Selenium is an essential trace mineral that the body builds into 25 selenoproteins, enzymes that control thyroid hormone activation, protect cells from oxidative damage and are needed for normal sperm. Most people in the US get well above the adult RDA of 55 mcg/day from food. Large trials show that extra selenium in well-nourished people did not prevent cancer, and the biggest of them, SELECT, raised concerns about type 2 diabetes and high-grade prostate cancer.
Selenium is often added to “testosterone support,” fertility and thyroid supplements aimed at lifters. The evidence follows a U-shaped pattern: people with low selenium status can benefit from correcting it, while people who already get enough gain little and may face harm at higher doses. The US upper limit is 400 mcg/day; the European limit, revised in 2023, is lower at 255 mcg/day. A single ounce of Brazil nuts can contain more than 500 mcg.
This page summarizes what the NIH Office of Dietary Supplements, the National Academies, the European Food Safety Authority and clinical trials report. It is for education only and is not medical advice.
| Property | Detail |
|---|---|
| Type | Essential trace mineral; dietary supplement ingredient |
| Main forms in supplements | Selenomethionine, selenium-enriched yeast, sodium selenite, sodium selenate; up to about 90% absorbed |
| Adult RDA (US) | 55 mcg/day for men and women; 60 mcg/day in pregnancy; 70 mcg/day in lactation |
| Upper limit | US: 400 mcg/day for adults. EU (EFSA 2023): 255 mcg/day for adults |
| Typical US intake | About 132 mcg/day from food in adult men and 94 mcg/day in adult women (NHANES 2017–March 2020) |
| Richest food source | Brazil nuts, averaging 544 mcg per ounce (6–8 nuts), with wide variation |
| Doses in cited trials | 100–300 mcg/day (thyroid, fertility, diabetes trials); 200 mcg/day as selenomethionine in SELECT |
| Half-life | No single value established for a nutrient held in many body pools; not listed here |
| WADA status | Not on the 2026 Prohibited List |
What it is
Selenium is a mineral found in soil and water. Plants take up inorganic selenium (selenite and selenate) and convert it to organic forms, mainly selenomethionine and selenocysteine. In food, selenium is present mostly as selenomethionine. It is readily absorbed, and absorption is largely unaffected by how much selenium a person already has (NIH ODS).
After absorption, both organic and inorganic selenium are converted to a common intermediate that the body uses to make selenocysteine, sometimes called the 21st amino acid. Selenocysteine is the active part of the 25 human selenoproteins, which include:
- Glutathione peroxidases, which break down hydrogen peroxide and lipid peroxides.
- Thioredoxin reductases, which keep other antioxidant systems working.
- Iodothyronine deiodinases, which convert the thyroid prohormone T4 into active T3.
- Selenoprotein P, the main carrier of selenium in blood.
About 28% to 46% of the body’s selenium is stored in skeletal muscle. The body keeps selenium in balance mainly through urinary excretion; at higher intakes, more is also lost through the breath and feces (NIH ODS).
Rayman’s 2012 review in The Lancet stresses the central point about selenium: its health effects follow a U-shaped curve. Extra selenium may help people with low status, while people with adequate-to-high status may be harmed and “should not take selenium supplements” (Rayman, Lancet 2012).
Measuring status
Plasma or serum selenium is the usual test. Concentrations of 8 mcg/dL or higher are usually considered sufficient in healthy people. Hair and nail selenium reflect intake over months to years. Glutathione peroxidase 3 and selenoprotein P can be used as functional markers, but inflammation can affect them (NIH ODS). In US NHANES data from 2011 to 2016, mean serum selenium in people aged 8 to 80 was 12.7 mcg/dL, comfortably above that cut-off.
Dietary reference intakes
The US Food and Nutrition Board set the following values (NIH ODS; IOM 2000):
| Group | RDA (mcg/day) | UL (mcg/day) |
|---|---|---|
| Infants 0–6 months | 15 (AI) | 45 |
| Infants 7–12 months | 20 (AI) | 60 |
| Children 1–3 years | 20 | 90 |
| Children 4–8 years | 30 | 150 |
| Ages 9–13 years | 40 | 280 |
| Ages 14 years and older | 55 | 400 |
| Pregnancy | 60 | 400 |
| Lactation | 70 | 400 |
The RDA (Recommended Dietary Allowance) covers the needs of nearly all (97%–98%) healthy people. The AI (Adequate Intake) is used when evidence is too thin to set an RDA. The UL (Tolerable Upper Intake Level) is the highest daily intake unlikely to cause harm. The US UL is based on hair and nail brittleness and loss, and it does not apply to people taking selenium under a physician’s care.
European values differ:
- Adequate intake. In 2014, EFSA’s nutrition panel set an AI of 70 mcg/day for adults, based on the intake at which plasma selenoprotein P levels off. It found no evidence of extra benefit above that intake. The AI for lactation is 85 mcg/day (EFSA NDA Panel, EFSA J 2014).
