Selenium: What It Does in the Body, and How Much You Really Need
A trace mineral with a big job: making the antioxidant enzymes that protect your cells from damage.
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Key takeaways
- Selenium is a trace mineral used to build selenoproteins, including antioxidant enzymes and thyroid hormone regulators
- Brazil nuts, seafood, organ meats, and eggs are among the richest food sources
- Both too little and too much selenium can cause health problems, and the safe range is narrower than for many nutrients
- Most people who eat a varied diet get enough selenium without needing a supplement
What is selenium?
Selenium is a trace mineral, meaning your body needs only small amounts of it to function.
It gets incorporated into specialized proteins called selenoproteins. These include glutathione peroxidase, an antioxidant enzyme, and enzymes that activate thyroid hormone.
Because plants take up selenium from soil, the selenium content of crops varies a lot depending on where they're grown. This is part of why selenium deficiency is more common in certain regions of the world, like parts of China and other areas with selenium-poor soil.
What does selenium do in the body?
Selenium's main job is structural: it's built directly into selenoproteins as the amino acid selenocysteine.
- Supports antioxidant defense by helping break down harmful peroxides before they damage cells
- Helps convert inactive thyroid hormone (T4) into its active form (T3)
- Supports sperm development and male reproductive health
- Plays a role in immune cell function
- May help regulate inflammation
Because of its role in thyroid hormone metabolism, selenium status is often discussed alongside iodine and thyroid health.
What does the evidence show?
Most of what's known about selenium's benefits comes from studying populations with frank deficiency, rather than from trials of supplementation in already-replete people.
In selenium-deficient regions, low intake has been linked to Keshan disease, a form of heart muscle damage, and Kashin-Beck disease, a joint disorder. These conditions are essentially unheard of in places with adequate dietary selenium.
Interest in selenium and cancer prevention grew after early observational work suggested a possible link, but large randomized trials in selenium-replete populations have generally not shown a protective effect, and some have raised concerns about excess intake.
Selenium supplementation has also been studied in autoimmune thyroid conditions, with some trials suggesting it may modestly reduce thyroid antibody levels, though results are mixed and it isn't considered a standalone treatment.
How much selenium do you need?
Recommended intakes for adults generally fall around 55 micrograms per day, with somewhat higher amounts suggested during pregnancy and breastfeeding.
Selenium is unusual among trace minerals in that a single food, Brazil nuts, can contain far more than a full day's worth in just one or two nuts. This makes it easy to overshoot intake if you eat them regularly alongside a supplement.
Is selenium safe? What are the risks of too much?
Selenium has one of the narrower safety margins among essential nutrients, meaning the gap between a helpful amount and too much is relatively small.
- Deficiency symptoms include muscle weakness, fatigue, and in severe cases heart or joint problems
- Excess intake (selenosis) can cause hair loss, brittle nails, gastrointestinal upset, and a garlic-like breath odor
- Chronic high intake has been linked to nerve damage in some cases
- Some large trials found higher rates of type 2 diabetes among people given selenium supplements who were not deficient to begin with
Because of this narrow margin, selenium is a nutrient where more is not automatically better, and supplementing without a documented low intake or deficiency isn't generally supported by current evidence.
Who might think about their selenium intake?
- People living in regions with selenium-poor soil and low dietary variety
- People following restrictive diets that exclude seafood, meat, eggs, and nuts
- People with diagnosed selenium deficiency or certain thyroid or autoimmune conditions, under medical guidance
- People who eat Brazil nuts regularly and also take a multivitamin or supplement containing selenium, who should be mindful of total intake
Bottom line
Selenium is essential for antioxidant defense and thyroid hormone activation, and a varied diet with seafood, organ meats, eggs, or even the occasional Brazil nut typically covers daily needs.
The evidence for supplementing beyond that, especially for cancer prevention, has not held up well in larger trials, and excess selenium carries real risks. For most people, food sources are the simpler and safer approach.
Frequently asked questions
What foods are highest in selenium?
Brazil nuts are exceptionally concentrated, often containing more selenium in one nut than a full day's recommended amount. Seafood, organ meats, and eggs are also good sources.
Can you get too much selenium from food alone?
It's possible if you regularly eat several Brazil nuts a day, since their selenium content is unusually high and variable. Most other food sources make excess intake from diet alone less likely.
Does selenium help thyroid function?
Selenium is needed to activate thyroid hormone, and some trials suggest supplementation may modestly lower thyroid antibodies in autoimmune thyroid conditions, though evidence is mixed.
Is selenium supplementation good for preventing cancer?
Early observational data suggested a possible link, but larger randomized trials in people with adequate selenium levels have generally not confirmed a protective effect.