contains the highest concentration of selenium (0.2–2 µg/g of tissue). Selenium is crucial for the synthesis, activation, and metabolism of thyroid hormones. It is part of the enzyme iodothyronine deiodinase, which converts the thyroid hormone thyroxine (T4) into its active (T3) and inactive (rT3) forms. Selenium deficiency can disrupt this balance. Its importance is especially noted in autoimmune thyroid diseases. Hashimoto’s thyroiditis is an autoimmune condition characterized by antibodies targeting either the protein thyroglobulin or the enzyme thyroid peroxidase, both essential for hormone production. Studies show that supplementation with 200 µg of selenium for several months can reduce antibody levels by 30–50%, improving overall well-being compared to placebo. Selenium supplementation during pregnancy can be beneficial, particularly in women with antibodies, reducing antibody levels and postpartum thyroid dysfunction. Some countries, such as Germany, recommend 200 µg of selenium daily during pregnancy and for 12 months postpartum for women with positive TPO antibodies. Low selenium intake is also linked to goiter and thyroid nodules. In Graves’ disease, selenium supplementation in patients receiving thyroid medication helps restore normal thyroid function faster.

Selenium in Food

Selenium content in food depends on soil selenium levels, soil type, agro-climatic conditions, and geographic location. Rich sources include meat, dairy, fish, seafood, grains, and nuts. Brazil nuts contain 0.2–512 µg/g, so caution is advised with mixed nut consumption to avoid exceeding the recommended daily intake. Other rich sources include eggs, yeast, bread, mushrooms, and garlic.

Zinc and Selenium

Zinc is a trace element involved in numerous enzymatic systems, cell growth, sexual maturation, reproduction, adaptation to darkness, and wound healing. It participates in vitamin metabolism and sensory functions (taste and smell) and influences brain function. Zinc and selenium are involved in over 300 metabolic reactions in the body and both support the immune system. Deficiency of selenium and zinc can lead to:

• Increased susceptibility to oxidative stress

• Inflammatory bowel disease

• Prolonged time to conception

• Increased inflammation

Proper thyroid function requires several elements besides iodine, including selenium, iron, zinc, copper, and calcium. Zinc, along with selenium, plays a key role in the pathogenesis of Kashin-Beck disease.

Conclusion

Selenium is an essential micronutrient known to science for over 200 years and remains a focus of research. It exists in both inorganic and organic forms, with the best absorption from organic selenomethionine in the duodenum. Major dietary sources include meat, fish, seafood, Brazil nuts, other nuts, and grains. Selenium is a component of selenoproteins that are incorporated into enzymes vital for numerous physiological processes, including glutathione peroxidase, thioredoxin reductase, and iodothyronine deiodinase. Selenium supports cardiovascular, immune, musculoskeletal, nervous, reproductive, and endocrine systems, with particular importance for thyroid function. It regulates thyroid hormone synthesis, reduces antibody levels in Hashimoto’s thyroiditis, and, together with vitamin E, protects cells from oxidative damage. Selenium often works synergistically with zinc, another mineral with antioxidant and immune-supporting roles.