Selenium

Selenium is an essential trace mineral with a biochemical role unlike any other: it’s built directly into proteins as selenocysteine — often called the 21st amino acid. Selenocysteine is encoded by a genetic “codon” that normally tells the cell to stop building a protein; when a special signal is present in the messenger RNA, that stop instruction is overridden and selenocysteine is inserted instead. Humans have 25 genes for these selenoproteins, and in most of them, selenocysteine sits at the active site, carrying out redox reactions that the body can’t perform as efficiently any other way. So selenium isn’t just a mineral the body uses — it’s built into the structure of key antioxidant and hormone-activating enzymes.

Key Mechanisms — The Selenoproteins

Glutathione peroxidases (GPx): humans have eight glutathione peroxidases, five of which contain selenium (GPx1, 2, 3, 4, and 6). These are the main selenium-dependent antioxidant enzymes: they neutralize hydrogen peroxide and fat hydroperoxides using glutathione, protecting cells and membranes from oxidative damage. GPx1 is the most abundant and is found throughout the body. GPx4 is the most unusual: it’s the only enzyme that can directly repair damaged phospholipids within cell membranes. When GPx4 fails, a form of regulated cell death called ferroptosis — driven by iron-dependent damage to membrane fats — can be triggered. Ferroptosis is being studied in stroke and heart attack damage, neurodegenerative diseases, and cancer.

Thioredoxin reductases: these selenoproteins recharge thioredoxin, which then restores damaged proteins to working order and supports DNA synthesis. The thioredoxin system and the glutathione system are the cell’s two main redox-maintenance networks — and selenium is essential to both.

Iodothyronine deiodinases (DIO1, DIO2, DIO3): these selenium-dependent enzymes control the activation and deactivation of thyroid hormones. DIO1 and DIO2 convert T4 (thyroxine — the main hormone released by the thyroid) into T3 (triiodothyronine — the more active form), while DIO3 deactivates both. The body prioritizes selenium for these enzymes, so thyroid hormone activation is protected in mild deficiency, but severe deficiency can impair it. This is why selenium and iodine work together: iodine builds thyroid hormones, and selenium helps activate them.

Selenoprotein P (SELENOP): the main selenium transport protein in the blood. When selenium is scarce, it preferentially delivers selenium to the brain and testes — so these organs are protected, and are among the last to lose selenium as status declines.

Selenoprotein W: concentrated in skeletal and heart muscle. Its exact function isn’t fully understood, but it’s thought to have an antioxidant role in muscle — and in livestock, selenium deficiency causes a muscle-wasting condition called white muscle disease.

RDA (Recommended Dietary Allowance)

* Adequate Intake (AI) for infants. Source: US National Academies, via the NIH Office of Dietary Supplements. The daily value used on food labels is 55µg.

ODA (Optimal Daily Allowance)

Aim for about 55–100µg a day, mainly from food. Where you live matters: in the US and Canada, where much of the food is grown in selenium-rich soil, typical diets provide around 100µg a day or more. In many parts of Europe, where soils are lower in selenium, intakes are often lower — sometimes below the RDA — and fish, seafood, eggs, meat, and the occasional Brazil nut become more important. The safe range is narrower than for most minerals: there’s no evidence of extra benefit above adequate intake, and in large trials, people who already had enough selenium and took 200µg a day for years gained no protection against cancer, and showed signs of a higher risk of type 2 diabetes.

Found In (per 100g)

Brazil nuts need a specific caution: their selenium content varies enormously, depending on the soil where the trees grew. USDA’s own data shows how much: its older figures list Brazil nuts at about 1,900µg per 100g — roughly 95µg in a single nut — while its newer analysis found about 280µg per 100g, closer to 15µg per nut. Published measurements range even more widely. So a small handful could provide anything from a modest amount to well over the daily upper limit. Eating one or two Brazil nuts a day, or a few a week, is a popular approach, but because you can’t know their content, it’s sensible not to rely on them alone or eat them in large amounts. Getting selenium from a mix of foods — seafood, eggs, meat, and grains — is more predictable.

Seafood, organ meats, and meat are consistently good sources. The selenium in plant foods reflects the soil they were grown in — wheat grown in selenium-rich soil can contain many times more than the same wheat grown in selenium-poor soil. The table’s grain and legume values reflect US crops, which are often relatively high in selenium.

Helps With

• Antioxidant defense, through the glutathione peroxidase family
• Protecting cell membranes from damage, via GPx4
• Thyroid hormone activation — converting T4 to active T3 through the deiodinases
• Immune function — both adaptive (T-cell) and innate immunity depend on selenium
• DNA synthesis and protection, through the thioredoxin system
• Brain health — selenoprotein P prioritizes selenium delivery to the brain
• Male fertility — selenium is needed for sperm structure and movement
• Thyroid eye disease — European guidelines recommend a six-month course of selenium for people with mild Graves’ orbitopathy, based on a trial that showed improvements in eye symptoms and quality of life
• Heart health in people with low selenium — in a Swedish trial in older adults with low selenium levels, selenium combined with coenzyme Q10 reduced deaths from cardiovascular disease; in people with adequate selenium, supplements haven’t shown heart benefits

For Athletes and Active People

Selenium is relevant for athletes where high oxidative stress, thyroid function, and muscle health meet.

