Selenium: Everything You Need to Know

Selenium is a trace mineral commonly found in the soil of most regions of the world.

Because it is a trace mineral, your body only needs it in small amounts. However, consuming too much or too little selenium can cause serious side effects and health conditions.

Once consumed and metabolized, selenium is inserted into various selenoproteins. These selenoproteins play essential roles in many processes in the human body, including immunity, metabolism, and the synthesis of hormones.

In food, selenium is found in both organic and inorganic forms. Organic selenium, or selenomethionine, is more readily absorbed than inorganic forms, like selenate or selenite. Dietary supplements typically contain selenomethionine.

Regarding human health, selenium is thought to act mostly as an antioxidant that may be useful in preventing conditions like type 2 diabetes, heart disease, and other metabolic disorders.

This article will provide an overview of common uses of selenium. It will also discuss selenium deficiency, toxicity, dosage, side effects, and more.

Uses of Selenium

Supplement use should be individualized and vetted by a healthcare professional, such as a registered dietitian, pharmacist, or healthcare provider. No supplement is intended to treat, cure, or prevent disease. 

Selenium is needed for basic human health. Although some people use selenium with the desire to improve certain health conditions, research results confirming any additional benefits of the mineral are mixed.

The use of selenium supplements is typically only necessary when recommended by a healthcare provider to treat a deficiency. Most people can obtain all the selenium they need through a normal diet.

Below is a look at some research surrounding common uses of selenium.

Thyroid Disease

In the human body, selenium can be found in the highest concentrations in the thyroid gland.

Selenium also plays a vital role in the synthesis of various enzymes that act as antioxidants and regulate thyroid hormones. A deficiency in selenium has been linked to hypothyroidism (underactive thyroid) and other thyroid diseases, like Graves' disease and Hashimoto's disease.

As healthy levels of selenium appear to be vital for thyroid health, several studies have looked at how selenium supplements may prevent thyroid disease.

According to one review, selenium supplementation may be especially beneficial for people with autoimmune thyroiditis, which affects the production of thyroid hormones essential to normal metabolism. Researchers also found that maintaining normal selenium levels helps prevent thyroid disease. However, selenium intake or supplementation should be closely monitored, as high selenium levels may be harmful.

Antioxidant Properties and Its Role In Heart Health

Selenium's role as an antioxidant has led some researchers to hypothesize that it may reduce the risk of heart disease. However, the scientific evidence is inconsistent.

According to one meta-analysis of various studies, taking selenium supplements does not affect the risk of cardiovascular (heart) disease. It's worth noting, though, that normal selenium blood levels were associated with a lower heart disease risk.

Another review found that, while selenium supplements alone didn't affect heart disease risk, adding selenium to other antioxidants did. Researchers found that when selenium was part of an antioxidant supplement, the risk for heart disease and all-cause mortality (death fro all sources) was lower than when selenium or antioxidants were taken separately. This led the researchers to believe that selenium is an essential part of any antioxidant supplement.

A third review examined how selenium status (how much selenium you have in your body) affects both the incidence and mortality of heart disease. It was concluded that high levels of selenium in the body are associated with a decreased risk of heart disease. However, researchers cautioned that excessive selenium intake may be harmful.

Clearly, more research is needed to support the role of selenium in heart disease.

Type 2 Diabetes

Some evidence suggests that selenium may help prevent and treat type 2 diabetes.

One small selenium study was performed on adult males and females with type 2 diabetes. In the study, participants followed a Mediterranean diet (an eating plan high in plant-based foods and healthy fats and low in red meat and processed foods) and took 200 micrograms of selenium per day on an empty stomach. By the end of the six-month trial, participants had significant reductions in fasting blood sugar, A1C, total cholesterol, and low density lipoprotein (LDL) cholesterol (considered "bad" cholesterol).

It should be noted, though, that we do not know for certain if these study results were due to selenium supplementation, adherence to the Mediterranean diet, or a combination of both.

Alternatively, some research has found a possible causal relationship between high selenium status and type 2 diabetes. Yet, the 2013–2018 National Health and Nutrition Examination Survey (NHANES) found no association between selenium status and the prevalence of diabetes.

In the end, there is not enough strong evidence that confirms the role of selenium in either the development or prevention of type 2 diabetes. More research is needed.

Immune Function

There is some belief that selenium levels may factor into COVID-19 infection and infection severity.

