Selenium: What It Does, Food Sources, Daily Needs, and Safety
Selenium supports thyroid hormone metabolism, antioxidant defenses, DNA synthesis, reproduction, and immune function, while foods such as seafood, meat, eggs, dairy, grains, and Brazil nuts can help meet daily needs.
Key takeaways
- Selenium is an essential trace mineral. The body needs only small amounts, but selenium participates in several important physiological processes.
- It supports thyroid hormone metabolism. Some selenium-dependent enzymes help convert thyroid hormone T4 into the active hormone T3.
- Food content can vary considerably. Selenium levels in plant foods depend heavily on the soil in which they are grown.
- Adults need 55 mcg per day. Requirements increase to 60 mcg during pregnancy and 70 mcg during breastfeeding.
- Too much selenium can be harmful. Long-term excessive intake can cause selenosis, including changes in hair and nails and other symptoms.
In this article
- What Is Selenium?
- What Does Selenium Do?
- How Much Selenium Do You Need?
- What Foods Contain Selenium?
- Why Does Selenium Content Vary So Much?
- Selenium and Thyroid Function
- What Is Selenium Deficiency?
- How Much Selenium Is Too Much?
- Do You Need a Selenium Supplement?
- Frequently Asked Questions
- Final thoughts
Selenium is an essential trace mineral that the body needs in very small amounts. It is incorporated into a group of proteins called selenoproteins, which are involved in protection against oxidative stress, thyroid hormone metabolism, DNA synthesis, reproduction and other physiological processes.
Most people can meet their selenium needs through food. However, selenium is one of the nutrients for which “more” is not necessarily better, because supplements and frequent consumption of extremely concentrated foods such as Brazil nuts can push intake too high.
What Is Selenium?
Selenium is an essential trace mineral, which means the body needs it but only in very small amounts. In the body, selenium is incorporated into 25 known selenoproteins, including glutathione peroxidases, thioredoxin reductases and selenoprotein P.
These proteins perform different functions and contribute to thyroid hormone metabolism, protection from oxidative stress, DNA synthesis and reproduction. Skeletal muscle contains an important share of the body’s selenium, while excess selenium is eliminated mainly through urine.
Selenium comes primarily from food. Seafood, meat, eggs and dairy products are common sources, while grains, nuts and other plant foods may provide very different amounts depending on where they were grown.
What Does Selenium Do?
Selenium does not perform all of its functions independently. Many of the biological roles associated with selenium come from the activity of the selenoproteins into which it is incorporated.
It helps protect cells from oxidative stress
Some selenoproteins are part of the body’s natural antioxidant systems. Glutathione peroxidases and thioredoxin reductases, for example, help control oxidative compounds generated during normal metabolism.
This role does not mean that very high doses provide greater antioxidant protection. When selenium intake is already adequate, additional supplementation does not necessarily increase selenoprotein activity or provide extra benefit.
It supports thyroid hormone metabolism
The thyroid contains a relatively high concentration of selenium. Selenium-dependent enzymes called deiodinases participate in thyroid hormone metabolism, including conversion of T4 into the active hormone T3.
Other selenoproteins help protect thyroid tissue from hydrogen peroxide generated during thyroid hormone production. Selenium and iodine therefore have different roles, but both are important for normal thyroid function.
It participates in DNA synthesis and reproduction
Selenoproteins are also involved in DNA synthesis and reproductive processes. Severe selenium deficiency has historically been associated with problems that include impaired male fertility.
This does not mean selenium supplements improve fertility in people whose intake is already adequate. Evidence on selenium supplementation for male fertility remains insufficiently clear.
It contributes to immune function
Selenium contributes to normal immune-system function through the activity of selenoproteins. Research suggests roles in T-cell and natural killer cell function, among other immune mechanisms.
An adequate intake is important for normal physiology, but this should not be confused with the claim that selenium supplements “boost” immunity. Benefits from supplementation are not established in the same way when selenium status is already sufficient.
How Much Selenium Do You Need?
