What Is Selenium and Why Is It Important for Health?

Selenium plays a central role among micronutrients, as it is involved in numerous physiological processes. From A for immunity to Z for cell metabolism, selenium is an essential nutrient for your health.

What is Selenium?

The vital trace element selenium occurs as a chemical element in the soil. After selenium has been absorbed by the metabolism of various plants, it is available for human consumption in the form of plant and animal sources of selenium.

What does selenium do in the body?

While the micronutrient is essential for the normal functioning of your immune system and thyroid gland, selenium also plays a role in reproduction and is needed for hair and nails.

Bioactive dietary supplements are suitable for avoiding nutritional deficiencies, both preventively and therapeutically in cases of diagnosed selenium deficiency.

Below, you will learn how versatile your body's need for this trace element is, how to supply your metabolism with sufficient selenium, and how to benefit optimally from its health effects.

What forms of selenium are there?

The trace element selenium rarely occurs in its elemental form, but mainly as an inorganic and organic compound in the oxidation states -2, +4, and +6. [1] 

Inorganic selenium: In addition to selenate and selenite, selenide is also one of the inorganic selenium compounds, although it is less common. While inorganic selenium is found in varying concentrations in soil and water, organic selenium occurs as a component of plants and animal tissue.

Organic selenium: Organic selenium compounds include selenocysteine and selenomethionine, combinations with amino acids.

Selenium, which is essential for nutrition, is part of more than two dozen selenoproteins (proteins containing selenium) that are indispensable for reproduction, immune support, thyroid function, and DNA synthesis.

Selenium is also incorporated into enzymes with antioxidant properties in the body. [2] 

Important forms of selenium at a glance:

• Selenomethionine
• Sodium selenite
• Selenocysteine
• Sodium selenate

Inorganic selenium: Occurrence in soil

Plants absorb inorganic selenate and selenite from the soil and convert them into organic forms.

Selenium in food: During plant conversion, inorganic selenium is converted into organic selenocysteine and selenomethionine as well as their methylated derivatives (biochemical modifications). These are available to your body as valuable sources of selenium in plant-based foods and also in animal-based foods such as eggs, meat, and fish.

Organic selenium: Occurrence in human and animal tissue

The skeletal muscles are the main storage site, as up to 50 percent of the selenium supply is found here. Selenium is also stored in the thyroid gland, heart, liver, kidneys, brain, and blood.

Organic selenium: Selenium occurs in organic form in animal and human tissue as selenium-containing amino acids – as L-selenomethionine and L-selenocysteine.

Selenium proteins: While selenocysteine is a specific component of selenoproteins, selenomethionine is incorporated into proteins in a non-specific manner. To date, 25 selenoproteins with various, mostly antioxidant properties have been identified. In addition, there are organic selenides such as dimethyl selenide. [1] 

Which selenium is best? L-selenomethionine has the advantage that the micronutrient is easily absorbed thanks to its connection with methionine. Inorganic sodium selenite, on the other hand, can be efficiently excreted by your body via urine and breath a preventive factor for selenium poisoning. [2] This aspect is particularly important when taking selenium-containing supplements in high doses over a long period of time.

What are the properties of selenium?

Selenium is essential from birth, as it supports numerous metabolic processes and later also spermatogenesis (sperm formation) in men. But there are numerous other functions of selenium that explain why it is considered healthy.

Selenium effect: [3] 

• Selenium contributes to normal thyroid function
• Selenium helps protect cells from oxidative stress
• Selenium contributes to the normal function of the immune system (physical defense)
• Selenium contributes to the maintenance of normal hair
• Selenium contributes to the maintenance of normal nails
• Selenium contributes to normal sperm formation (formation of sperm cells)

Selenium contributes to the maintenance of normal hair and nails

A selenium deficiency can lead to reduced hair growth, hair loss, and white spots on the nails.

Dietary supplements, hair shampoo, and more: In addition to dietary supplements, hair care products such as shampoo and hair tonic, nail care products, and hand cream are also enriched with selenium.

Selenium and hair: In this regard, the antioxidant properties of the trace element come into play at the hair follicles, reducing cell damage caused by free radicals.

