What Selenium Does

Selenium is a trace mineral your body needs to build and run proteins called selenoproteins. These proteins do two main jobs: they protect your cells from damage caused by free radicals (unstable molecules that accumulate over time), and they help regulate your thyroid and immune system. You need only small amounts — the recommended daily intake is 55 micrograms for adults — but that small amount has outsized effects on how your cells function.

Your body cannot make selenium on its own, so it has to come from food. The amount in any given food depends on how much selenium is in the soil where it was grown. Regions with selenium-rich soil — parts of the Great Plains, for example — produce foods naturally higher in the mineral. This geographic variation is why some people get plenty of selenium from their diet while others may fall short, even if they eat similar foods.

Key Takeaways

  • Selenium is essential for making selenoproteins, which protect cells from oxidative stress and support thyroid and immune function.
  • Most people in North America meet their selenium needs through food, particularly from Brazil nuts, seafood, poultry, and eggs.
  • Selenium deficiency is rare in developed countries but can occur in people with certain digestive disorders or those on dialysis.
  • Research shows selenium supports thyroid health and may play a role in reducing inflammation, though evidence for other claims remains preliminary.
  • Excess selenium from supplements can cause toxicity; the safe upper limit is 400 micrograms daily for adults.

How Selenium Protects Your Cells

The main protective mechanism involves selenoproteins called glutathione peroxidases. These enzymes neutralize hydrogen peroxide and other reactive oxygen species before they can damage cell membranes, DNA, and proteins. Without adequate selenium, your cells lose this defense and accumulate damage over time — a process called oxidative stress that is linked to aging and chronic disease.

This protective role is why selenium deficiency has been associated with increased risk of certain cancers, heart disease, and cognitive decline in observational studies. However, observational data shows correlation, not causation — people with low selenium may differ in many other ways from those with adequate levels. Randomized controlled trials testing whether selenium supplements prevent these diseases have produced mixed results, and most major health organizations do not recommend supplementing for disease prevention in people who already meet their daily needs.

Selenium and Thyroid Function

Your thyroid contains more selenium per gram of tissue than any other organ in your body. Selenoproteins called thioredoxin reductases protect thyroid cells from the oxidative damage that occurs during hormone production. Selenium also helps convert thyroid hormone into its active form, so deficiency can impair thyroid function even when iodine intake is adequate.

Research in people with autoimmune thyroid disease (Hashimoto's thyroiditis) shows that selenium supplementation may reduce antibody levels and improve symptoms in some cases, though the effect is modest and not universal. A 2017 review in the journal Nutrients found that selenium improved thyroid peroxidase antibody levels in about half of the trials reviewed, but the clinical significance of these changes remains unclear. If you have thyroid disease, discussing selenium status with your doctor makes sense, but supplementing without testing is not standard practice.

Selenium in Food vs. Supplements

Brazil nuts are by far the richest source of selenium — a single nut can contain 50 to 100 micrograms, which exceeds the daily requirement in one serving. Other reliable sources include oysters and other shellfish (40 to 50 micrograms per 3-ounce serving), turkey and chicken (22 to 35 micrograms per 3-ounce serving), and eggs (15 to 20 micrograms per large egg). Whole grains, legumes, and dairy products also contribute selenium, though amounts vary by region.

Most people in North America meet their selenium needs through food alone. Supplements are available as selenomethionine (the form found in plant-based foods) or sodium selenite (a synthetic form). Both are absorbed reasonably well, though selenomethionine may be retained in the body longer. If you take a multivitamin, check the label — many contain 50 to 100 micrograms of selenium, which is close to or exceeds the daily need. Combining food sources with a multivitamin can push intake above safe levels.

Who May Need More Selenium

Selenium deficiency is rare in developed countries but occurs in specific populations. People with Crohn's disease, celiac disease, or cystic fibrosis may absorb less selenium from food. Those on long-term parenteral nutrition (intravenous feeding) or hemodialysis can lose selenium or receive inadequate amounts. Residents of regions with severely selenium-depleted soil — parts of China, for example — face higher deficiency risk.

If you fall into one of these groups, a blood test can measure your selenium status. Plasma selenium below 70 nanograms per milliliter is considered deficient. Your doctor can recommend supplementation if testing shows a shortfall. For people without these risk factors, testing is not routine and supplementing without evidence of deficiency is not recommended.

Risks of Too Much Selenium

Selenium toxicity, called selenosis, occurs when intake exceeds 400 micrograms daily over extended periods. Early signs include a garlic odor on the breath and skin, brittle nails, and hair loss. Chronic excess can damage the nervous system and cause joint pain. Because selenium accumulates in the body, toxicity develops gradually and can persist even after intake drops.

Cases of selenosis have occurred in people taking high-dose supplements or consuming very large amounts of Brazil nuts regularly. The risk from food alone is low because even selenium-rich foods contain moderate amounts. If you take supplements, keep total intake (from multivitamins, single-nutrient supplements, and food) below 400 micrograms daily. People with kidney disease should be especially cautious, as the kidneys help regulate selenium excretion.

What the Research Actually Shows

Selenium's role in preventing cancer, heart disease, and cognitive decline remains an open question. The Selenium and Vitamin E Cancer Prevention Trial (SELECT), a large randomized trial published in 2009, found that selenium supplements did not reduce prostate cancer risk in men with adequate baseline selenium levels. Other trials have shown mixed results, and meta-analyses suggest that supplementation may help only in people who start with very low levels.

For immune function, animal studies show that selenium deficiency impairs the ability to fight infections, but human trials are limited. Observational data links low selenium to worse outcomes in people with HIV, but whether supplementation improves outcomes is not yet clear. The takeaway: selenium is necessary for immune function, but more is not necessarily better, and supplementing beyond your daily need has not been proven to strengthen immunity in people who are already replete.

Frequently Asked Questions

How much selenium do I need daily?

The recommended dietary allowance is 55 micrograms for adults. Pregnant and breastfeeding women need 60 and 70 micrograms respectively. Most people in North America meet this through food alone, particularly if they eat seafood, poultry, eggs, or Brazil nuts regularly.

Can I get too much selenium from food?

Toxicity from food is very rare. You would need to eat dozens of Brazil nuts daily for weeks to approach toxic levels. The risk is higher with supplements, especially if you combine a high-dose supplement with a multivitamin and selenium-rich foods.

Should I take a selenium supplement?

If you eat a varied diet including seafood, poultry, eggs, or whole grains, you likely have adequate selenium. Supplementation makes sense only if testing shows deficiency or if you have a condition that impairs absorption. Talk with your doctor before starting a supplement.

Does selenium prevent cancer?

Large randomized trials have not found that selenium supplements prevent cancer in people with adequate baseline levels. Observational studies suggest low selenium may increase risk, but this does not mean supplementing will reduce it. Meeting your daily need through food is the evidence-based approach.

Is selenomethionine better than sodium selenite?

Both forms are absorbed and used by the body. Selenomethionine may be retained slightly longer, but the practical difference is small. Choose based on what your supplement contains and whether you tolerate it well.