Selenium

Selenium is a minor mineral that comes in many forms - two forms found in the body are seleno-methionine (selenium analogue of sulfur-containing amino acid) and selenite.¹ Selenium types can be found in inorganic or organic states; seleno-methionine is inorganic and selenite is organic.¹ Other forms include selenate, selenophosphate, and selenocysteine.¹ Selenium is known for its involvement in antioxidant complexes among other functions.¹

Absorption and Excretion: Selenium is absorbed in the small intestine, specifically the duodenum.¹ Selenium (as seleno-methionine and selenite) absorption rate varies between 50-100% within the small intestine; selenium is not absorbed in the stomach.¹ The kidneys regulate selenium excretion through urine (50-60% loss) depending on the amount of selenium stored in the body - selenium is also excreted in feces (40-50%).¹

Absorption of selenium is dependent on other nutrients. For example, heavy metals and sulfates increase the need for selenium in the diet because it substitute selenium.¹ Vitamin A, vitamin C, and vitamin E decrease the need for selenium in the diet because it spares selenium.¹ 

Metabolism and Circulation: Seleno-methionine is bonded to a plasma protein called selenoprotein P once in circulation. Selenoprotein P acts as an antioxidant that transports and stores selenium.¹ Selenite is incorporated into a metalloenzyme called glutathione peroxidase (abbreviated as GPX) which serves many roles and cones in many forms (GPX1, GPX2, GPX3, GPX4).¹

Functions:  

Selenium within the metalloenzyme glutathione peroxidase (GPX) plays a huge role as an antioxidant which includes the following: 

• Reduces organic and hydrogen peroxidases found as waste or by-products within cells such as phagocytes (leukocytes and macrophages) to prevent autodestruction of cells.¹

• Prevents platelet aggregation by reducing fatty acid peroxidase formation and increasing prostacyclin (anti-aggregation factor) compared to thromboxane (pro-aggregation factor); therefore, selenium is linked to reduced clots and atherosclerosis.¹

• Prevents free radical buildup by stabilizing cell membranes, especially in red blood cells that contain an abundance of oxygen. By reducing free radicals, selenium intake is associated with preventing cellular damage that can lead to malignancy.¹

A group of enzymes called thioredoxin reductases contains selenocysteine; they, along with other molecules, are responsible for the regeneration of antioxidants by reducing substrates such as vitamin C and vitamin E.¹

Other enzymes containing selenium is iodothyronine deiodinase  (plays a role in converting thyroxine [T4]  into triiodothyronine [T3]) and selenoprotein W (unknown function within muscles but deficiency causes white muscle disease in animals).¹

Dietary Reference Intake (DRI) - The recommended daily allowance (RDA) of selenium for males and females aged 14-70+ years old is 55 micrograms/day.²

Deficiency/Toxicity- Selenium deficiency is typically found in infants and children causing a disease called Keshan disease; Keshan disease is characterized by muscle degeneration, cataracts, red blood cell damage, and cardiomyopathy.¹ Kashin-Beck disease can develop if skeletal cartilage is affected but may be caused by other molecules such as mycotoxins.¹ Foods grown in selenium-poor soil can lead to deficiency - these diseases are common in some regions of China.¹

Selenium toxicity can come in three forms: acute selenosis, subacute selenosis, and chronic selenosis.¹ Acute selenosis occurs when excess amounts of selenium are consumed in a short time period and subacute selenosis occurs when large amounts are ingested over a long time.¹ Chronic selenosis occurs when moderate amounts of selenium are consumed over a long period of time.¹ Symptoms are dependent on the type of toxicity but includes myocarditis, vision impairment, liver dysfunction, and nerve dysfunction.¹

Dietary Sources of Selenium

Selenium found in foods is dependent on the amount of selenium found in soil or water sources.¹ Some regions have poor selenium concentrations within the soil while other regions have excessive selenium within the soil. This can cause issues with selenium deficiency and toxicity; however, regions or countries with varied international and domestic food distribution do not display symptoms compared to areas solely dependent on the local supply of crops/foods.¹ Selenium is found in adequate quantities within protein-rich foods but is also found in grains, dairy, and some produce.²

Examples of foods containing selenium include:

Meat/Seafood/Animal Product- tuna, sardines, pork, beef, turkey, chicken, egg

Dairy - cottage cheese, yogurt, milk  

Legumes - lentils, beans

Grains/Nuts - Brazil nuts, oatmeal, rice 

Vegetables/Fruit - green peas, bananas, potatoes

Source(s):

1. Denise M Medeiros and Robert E.C. Wildman, Advanced Human Nutrition, 4th ed. (Burlington, MA: Jones & Bartlett, 2019).

2. https://ods.od.nih.gov/factsheets/Selenium-HealthProfessional/