Selenium and nails: With regard to nails, selenium's ability to reduce oxidative stress at the matrix (nail root) is also relevant.

Selenium contributes to normal thyroid function

Your thyroid gland forms a hormonal control center through interaction with the hypothalamus and pituitary gland. [4] 

Influence of selenium on thyroid function: Selenium is involved in the synthesis of thyroid hormones and acts as a building block for certain enzymes involved in hormone production. [5] Outside the thyroid gland, selenium supports the conversion of thyroxine (T4) into its active form, triiodothyronine (T3). [6] 

Risk of selenium deficiency: In a deficiency situation, the remaining reserves are concentrated in the tissue of the thyroid gland and brain.

Selenium-containing dietary supplements can have a supportive effect in cases of thyroid dysfunction.

Selenium helps protect cells from oxidative stress

Just like the vitamin trio ACE, selenium is an antioxidant, as it is a component of enzymes that protect your body from cell damage. [7] Some selenoproteins are also involved in DNA synthesis and the repair of DNA damage. [8] 

We are constantly exposed to free radicals from exhaust fumes and other environmental toxins, as well as UV light and aggressive oxygen molecules from metabolic waste products, alcohol, medications, and nicotine.

Selenium helps protect cells from oxidative stress by neutralizing harmful oxygen molecules. Even physical activity, which generally has a positive effect on health and well-being, generates oxidative stress at the cellular level.

Selenium contributes to normal immune system function

Selenium can modulate the immune response and support the synthesis of antibodies, lymphocyte division, and T-cell activity. Selenoproteins, in turn, play a role in immune function by modulating the proliferation and activity of immune cells. [8] 

Selenium and immune defense: Long-term deficiency impairs the function of the immune system. [6] In addition, there is debate about the extent to which selenium has anti-inflammatory properties.

Selenium contributes to normal sperm formation

Selenium plays a crucial role in reproduction: Chronic selenium deficiency disrupts sperm synthesis. [6] 

Selenium and male fertility: Selenium influences male fertility because it is involved in the formation, development, and motility of sperm, as well as in the synthesis of testosterone.

Selenium and female fertility: There is debate as to whether selenium can also improve female reproductive capacity and whether selenium deficiency is associated with an increased likelihood of premature births and miscarriages. [9] 

What should be considered when dosing selenium?

The German Nutrition Society (DGE) provides estimated values for the recommended daily intake of selenium for different age groups.

DGE reference values for daily selenium requirements: [10] 

• Infants under 4 months: 10 µg
• Infants 4 to 11 months: 15 µg
• Children aged 1 to 3 years: 15 µg
• Children aged 4 to 6 years: 20 µg
• Children aged 7 to 9 years: 30 µg
• Children aged 10 to 12 years: 45 µg
• Teenagers aged 13 to 14 years: 60 µg

Gender-specific selenium dosage: From the age of 15, the daily selenium requirement varies. While the daily selenium dose for male teenagers and adults is a uniform 70 µg and 60 µg for female teenagers and adults, the selenium requirement increases to 75 µg during breastfeeding. [10] 

When is there a risk of selenium overdose?

Selenium overdose is unlikely, but supplements should not be taken in excessive doses.

Selenium overdose: Undesirable effects may occur in the digestive tract, skin, heart, kidneys, and nervous system.

Selenium overdose symptoms:  [10] [11] [12] 

• Metallic taste in the mouth
• Digestive disorders
• Neurological symptoms
• Hair loss and nail loss
• Body odor and bad breath
• Skin rashes and facial redness
• Liver damage
• Stained teeth
• Muscle pain
• Fatigue and irritability
• Shortness of breath
• Heart attack and heart failure
• Kidney failure

Selenium poisoning: The most common clinical symptoms of a rare, chronically elevated selenium intake (selenosis) include hair and nail loss or brittleness.