Selenium requirements are measured in micrograms (mcg) rather than milligrams. For adults, the U.S. Recommended Dietary Allowance is just 55 mcg per day.
| Age / Life Stage | Daily Selenium Need |
|---|---|
| 1–3 years | 20 mcg |
| 4–8 years | 30 mcg |
| 9–13 years | 40 mcg |
| 14–18 years | 55 mcg |
| Adults 19+ | 55 mcg |
| Pregnancy | 60 mcg |
| Breastfeeding | 70 mcg |
Needs are the same for adult men and women except during pregnancy and breastfeeding. During breastfeeding, the recommendation rises to 70 mcg per day.
These values represent recommended daily intake, not the maximum safe amount. With selenium, the difference between the RDA and the upper limit is especially important because excessive intake can become toxic.
What Foods Contain Selenium?
Selenium occurs in many foods. Brazil nuts, seafood, meat, poultry and organ meats tend to be among the richest sources, while eggs, dairy products and grains also contribute.
| Food | Serving | Approximate Selenium |
|---|---|---|
| Brazil nut | 1 nut | ~68–91 mcg |
| Tuna | 85 g | ~90 mcg |
| Sardines | 85 g | ~45 mcg |
| Shrimp | 85 g | ~40 mcg |
| Beef | 85 g | ~30 mcg |
| Turkey | 85 g | ~25 mcg |
| Chicken | 85 g | ~20–25 mcg |
| Egg | 1 large | ~15 mcg |
| Cottage cheese | 1 cup | ~20 mcg |
| Brown rice | 1 cup | ~20 mcg |
| Whole-wheat bread | 1 slice | ~10–15 mcg |
These values are approximate and can vary substantially. Variation is particularly important in plant foods because the amount of selenium accumulated by the plant depends on the concentration and chemical form of selenium in the soil.
What about Brazil nuts?
Brazil nuts are unusual because they can contain extremely large amounts of selenium. NIH estimates that one Brazil nut may provide about 68–91 mcg, meaning a single nut can already exceed the adult RDA of 55 mcg.
The actual selenium content can vary widely between nuts. Regularly eating large quantities can therefore push total intake above recommended safety limits.
Why Does Selenium Content Vary So Much?
Plants obtain selenium from soil, and selenium concentrations differ greatly between geographic regions. Soil chemistry, pH and the chemical form of selenium can all influence how much plants are able to absorb.
This variation can continue through the food chain. Selenium concentrations in animal foods can also be influenced by the selenium content of animal feed.
For this reason, a nutrition-table value for a plant food should be treated as an estimate rather than an exact amount found in every sample. Eating a varied diet also reduces dependence on a single highly variable source.
Selenium and Thyroid Function
Selenium’s relationship with the thyroid is one reason the mineral has become popular in supplements. Selenium is genuinely important for thyroid hormone metabolism, including the conversion of T4 to T3.
However, this does not mean selenium supplementation automatically treats thyroid disorders. Some studies have observed associations between low selenium status and certain thyroid conditions, while some trials have reported reductions in specific thyroid antibodies after supplementation.
Results for thyroid function and quality of life have been mixed. The physiological role of selenium in thyroid function is well established, but the benefits of selenium supplements for thyroid disease are much less certain, and more research is needed.
What Is Selenium Deficiency?
Selenium deficiency is uncommon in the United States and Canada and does not usually occur in people eating a varied diet. Risk may be higher in geographic areas where soils contain very little selenium.
People who may be at greater risk include those who rely heavily on local foods grown in selenium-poor regions, people receiving kidney dialysis, and people living with HIV.
Severe selenium deficiency has historically been associated with Keshan disease, a form of cardiomyopathy reported in certain regions with very low selenium intake. It has also been associated with Kashin-Beck disease and with problems involving male fertility.
These examples do not mean higher-than-needed selenium intake prevents these conditions in the general population. They demonstrate why an adequate amount of selenium is necessary for normal physiological function.
How Much Selenium Is Too Much?
Selenium has a relatively narrow safety margin compared with many other nutrients. Chronic exposure to excessive amounts can cause selenosis.