- Upper limit. In 2023, EFSA set a UL of 255 mcg/day for all adults, including pregnant and lactating women. It used alopecia (hair loss) as the critical effect, took a lowest-observed-adverse-effect level of 330 mcg/day from SELECT, and applied an uncertainty factor of 1.3. EFSA said adults are unlikely to exceed the UL except regular users of high-dose supplements or regular Brazil nut eaters (EFSA NDA Panel, EFSA J 2023).
Food sources and supplements
Selenium is bound to protein, so protein-rich foods tend to be the best sources. The ODS table, based on USDA data, lists these examples (NIH ODS):
| Food | Selenium per serving |
|---|---|
| Brazil nuts, 1 ounce (6–8 nuts) | 544 mcg |
| Tuna, yellowfin, cooked, 3 ounces | 92 mcg |
| Sardines, canned in oil, 3 ounces | 45 mcg |
| Shrimp, cooked, 3 ounces | 42 mcg |
| Beef steak or pork chop, 3 ounces | 37 mcg |
| Chicken, light meat, roasted, 3 ounces | 22 mcg |
| Egg, hard-boiled, 1 large | 15 mcg |
| Brown rice, cooked, 1 cup | 12 mcg |
Brazil nuts vary widely. The ODS gives a range of 68–91 mcg per nut and warns that eating them regularly could cause selenium toxicity. The selenium content of plant foods depends on local soil, while animal products are more consistent because animals regulate their tissue levels and livestock feed is usually supplemented.
In US NHANES data from 2017 to March 2020, adult men averaged 132 mcg/day from food and beverages (145 mcg/day including supplements), and adult women averaged 94 mcg/day (102 mcg/day). About 17% to 19% of adults take a supplement that contains selenium. Multivitamins often contain 55 mcg, combination products 50–200 mcg, and selenium-only products typically 100–400 mcg (NIH ODS).
Deficiency is very rare in the US and Canada. Groups at higher risk include people in low-selenium regions (parts of China and some European countries, especially people on vegan diets), people on long-term kidney dialysis and people living with HIV. Severe deficiency is linked to Keshan disease, a heart-muscle disease first described in selenium-poor areas of China, where adults ate no more than 10 mcg/day (NIH ODS).
What the research shows
Prostate cancer and the SELECT trial
The idea that selenium prevents cancer came largely from the Nutritional Prevention of Cancer (NPC) trial. In 1,312 US adults with a history of skin cancer, 200 mcg/day of selenium yeast did not reduce skin cancer recurrence. A secondary analysis found 49% fewer prostate cancers in men taking selenium, with the effect concentrated in men with the lowest baseline plasma selenium (below 10.6 mcg/dL) (NIH ODS).
The Selenium and Vitamin E Cancer Prevention Trial (SELECT) was designed to test that finding directly:
- Design. 35,533 relatively healthy men from 427 sites in the US, Canada and Puerto Rico were randomized between 2001 and 2004 to 200 mcg/day of selenium (as L-selenomethionine), 400 IU/day of vitamin E, both, or placebo (Lippman et al., JAMA 2009).
- First report (2009). After a median 5.46 years, the hazard ratio for prostate cancer with selenium alone was 1.04 (99% CI 0.87–1.24). No other prespecified cancer outcome differed. The supplements were stopped early.
- Extended follow-up (2011). With more cases, selenium alone still had no significant effect (HR 1.09; 99% CI 0.93–1.27), but vitamin E alone significantly increased prostate cancer (HR 1.17; 99% CI 1.004–1.36) (Klein et al., JAMA 2011).
- Baseline status analysis (2014). In a case-cohort study of 1,739 prostate cancer cases and 3,117 comparison men, selenium had no effect in men with lower baseline toenail selenium. In men with higher baseline status (60th percentile or above), selenium supplements increased the risk of high-grade prostate cancer by 91% (P = .007). The authors concluded that men should avoid selenium or vitamin E supplements at doses above recommended intakes (Kristal et al., J Natl Cancer Inst 2014).
A 2018 Cochrane review pooled 10 randomized trials with 27,232 participants (94% men). Selenium at 200–500 mcg/day for 2 to 10 years had little to no effect on cancer incidence or cancer death, with high-certainty evidence for prostate, colorectal, lung and bladder cancer (Vinceti et al., Cochrane 2018). In 2009, the FDA concluded it is “highly unlikely” that selenium supplements reduce prostate cancer risk (NIH ODS).
Type 2 diabetes signal
Two large trials raised a diabetes concern:
- NPC trial. In a secondary analysis of 1,202 people without diabetes at baseline, type 2 diabetes developed in 58 people on 200 mcg/day of selenium vs 39 on placebo over an average of 7.7 years (HR 1.55; 95% CI 1.03–2.33). Risk was highest in people who started with the most selenium in their blood (top tertile HR 2.70) (Stranges et al., Ann Intern Med 2007).
