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Selenium 101: Testing, Top Foods, and Supplements

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Selenium 101: Testing, Top Foods, and Supplements

Selenium is the 34th element on the periodic table and is typically found in areas with dryer soil. In the United States, the highest amount of selenium is found in Wyoming and South Dakota. Selenium is essential for both animals and humans as it plays a pivotal role in many biological processes. This article will discuss what selenium is, its role in the body, how to test for it, and how to ensure proper amounts in your diet.


What is Selenium?

Selenium can exist as a silvery metal or a red powder. Named after the Greek goddess of the moon, Selene, selenium is most commonly used industrially as a glass additive. It’s also used to make stainless steel and as a coloring agent for paint, plastics, and ceramics. The majority of selenium produced is a byproduct of copper refining. 

Selenium is a required nutrient for humans. The human body contains around 14 milligrams of selenium, with 28-46% of selenium stored in the body located in the skeletal muscle. 

What is Selenium's Role in The Body?

Selenium is a part of enzymes called selenoproteins. Over 24 different types of selenoproteins in the body play a role in thyroid, reproductive, immune processes, DNA creation, and antioxidant actions. The types of selenoproteins are glutathione peroxidases, iodothyronine deiodinases, selenophosphate synthetase 2, Methionine-R-sulfoxide reductase B1, 15 kDa selenoprotein, and selenoprotein K, M, N, P, S, T, and W. Let’s discuss some of the most well-known selenoproteins. 

Glutathione Peroxidases

Glutathione peroxidases are one type of selenoprotein. These enzymes have potent antioxidant properties. Antioxidants neutralize reactive oxygen species (ROS), which, if not neutralized, can cause inflammation and DNA damage, among other things. Glutathione peroxidases, along with other selenoproteins, are especially important for male fertility as sperm production and function are dependent upon protection from these enzymes.  

Iodothyronine Deiodinase

Iodothyronine deiodinase enzymes are another type of selenoprotein involved in thyroid physiology. These enzymes are essential in the conversion of the inactive thyroid hormone thyroxine (T4) into the more active hormone triiodothyronine (T3). 

Selenoprotein P

Selenoprotein P regulates selenium levels in the testes and brain and has both direct and indirect properties, as it activates other antioxidants, including the above-mentioned glutathione peroxidases. Additionally, selenoprotein P may aid in blood sugar regulation, including insulin sensitivity.

Selenoprotein W

Selenoprotein W is found in high concentrations in the skeletal and cardiac muscles. It has been found to protect neurons from cell death. 

Methionine-R-sulfoxide reductase B1 (MSRB-1)

MSRB-1 aids in resuscitating proteins that free radicals have damaged. Additionally, MSRB-1 is required for the proper function of macrophages, a type of white blood cell.

Selenoprotein S

Selenoprotein S plays a role in immune and inflammatory regulation. Genetic polymorphisms that lead to a change in the function of the selenoprotein S gene have been associated with increased susceptibility to coronary artery disease, gastrointestinal cancers, preeclampsia, and other conditions. 

How to Test Selenium Levels

Selenium levels can be tested in the blood. The reference range for serum selenium levels is 120-160 micrograms/liter, although this may vary between labs. 

Additionally, selenium is found on micronutrient panels such as the Micronutrient Test by SpectraCell Laboratories

Early symptoms of chronically high levels of selenium, known as selenosis, may cause a metallic taste in the mouth and a garlic-odor breath. Brittle hair and nails and hair and nail loss are the most common symptoms of selenosis. Other symptoms include mottled teeth, skin rashes, nausea, diarrhea, fatigue, irritability, and nervous system abnormalities. Selenium toxicity can cause symptoms ranging from hair loss, facial flushing, muscle tenderness, and lightheadedness to acute respiratory distress syndrome, myocardial infarction, kidney failure, cardiac failure, and death. There have been documented cases of toxicity as a result of improper supplement labeling, leading to unknowingly extremely high doses. It is also thought that regular consumption of Brazil nuts may also lead to toxicity due to their high selenium content. 

Selenium deficiency is most commonly seen in people with low animal protein diets who live in areas of low selenium content in the soil. People with HIV and those undergoing kidney dialysis are also at a higher risk of selenium deficiency. Selenium deficiency in isolation may not cause any symptoms. Instead, selenium deficiency paired with other imbalances or stressors may result in symptoms. Selenium deficiency can worsen iodine deficiencies, raising the risk for an iodine deficiency condition called cretinism which results in severe neurological and physical developmental delays in infants. Selenium deficiency may also affect male fertility and a type of osteoarthritis called Kashin-Beck disease.

How to Make Sure You are Getting Enough Selenium in Your Diet 

The Institute of Medicine of the National Academies established a Food and Nutrition Board, which created recommendations for vitamins and minerals based on research, age, sex, and other factors. Recommended Dietary Allowances (RDAs) are one such measurement that has been created and is defined as an amount of a given nutrient that would meet nutritional requirements for the majority of healthy people. Additionally, tolerable upper intake levels (ULs), the largest amount of a given nutrient that will not cause any adverse effects, have also been set for selenium. Below are the RDAs for Selenium based on sex and age:

Selenium RDA

Due to the lack of sufficient evidence, RDAs were not created for those under one year of age. In place of RDAs, Adequate Intakes (AIs) were created, which are levels that are assumed adequate to meet nutritional adequacy. The AIs for males and females under six months of age are 15 mcg and 20 mcg for those 7 to 12 months of age. 

Additionally, pregnant and lactating women often have different nutrient requirements due to higher demands. The RDA of selenium for pregnant women is 60 mcg and 70 mcg for lactating women. The UL for both pregnant and lactating women is 400 mcg. 


Since selenium is found naturally in the soil, many foods contain selenium. Below is a table of the top selenium-containing foods. 

Top Ten Selenium-Containing Foods


Selenium is found in multivitamins and is also available as a singular supplement. Multivitamin dosages tend to mirror RDAs discussed above. Stand-alone selenium supplements are also offered in these dosages, in addition to 200mcg, as studies have shown 200mcg to be beneficial for certain conditions. However, caution is advised with long-term supplementation at 200 mcg as this dose has been associated with increasing the risk of diabetes.



Selenium is an important mineral involved in various processes in the body. Inadequate amounts of selenium can alter physiological functioning, and excessive amounts can have grave repercussions. Therefore, testing selenium levels, especially prior to or during supplementation, can be helpful to ensure levels are appropriate. 

The information provided is not intended to be a substitute for professional medical advice. Always consult with your doctor or other qualified healthcare provider before taking any dietary supplement or making any changes to your diet or exercise routine.
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