Selenium has been found to have antioxidant and anti-inflammatory effects, as well as an influence on immune function. This may explain why one study found that regions of China with higher levels of selenium in the soil had decreased rates of COVID-19 infections.

A systematic review noted that various studies have correlated a deficiency in selenium to more severe infections from COVID-19. One study included in the review found that high levels of selenium in the urine were found in fatal cases. However, other studies have found no link between selenium levels and COVID-19 infections.

Research should continue in this area with the hope of finding more consistent results.

Selenium Deficiency

Although rare in the United States, a selenium deficiency may lead to serious health issues.

Selenium deficiency has been linked to such conditions as heart disease, cancer, male infertility, cognitive decline, and inflammation. Selenium is also known to be important for normal development, immunity, and delaying the progression of AIDS in patients with human immunodeficiency virus (HIV).

You may develop a selenium deficiency for various reasons.

What Causes a Selenium Deficiency?

A selenium deficiency may occur due to diet, where you live, or other factors.

Possible causes of selenium deficiency include:

• Diet: Selenium is found in Brazil nuts, grains, meat, fish, and eggs. If you do not eat enough of these and other sources of selenium, you may become deficient.
• Where you live: In some regions of the world, there is not enough selenium in the soil to meet nutrient needs through a plant-based diet. In the United States, vegetables are grown in soil that contains adequate levels of selenium, which means vegans and vegetarians typically get enough of the mineral.
• Kidney dialysis: People undergoing dialysis due to kidney disease may become deficient in selenium. This is because some selenium is removed from the blood during dialysis.
• HIV status: Selenium deficiency is possible in people living with HIV who experience diarrhea and malabsorption (an inability to absorb nutrients from food) caused by the disease. In developing countries, where HIV rates are high, selenium deficiency may be due to inadequate intake.

How Do I Know If I Have a Selenium Deficiency?

A deficiency in selenium may lead to certain illnesses, including:

• Keshan disease (an abnormality of the heart muscle)
• Kashin-Beck disease (a type of joint and bone disease)
• Myxedematous endemic cretinism (an intellectual disability mostly affecting infants)

Additionally, selenium deficiency may be associated with male infertility. Some research also suggests that a deficiency in selenium may negatively impact the nervous system and lead to increased anxiety or the development of depression or Alzheimer's disease.

A healthcare provider can properly identify a selenium deficiency through an exam and lab tests.

Are There Side Effects of Taking Selenium?

Taking selenium supplements or consuming too much selenium in food may lead to side effects. Side effects of selenium can be mild or severe.

Common Side Effects

You are more likely to experience side effects if you ingest too much selenium, either acutely or chronically.

Common side effects of selenium include:

• Nausea
• Diarrhea
• Nail/hair loss or brittleness
• Skin rash
• Fatigue
• Irritability
• Foul breath
• Metallic taste in the mouth
• Discolored teeth

If you experience side effects while taking selenium supplements, stop use and speak with a healthcare provider. Mild side effects should resolve on their own with time and treatment.

Severe Side Effects

Selenium in amounts commonly found in foods is not thought to cause side effects.

However, taking extremely high doses of selenium may cause severe side effects, such as:

• Difficulty breathing
• Tremors
• Kidney failure
• Heart attack
• Heart failure

You should seek immediate medical attention if you experience these or other severe side effects after using selenium.

Precautions

Selenium is an essential nutrient throughout the life cycle. This means you must consume enough selenium through food or supplements for health. However, precautions should be taken when using selenium supplements.

How you take selenium supplements affects their safety. If you take a high dose of selenium or take selenium for too long, you may experience various side effects.

Talk with a healthcare provider before using selenium supplements, especially if you have a preexisting medical condition or are taking any medications.

Dosage: How Much Selenium Should I Take?

There are guidelines called Dietary Reference Intakes (DRIs) for taking selenium daily. DRIs provide dosage information for people of all ages and genders based on scientific evidence. Essential nutrients, like selenium, have DRIs.

The Recommended Daily Allowances (RDAs) for selenium are as follows:

• Children 1 to 3 years old: 20 micrograms/day
• Children 4 to 8 years old: 30 micrograms/day
• Children 9 to 13 years old: 40 micrograms/day
• Males 14 years old and older: 55 micrograms/day
• Females 14 years old and older: 55 micrograms/day
• People who are pregnant: 60 micrograms/day
• People who are lactating: 70 micrograms/day

An Adequate Intake (AI) (an average nutrient intake for adequate needs) has been established for infants. From birth to 6 months old, infants need 15 micrograms/day of selenium, while older babies ages 7 to 12 months need 20 micrograms/day.

