Selenium is a mineral your body uses to make proteins that protect your cells and support your immune system

Selenium is a trace mineral, meaning you need only small amounts of it—but those small amounts do important work. Your body uses selenium to build selenoproteins, which are proteins that act as antioxidants. Antioxidants help protect your cells from damage caused by unstable molecules called free radicals. Selenium also plays a role in how your thyroid works and in keeping your immune system functioning.

You get selenium from food, and most people in the United States eat enough of it without thinking about it. Unlike some minerals, selenium deficiency is rare in developed countries. That said, understanding what selenium does and which foods contain it can help you make informed choices about your diet.

Key Takeaways

  • Selenium helps your body make proteins that protect cells from damage and support immune function.
  • Most people get enough selenium from everyday foods like Brazil nuts, fish, eggs, and whole grains.
  • The recommended daily amount is 55 micrograms for adults, which is a very small quantity.
  • Selenium deficiency is uncommon in the United States but can occur in people with certain health conditions or who live in areas with selenium-poor soil.
  • Too much selenium from supplements can cause problems, so getting it from food is usually the safest approach.

How much selenium you need each day

The recommended dietary allowance (RDA) for selenium is 55 micrograms per day for adults. To put that in perspective, a single Brazil nut contains about 95 micrograms, so you can meet your daily need with just one or two Brazil nuts—though eating them regularly is not necessary.

Pregnant and breastfeeding people need the same amount as other adults. Children need less: infants need 15 micrograms daily, and the amount increases gradually through childhood until reaching the adult level at age 14.

These are average needs for healthy people. If you have a health condition that affects how your body absorbs minerals, or if you take certain medications, your needs might be different. A doctor or registered dietitian can tell you whether your situation calls for more attention to selenium intake.

Foods that contain selenium

Selenium is found in both animal and plant foods. The amount in plant foods depends on how much selenium is in the soil where they grew, which varies by region. Animal foods tend to have more consistent selenium levels because animals eat a variety of plants from different areas.

Animal sources include fish (especially tuna and halibut), shellfish, beef, turkey, chicken, eggs, and dairy products. A 3-ounce serving of cooked tuna or halibut contains roughly 40 to 60 micrograms of selenium.

Plant sources include Brazil nuts (the richest source), whole wheat bread, brown rice, oats, beans, lentils, and mushrooms. Whole grains and legumes are reliable sources because they are grown in many regions and you eat them regularly.

If you eat a varied diet that includes protein, whole grains, and vegetables, you are almost certainly meeting your selenium needs without tracking it.

Why your body needs selenium

Selenium's main job is to help your body make selenoproteins. These proteins work as antioxidants, meaning they neutralize free radicals—unstable molecules that can damage cells and contribute to aging and disease. By protecting cells from this damage, selenoproteins may help reduce the risk of certain chronic conditions.

Selenium also supports your thyroid gland, which controls your metabolism and energy levels. The thyroid contains more selenium per unit of tissue than any other organ in your body. Selenoproteins help the thyroid produce and use thyroid hormones correctly.

Additionally, selenium plays a role in immune function. It helps immune cells work properly and may help your body respond to infections. Some research suggests that adequate selenium supports the health of your reproductive system as well, though more research is ongoing.

What happens if you do not get enough selenium

Selenium deficiency is uncommon in developed countries because selenium is widely available in food. However, it can occur in people with certain conditions. Crohn's disease, celiac disease, and other digestive disorders that affect nutrient absorption can lower selenium levels. People on dialysis for kidney disease may also have low selenium.

In areas of the world where soil is very low in selenium—parts of China, for example—deficiency is more common. Severe deficiency can cause muscle weakness, fatigue, and mood changes. Over time, it may affect thyroid function and immune health.

If you have a condition that affects nutrient absorption, or if you follow a very restricted diet, talk with a doctor or dietitian about whether you need to pay special attention to selenium intake.

Selenium supplements and safety

Most people do not need selenium supplements because they get enough from food. Supplements are available in different forms, including selenomethionine (the form found naturally in foods) and sodium selenite (a synthetic form). Both are absorbed by the body, though selenomethionine may be retained longer.

Taking too much selenium can cause problems. The upper limit for adults is 400 micrograms per day. Long-term intake above this level can cause selenosis, a condition marked by brittle nails, hair loss, nausea, and neurological symptoms. This is rare and usually happens only when people take high-dose supplements for extended periods.

If you are considering a selenium supplement, talk with a doctor or dietitian first. They can help you figure out whether you actually need one and what dose would be appropriate for your situation. For most people, getting selenium from food is the safest and most effective approach.

Selenium and specific health conditions

Research has explored whether selenium plays a role in preventing or managing certain health conditions. Some studies suggest that adequate selenium may support heart health, bone health, and cognitive function, though the evidence is still developing and more research is needed.

For thyroid health specifically, selenium is essential because the thyroid uses it to make selenoproteins that regulate thyroid hormone production. People with thyroid conditions sometimes have lower selenium levels, and some research suggests that adequate selenium may support thyroid function. However, taking selenium supplements does not treat thyroid disease—that requires medical care from a doctor.

If you have a health condition and are wondering whether selenium plays a role in your care, ask your doctor or a registered dietitian. They can explain what the current evidence shows and whether paying attention to your selenium intake makes sense for your situation.

Frequently Asked Questions

Can I get too much selenium from eating Brazil nuts?

Brazil nuts are very high in selenium—one nut can contain 95 micrograms or more. Eating a handful daily could push you above the safe upper limit of 400 micrograms per day. Eating one or two Brazil nuts occasionally is fine, but they should not be a daily snack if you are also taking supplements or eating other selenium-rich foods regularly.

Do I need a selenium supplement if I eat fish and whole grains?

Probably not. Fish and whole grains are reliable sources of selenium, and eating them regularly usually provides enough. Supplements are most useful for people with digestive conditions that affect nutrient absorption or those whose doctors have identified a deficiency. For others, food is the better choice.

Does selenium help prevent cancer?

Some research has explored whether selenium reduces cancer risk, but the evidence is mixed and ongoing. Adequate selenium is important for overall health, but taking supplements specifically to prevent cancer is not supported by strong evidence. Eating a varied diet with plenty of vegetables, fruits, whole grains, and protein is a better approach to cancer prevention.

What if I follow a vegan diet—can I get enough selenium?

Yes. Plant foods like whole grains, legumes, nuts, and seeds contain selenium, though the amount varies by region. Brazil nuts, oats, brown rice, lentils, and mushrooms are good vegan sources. Eating a variety of these foods throughout the week should provide enough selenium without supplements.

Is selenium in drinking water?

Selenium can be present in drinking water, but the amount varies widely depending on where you live and the mineral content of your local water supply. Water is not a reliable source of selenium, so you should focus on getting it from food instead.