Possible signs of excessive selenium intake include:
- garlic-like breath odor
- metallic taste
- nausea and diarrhea
- skin rashes
- brittle hair and nails
- hair or nail loss
- irritability
- nervous-system problems
In the United States, NIH and the National Academies use a Tolerable Upper Intake Level of 400 mcg per day for adults. This includes selenium from food, beverages and supplements.
In Europe, EFSA reevaluated selenium safety and in 2023 established a lower adult UL of 255 mcg per day, including during pregnancy and breastfeeding. EFSA specifically identified high-dose supplements and frequent Brazil nut consumption as situations in which this limit may be exceeded.
Neither value should be confused with the adult RDA of 55 mcg. An UL is a safety ceiling, not a recommended intake or nutritional target.
Do You Need a Selenium Supplement?
Most people who eat a varied diet can obtain enough selenium from food. Selenium is available as a stand-alone supplement and in many multivitamin-mineral products, often in forms such as selenomethionine or sodium selenate.
For someone with adequate intake, more selenium does not automatically provide extra benefits. Clinical trials have not demonstrated that selenium supplementation prevents prostate cancer or other cancers.
Supplements also deserve caution because the difference between daily requirements and potentially excessive intake is relatively small. This becomes especially important when a selenium supplement is combined with a multivitamin and selenium-rich foods such as Brazil nuts.
Frequently Asked Questions
What is selenium good for?
Selenium is required to form selenoproteins involved in thyroid hormone metabolism, protection from oxidative stress, DNA synthesis, reproduction and normal immune function. These are established physiological roles of selenium in the body.
These functions require an adequate amount, not extremely high doses. Taking more selenium than needed does not automatically intensify these processes.
How much selenium do you need per day?
Adults have an RDA of 55 mcg per day. Requirements increase to 60 mcg during pregnancy and 70 mcg during breastfeeding.
These values are much lower than upper safety limits. The RDA represents the amount recommended to meet the needs of most healthy people.
What are the best food sources of selenium?
Brazil nuts, seafood, meat, poultry, eggs and some dairy products and grains provide selenium. Protein-rich foods tend to be among the more reliable dietary sources.
Plant-food concentrations can vary widely depending on the soil in which they were grown. Nutrition-table values should therefore be considered approximate.
How many Brazil nuts should you eat for selenium?
You do not need Brazil nuts specifically to get enough selenium. A single Brazil nut may contain approximately 68–91 mcg, already more than the adult RDA of 55 mcg, and the amount can vary substantially.
This unusually high concentration is exactly why “more is better” does not apply to Brazil nuts. Regularly consuming large amounts can contribute to excessive selenium intake.
Is selenium good for the thyroid?
Selenium is required for normal thyroid function and thyroid hormone metabolism. Some selenoproteins are directly involved in conversion of T4 into T3.
That does not mean everyone with a thyroid condition needs a selenium supplement. Trials of selenium supplementation in thyroid disorders have produced mixed results.
Can you take too much selenium?
Yes. Long-term excessive intake can cause selenosis, with symptoms including hair and nail changes, metallic taste, garlic-like breath, nausea and neurological problems.
Upper limits differ between authorities. The U.S. adult UL is 400 mcg per day, while EFSA currently uses 255 mcg per day for adults in Europe.
Final thoughts
Selenium is needed only in very small quantities, but it supports several important processes through selenoproteins. These include thyroid hormone metabolism, antioxidant defense systems, DNA synthesis, reproduction and immune function.
Most people can obtain enough selenium through food, and adults need only 55 mcg per day. Seafood, meat, poultry, eggs, dairy products, grains and some nuts can all contribute, although plant-food selenium levels can vary substantially depending on soil.
With selenium, higher intake is not automatically beneficial. Brazil nuts and high-dose supplements can provide surprisingly large amounts, and chronic excess can cause selenosis, making total intake and supplement dose important considerations.
References
- NIH Office of Dietary Supplements — Selenium: Fact Sheet for Health Professionals ods.od.nih.gov
- NIH Office of Dietary Supplements — Selenium: Fact Sheet for Consumers ods.od.nih.gov
- NIH Office of Dietary Supplements — Immune Function and Infectious Diseases ods.od.nih.gov
- European Food Safety Authority — Dietary Reference Values efsa.europa.eu
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