Tolerable Upper Intake Level (UL) of selenium: According to the European Food Safety Authority (EFSA), a daily intake of 255 µg of selenium is considered tolerable for adults without risk of harmful side effects. [13] 

Caution with Brazil nuts: Consuming more than 1 to 2 Brazil nuts per day should be avoided, as the nuts contain high doses of selenium and radioactive radium, and health risks cannot be ruled out. [7] 

Selenium in food and as a dietary supplement

Selenium levels vary due to various factors. In Europe, for example, the soil is considered to be low in selenium. [7] 

A sufficient intake of selenium can be supported by selenium capsules to prevent nutritional deficiencies. For optimal bioavailability, the selenium compound is crucial when selecting a suitable supplement.

The selenium content in food depends on various factors

The selenium concentration in food depends not only on the selenium content of the soil, but also on other factors.

• Influence on the selenium content of plant sources: The soil pH value and the presence of organic soil substances influence the selenium content. The form of selenium also determines whether it is optimally absorbed by the plant.
• Influence on the selenium content of animal sources: The selenium content in meat and other animal products varies depending on the selenium content of the feed. However, animal feed may be enriched with selenium in the European Union. [14]

Selenium in food: The best sources

Foods rich in selenium include nuts, legumes, and fish, as well as vegetables, eggs, and meat.

Getting enough selenium is not a problem with a vegan diet if legumes, nuts, rice, and other foods are eaten regularly.

Whole grain products rich in fiber are also a good source of selenium. However, according to the DGE, grain products from the USA are richer in selenium than European grains. [7] 

Selenium in food: The following overviews provide a helpful summary of which foods are suitable for meeting your selenium requirements.

TOP 10 vegan sources of selenium:

• Coconut up to 810 µg in 100 g
• Brazil nuts 100 to 200 µg in 100 g
• Soybeans 19 µg in 100 g
• White beans 14 µg in 100 g
• Oatmeal 10 µg in 100 g
• Brown rice 10 µg in 100 g
• Lentils 9.8 µg in 100 g
• Chickpeas 9 µg in 100 g
• Bell peppers 4.3 µg in 100 g
• White cabbage 2.4 µg in 100 g

TOP 10 animal sources of selenium:

• Tuna 82 µg in 100 g
• Sardines 58 µg in 100 g
• Pork liver 53 µg in 100 g
• Shrimp 50 µg in 100 g
• Redfish 44 µg in 100 g
• Chicken liver 40 µg in 100 g
• Mackerel 39 µg in 100 g
• Beef 20 to 40
• Emmental cheese 11 µg in 100 g
• Eggs 10 µg in 100 g

Selenium as a dietary supplement: The selenium compound is crucial

Selenium is available as a multivitamin and as a single supplement – often as selenium methionine or in the form of enriched yeast and as sodium selenite or sodium selenate.

Optimal selenium compound: Selenium methionine is highly bioavailable and is recommended as a dietary supplement.

The dosage form is not necessarily important

For optimal bioavailability, it is not primarily the dosage form that is crucial, but the respective compound.

 Buying selenium: When buying selenium, make sure that single preparations and selenium complexes contain bioactive selenium methionine.

Avoid overdosing, as only selenite and selenate are optimally excreted in excess. [2] 

Taking selenium in combination with other nutrients: It's all about the combination

When taking dietary supplements, interactions between micronutrients can occur. While unfavorable combinations with selenium should be avoided, you can also benefit from synergistic effects.

These nutrients form a good synergy with selenium

To improve bioavailability, it makes sense to combine selenium.

• Selenium and vitamin E to support cells: Selenium and vitamin E help protect cells from oxidative stress and have synergistic properties. [15]
• Selenium and zinc for your immune system: Both micronutrients contribute to the normal functioning of the immune system. Zinc does not compete with selenium, but rather has a synergistic effect.
• Selenium and iodine for your thyroid: While the duo contributes to normal thyroid function, iodine also contributes to normal thyroid hormone production and normal energy metabolism.
• Selenium and vitamin C for immune support: Both micronutrients contribute to the normal function of the immune system and help protect cells from oxidative stress.

Caution: Inorganic sodium selenite should not be taken together with vitamin C, as it reduces sodium selenite to an ineffective form. [2] 

These nutrients can impair the absorption of selenium

In addition to the synergistic effects of selenium, there are also unfavorable interactions.