Antioxidant enzyme adaptation: aerobic training increases mitochondrial density and the production of reactive oxygen species. The body responds by increasing its own antioxidant enzymes, including GPx — and adequate selenium is needed for that adaptation. This is different from taking an antioxidant pill: selenium supports the body’s own tightly regulated enzyme systems.

An important caveat: high-dose antioxidant supplements, such as vitamin C and vitamin E, may blunt some training adaptations by dampening the reactive oxygen species signals that drive them. Adequate selenium supports GPx without this concern, because the body regulates how much of these enzymes it makes — but there’s no benefit in pushing selenium intake beyond adequate levels.

Muscle tissue: selenoprotein W is concentrated in skeletal and heart muscle, where it’s thought to help protect muscle cells from oxidative damage.

Thyroid and metabolism: for athletes with unexplained fatigue, poor cold tolerance, slow recovery, or sluggish metabolism, thyroid function is worth checking with a doctor — and adequate selenium and iodine intake are part of the picture. This is most relevant for athletes in low-selenium regions, such as much of Europe, and those on restrictive diets.

Selenium and mercury: eating a lot of fish — common among athletes for omega-3 and protein — brings some mercury exposure. Selenium binds mercury tightly (forming mercury selenide), which is thought to reduce mercury’s toxicity, and most ocean fish contain more selenium than mercury. Some researchers believe this explains why the health benefits of fish usually outweigh its mercury risks. It doesn’t eliminate the risk, though — limits on high-mercury fish such as swordfish, shark, and king mackerel still apply, especially in pregnancy.

Insufficient Doses

Deficiency risk is higher in:

• People in regions with selenium-poor soils — parts of China, much of Europe (including the UK), Russia, and New Zealand. Unlike most mineral deficiencies, which reflect individual diet quality, low selenium can affect a whole population eating locally grown food. Finland has added selenium to agricultural fertilizers since 1984, which substantially raised the population’s intake
• Vegans and vegetarians in low-selenium regions — plant foods directly reflect soil levels, while animal foods provide a more consistent supply
• People with malabsorption conditions, such as Crohn’s disease or celiac disease
• People on long-term kidney dialysis
• People living with HIV, who are more likely to have low selenium levels
• People on long-term intravenous nutrition without enough selenium

Keshan disease: the most dramatic documented consequence of severe selenium deficiency — a heart muscle disease first identified in Keshan County, in northeastern China, in an area with some of the world’s lowest soil selenium. It causes an enlarged, weakened heart, heart rhythm problems, and heart failure, mainly in children and women of childbearing age. A common virus (coxsackievirus) appears to play a role: in selenium-deficient animals, the virus can mutate into a form that damages the heart. Large-scale selenium supplementation in China from the 1970s largely eliminated the disease.

Kashin-Beck disease: a degenerative joint disease found in selenium-poor regions of China, Tibet, Siberia, and North Korea. It affects growing children, causing joint deformity, short stature, and early osteoarthritis. Low selenium is a major factor, though iodine deficiency and fungal toxins in contaminated grain probably contribute.

Possible signs of deficiency:

• Muscle weakness and pain
• Weakened immune response and more frequent infections
• Hair and nail changes, such as whitened nail beds
• Hypothyroid-like symptoms — fatigue, weight gain, feeling cold — in severe deficiency, especially combined with low iodine
• Male infertility — reduced sperm movement
• In severe deficiency: heart muscle disease (as in Keshan disease)

Excessive Doses

Upper limit (UL): 400µg a day for adults and teenagers, from all sources. The limits for younger children are lower:

Source: US National Academies, via the NIH Office of Dietary Supplements.

Selenosis is the condition caused by chronic selenium excess. Because the gap between adequate intake and the upper limit is relatively small, it’s a realistic risk with high-dose supplements or very heavy Brazil nut consumption. In 2008, more than 200 people in the US developed selenosis after taking a liquid supplement that, through a manufacturing error, contained about 200 times the labeled amount of selenium.

Symptoms of selenosis:

• Garlic-like breath and a metallic taste — caused by an exhaled selenium compound (dimethyl selenide)
• Hair loss — often the most distinctive sign
• Brittle, discolored, or deformed nails
• Skin rashes
• Nausea, diarrhea, and abdominal pain
• Fatigue, irritability, and joint pain
• Nerve damage — numbness, tingling, and pain in the hands and feet
• In severe cases: serious neurological problems, breathing difficulties, and heart and kidney failure

On selenium, cancer, and diabetes: population studies in the 1980s and 1990s suggested lower cancer rates where selenium intakes were higher, and an early trial (the Nutritional Prevention of Cancer trial) hinted at fewer prostate cancers with 200µg a day of selenium yeast. This sparked major interest — but larger trials didn’t confirm it. The huge SELECT trial gave 35,000 men 200µg a day of selenomethionine for several years and found no reduction in prostate or other cancers. In a later analysis, men who already had high selenium levels and took the supplement had a higher risk of aggressive prostate cancer. Both trials also raised concerns about type 2 diabetes: in the Nutritional Prevention of Cancer trial, selenium significantly increased the risk, and in SELECT, there was a non-significant trend in the same direction. A Cochrane review concluded that selenium supplements don’t reduce cancer risk. The evidence supports getting enough selenium — not maximizing it.