Selenium supplements tend to come in doses much larger than these RDAs. Work with a healthcare provider before taking selenium supplements to determine the proper dose. Typically, you can get all the selenium you need through your diet.

Can I Take Too Much Selenium?

Selenium supplements can be toxic if you take too much.

Mild cases of selenium toxicity may result in:

• Stomach pain
• Nausea
• Vomiting
• Garlic-smelling breath

In severe cases, taking too much selenium may lead to:

• Vomiting blood
• Renal failure
• Cardiac symptoms
• Death

Because selenium can become toxic, tolerable upper limit levels (ULs) have been established. The UL pertains to all food, drinks, and supplements containing selenium. Do not exceed the following ULs for selenium:

• Birth to 6 months: 45 micrograms
• Infants 7 to 12 months: 60 micrograms
• Children 1 to 3 years: 90 micrograms
• Children 4 to 8 years: 150 micrograms
• Children 9 to 13 years: 280 micrograms
• Teens 14 to 18 years: 400 micrograms
• Adults: 400 micrograms

To prevent toxicity, never exceed dose recommendations for selenium supplements, and keep these ULs in mind.

Interactions

Selenium may interact with certain medications, supplements, or nutrients. These interactions may alter the way selenium or your medications work.

There is a possible interaction between selenium and cisplatin, a type of chemotherapy used to treat cancer. Cisplatin may reduce the amount of selenium in your body. But there is also some evidence that selenium may decrease the toxic side effects of cisplatin.

Evidence supporting other selenium interactions is weak or nonexistent. Despite this, additional interactions may exist for selenium. Tell a healthcare provider what medications and supplements you take before starting selenium.

Also, it is essential to carefully read the ingredients list and nutrition facts panel of a new supplement to know which ingredients and how much of each ingredient is included. Please review supplement labels with a healthcare provider to discuss any potential interactions with foods, other supplements, and medications. 

How to Store Selenium Supplements

Keep selenium supplements out of direct sunlight and store them in a cool, dry place. It's important that supplements do not overheat.

It's also important to keep selenium supplements out of sight and reach of children and pets who may accidentally ingest them.

Discard any remaining selenium supplements once they reach their expiration date or as indicated on the packaging.

Sources of Selenium & What to Look For

The best way to get selenium is through your diet. Selenium supplements may contain higher levels of the mineral than you need. Getting selenium through food rather than supplements is thought to be safer and more effective.

Food Sources of Selenium

Selenium is found in both plant and animal foods. The amount of selenium in plant-based foods may depend on how much selenium was in the soil in which they were grown. This means that selenium levels may vary depending on your country or region.

Animal-based foods tend to contain more selenium than vegetables, cereals, and fruits. Food sources of selenium include:

• Cereals
• Grains
• Fish
• Meat
• Dairy products
• Eggs
• Legumes
• Brazil nuts
• Cruciferous vegetables
• Garlic
• Onions
• Potatoes
• Tomatoes
• Leafy green vegetables
• Various fruits

Remember that you only need selenium in small amounts. Many of these and other selenium-containing foods can provide your daily selenium needs in just one serving.

Selenium Supplements

If a healthcare provider has recommended that you use selenium, there are a few things to keep in mind when looking for the right supplement.

Selenium supplements are mostly available as capsules, tablets, and liquid extracts. Some supplements may contain other nutrients or ingredients, so read the nutrition label and ingredient list to know what you're getting.

You can find selenium supplements that are organic, vegan, and/or gluten-free.

Selenium is often added to daily multivitamins. It may be present as selenomethionine, sodium selenate, or other forms in dietary supplements.

If possible, look for selenium supplements that have been approved by a third party like USP, ConsumerLab.com, or NSF. Dietary supplements are not well-regulated in the United States, and these third parties ensure that supplements actually contain what is listed on the label.

Summary

Selenium is an essential mineral that you need in small amounts. It's important to many aspects of your health, including metabolism and the synthesis of hormones.

It's important to get the right amount of daily selenium, as either too much or too little can cause health issues. Food is the best source of selenium.

If you're thinking of using selenium supplements, talk with a healthcare provider first to learn how to use them safely.