Avoid the following combinations:

• Calcium and iron reduce the availability of selenium: Since high amounts of calcium and iron reduce selenium absorption, you should not combine these micronutrients. Consume calcium- and iron-rich meals at a sufficient interval, at least 2 hours from selenium intake.
• Phytic acid and sulfur-containing foods inhibit selenium absorption: Phytic acid from whole grains and legumes, as well as garlic, onions, and cabbage vegetables, can also reduce selenium availability.

When is selenium deficiency diagnosed?

A distinction is made between acute and chronic selenium deficiency. Depending on the severity and cause, characteristic selenium deficiency symptoms occur.

Diagnosis of selenium deficiency: A selenium deficiency can be detected through laboratory diagnostic analyses. A value of less than 60 µg/liter of blood plasma is considered an indication of selenium deficiency.

What contributes to selenium deficiency?

There are various factors that increase the risk of selenium deficiency.

Causes of selenium deficiency:

• Metabolic disorders (see “Risk groups for selenium deficiency”)
• Interactions with medications and dietary supplements
• Selenium-deficient soils
• Vegan diet without intake of plant sources such as nuts, legumes, etc.
• Interactions with food components

Causes of selenium deficiency: Soil content and diet can have an influence

The primary source of selenium is food, followed by water and air.

• Varying selenium content: People around the world consume different amounts of selenium, depending on the region they live in. This is why selenium levels can vary greatly within the same food. Accordingly, some population groups have an increased risk of selenium deficiency.
• Nutritional tips for adequate selenium intake: Individual dietary habits play a decisive role in ensuring adequate selenium intake. You should consume the dietary supplements listed under “Selenium in food: the best sources” on a regular basis.
• Optimized selenium intake: Also consider the tips presented in this selenium guide on synergistic and undesirable effects with micronutrients and the like.

What are the symptoms of selenium deficiency?

Depending on the severity of the selenium deficiency, the symptoms of temporary selenium deficiency differ from those of chronic selenium deficiency.

General symptoms of temporary selenium deficiency

There are selenium deficiency symptoms that can occur even with an initial undersupply.

Symptoms of selenium deficiency:

• Fatigue and exhaustion
• Muscle weakness
• Hair loss and brittle nails
• Pale, dry skin
• Concentration problems
• Susceptibility to infections

Specific symptoms may occur in cases of severe or prolonged selenium deficiency

If an initial selenium deficiency is not corrected, chronic selenium deficiency can develop with specific symptoms such as reduced fertility and dysfunction of the immune system and thyroid gland.

Treatment of selenium deficiency: Therapy involves selenium supplementation. Selenium status should then be checked regularly.

Risk groups for selenium deficiency: These groups of people may be affected

In Europe, selenium deficiency is usually only due to certain diseases. The following risk groups are among those most likely to have an insufficient selenium intake.

Selenium deficiency risk groups: [7] 

• People with selenium utilization disorders*
• Increased loss of selenium*
• Dialysis patients

*including chronic inflammatory bowel disease, renal insufficiency, and cystic fibrosis

People in selenium-deficient regions: Selenium intake only needs to be monitored in certain cases

Selenium consumption in North America is well above the RDA value. However, people in some other countries where the diet is primarily based on vegetables from selenium-deficient growing regions are at risk of developing deficiency symptoms.

Selenium deficiency in Asia: The lowest selenium intake worldwide is found in certain parts of China, where large sections of the population are vegetarian and the selenium content of the soil is low.

Selenium deficiency can be detected by various tests

Typically, the diagnosis is made by a selenium blood test at the doctor's office.

Testing selenium with biomarkers: Selenium concentration can also be measured via selenoproteins in serum, erythrocytes, and various tissues. In addition, there are analyses of hair, nails, and urine. [16] [17] 

Selenium testing at a glance:

• Selenium blood test
• Determination of selenoproteins
• Nail or hair analysis
• Urine sample

Evaluation of the analyses: Blood and urine concentrations relate to recent selenium intake. Hair or nail analyses are suitable for long-term evaluation.

Selenium self-test: Self-diagnosis should be avoided due to possible measurement errors.

Interactions with medications: When should selenium not be taken?

To rule out selenium interactions, people taking medication should only use selenium after consulting a doctor.

Selenium and medications: In addition, some medications can have a negative effect on your selenium levels.