Different Types

• Selenomethionine — selenium in the form of the amino acid methionine; the main form in food and very well absorbed. Because the body can build it into proteins in place of methionine, it accumulates in tissues over time — which makes it effective at raising selenium levels, but also means high doses build up.
• Selenium Yeast — yeast grown in a selenium-rich medium; contains mostly selenomethionine and is similarly well absorbed. The form used in the Nutritional Prevention of Cancer trial.
• Se-methylselenocysteine — a form found naturally in garlic, onions, leeks, and broccoli grown in selenium-rich soil; studied in the laboratory for anti-cancer effects, with little human evidence.
• Sodium Selenite — an inorganic form, somewhat less well absorbed than selenomethionine, but readily used to make selenoproteins and not stored in body proteins. Used in many lower-cost supplements, in intravenous nutrition, and in the thyroid eye disease trial.
• Sodium Selenate — an inorganic form that’s well absorbed but excreted in urine more quickly; used in some supplements and fortified products.

Which form for your goal:

How to Take It

Most people in North America don’t need a selenium supplement. If your intake is likely to be low — for example, if you live in a low-selenium region and eat little seafood, eggs, or meat — a modest supplement of about 50–100µg a day, or the amount in a multivitamin, is usually plenty. Avoid long-term doses of 200µg a day or more if your selenium intake is already adequate, given the trial evidence on diabetes and prostate cancer.

Check your total intake from all sources before supplementing: multivitamins often contain 50–100µg, food provides more, and a standalone supplement on top could push you toward or past the 400µg upper limit — particularly if you also eat Brazil nuts regularly.

Brazil nuts are a highly variable source, so don’t rely on them as your only selenium strategy. If you eat them, keep it to one or two at a time.

Selenium can be taken with or without food, and timing around training doesn’t matter, because selenium works through the body’s own enzyme systems rather than as an immediate dose.

If you’re paying attention to iodine for thyroid health, make sure your selenium intake is adequate too — the enzymes that activate thyroid hormones depend on it.

Interactions

The chemotherapy drug cisplatin can lower selenium levels. Very high doses of vitamin C taken at the same time may reduce the absorption of sodium selenite, so it’s reasonable to take them a few hours apart. Heavy metals such as mercury, cadmium, and arsenic bind selenium, which is part of why selenium helps protect against mercury toxicity — but it also means very high heavy metal exposure can reduce selenium’s availability.

Synergy

• Works with iodine in thyroid health — iodine builds T4, and selenium-dependent enzymes convert it to active T3.
• Works with vitamin E as complementary antioxidant systems — vitamin E stops chain reactions in membrane fats, and selenium-dependent GPx enzymes clear the resulting peroxides.
• Works with zinc and copper in the broader antioxidant enzyme network — the GPx, SOD, and catalase systems complement each other.

Antagonistic Interactions

• Mercury, cadmium, and arsenic bind selenium and reduce its availability — relevant mainly with high heavy metal exposure.
• Long-term high-dose selenium supplements, in people who already have enough, have been linked with higher type 2 diabetes risk.

Food values: USDA FoodData Central — Brazil nuts, raw (Foundation Foods, FDC ID 2515373; older SR Legacy figure cited in the text for comparison); Pacific oysters, cooked (SR Legacy, 174250); lamb liver, braised (SR Legacy, 172532); yellowfin tuna, cooked (SR Legacy, 172006); blue mussels, cooked (SR Legacy, 174217); light tuna, canned in water (Foundation Foods, 334194); clams, cooked (SR Legacy, 171975); halibut, cooked (SR Legacy, 174201); sardines, canned in oil (SR Legacy, 175139); shrimp, cooked (SR Legacy, 171971); pork top loin chop, broiled (SR Legacy, 167841); Atlantic salmon, farmed, cooked (SR Legacy, 175168); chicken breast, braised (Foundation Foods, 331960); egg, hard-boiled (SR Legacy, 173424); ground beef, 80% lean, cooked (SR Legacy, 171797); sunflower seeds, raw (Foundation Foods, 2515381); cottage cheese, low-fat (Foundation Foods, 328841); brown rice, cooked (SR Legacy, 169704); oats, cooked (SR Legacy, 173905); lentils, cooked (SR Legacy, 172421). RDA and upper limits from the US National Academies, via the NIH Office of Dietary Supplements. % Daily Values use the FDA’s current Daily Value of 55µg.