- SELECT. The selenium-only group had a statistically nonsignificant increase in type 2 diabetes (relative risk 1.07; 99% CI 0.94–1.22; P = .16) (Lippman et al. 2009).
A 2014 meta-analysis of 4 randomized trials with 20,294 participants found a pooled relative risk of 1.09 (95% CI 0.99–1.20), not statistically significant, and concluded the results do not support selenium for diabetes prevention (Mao et al., Endocrine 2014). In the Danish PRECISE trial, in older adults with relatively low selenium status, 100–300 mcg/day for 2 years did not change HbA1c compared with placebo; the authors said the results fit a possible U-shaped effect on glucose metabolism (Stranges et al., Diabetes Obes Metab 2019).
Taken together, selenium supplements have not been shown to cause diabetes, but they do not prevent it, and the signal is strongest in people who already had high selenium levels.
Cardiovascular disease
SELECT found no effect of 200 mcg/day on cardiovascular events over 7 to 12 years. A Cochrane review of SELECT and 11 smaller trials, covering 19,715 mostly male adults, also found no reduction in fatal or nonfatal cardiovascular events (NIH ODS).
Thyroid
The thyroid holds more selenium per gram than any other organ. Selenoproteins convert T4 to T3 and protect the gland from the hydrogen peroxide made during hormone production (NIH ODS).
- Healthy older adults. In a 6-month trial of 368 people aged 60–74, 100–300 mcg/day raised plasma selenium but did not change thyroid function (NIH ODS).
- Hashimoto thyroiditis. A 2024 meta-analysis of 35 studies found that selenium lowered TSH in patients not on thyroid hormone (SMD −0.21; 7 cohorts, 869 participants) and lowered thyroid peroxidase antibodies (SMD −0.96; 29 cohorts, 2,358 participants), with adverse effects similar to control. Free T4, free T3 and thyroid volume did not change, and the certainty of evidence was moderate (Huwiler et al., Thyroid 2024). An earlier review of 9 trials limited to selenium alone found no effect on TSH, ultrasound findings or quality of life (NIH ODS). Lower antibody levels have not been shown to translate into better long-term outcomes.
- Mild Graves’ orbitopathy. In a randomized trial of 159 patients, 100 mcg of selenium twice daily for 6 months improved quality of life (P under 0.001) and eye involvement (P = 0.01) and slowed progression compared with placebo (Marcocci et al., N Engl J Med 2011). The trial was done in Europe, where selenium intakes are lower than in the US.
- Pregnancy. In 2017, the American Thyroid Association made a weak recommendation against selenium for pregnant women with thyroid peroxidase antibodies (NIH ODS).
Sperm and male fertility
A selenoprotein, phospholipid hydroperoxide glutathione peroxidase, is a structural part of mature sperm. Both low and high semen selenium have been linked to infertility (NIH ODS). Trial results depend on starting status:
| Study | Who | What was given | Result |
|---|---|---|---|
| Scott et al., Br J Urol 1998 | 69 subfertile Scottish men with low motility (low-selenium population) | Selenium, with or without vitamins A, C and E, or placebo for 3 months | Combined selenium groups: motility improved (P = 0.023); sperm count unchanged; 5 men (11%) fathered a child vs none on placebo |
| Hawkes & Turek, J Androl 2001 | 11 healthy men on a controlled diet | 13 or 297 mcg/day of selenium from food for 99 days | High-selenium group: motile sperm fell 32% by week 13; T3 fell and TSH rose; serum androgens unchanged |
| Hawkes et al., J Androl 2009 | 42 healthy US men | 300 mcg/day of selenium yeast or placebo for 48 weeks | No effect on sperm count, motility or morphology, sperm selenium or serum androgens |
| Steiner et al., Fertil Steril 2020 (MOXI) | 174 men with male factor infertility | Antioxidant mix including 200 mcg selenium, plus vitamins C and E, carnitine, zinc, folic acid and lycopene, vs placebo | No improvement in semen quality or DNA fragmentation; live birth 15% vs 24% (not significantly different) |
A 2025 meta-analysis of 50 placebo-controlled trials found that selenium improved sperm motility, but no supplement improved pregnancy or live birth rates, and the certainty of evidence was generally low or very low (Michaelsen et al., Nutrients 2025).
Testosterone and lifting
Selenium is often listed as a testosterone-support ingredient. In the trials above in healthy, selenium-replete men, neither 297 mcg/day from food nor 300 mcg/day from yeast changed serum androgen levels (Hawkes & Turek 2001; Hawkes et al. 2009). We found no placebo-controlled trial showing that selenium raises testosterone, muscle mass or strength in people who are not deficient.
Side effects and risks
Chronic excess (selenosis)
Early signs of too much selenium are a garlic odor on the breath and a metallic taste. The most common signs of chronic excess are hair loss and brittle or lost nails. Others include skin rash, nausea, diarrhea, fatigue, irritability and nervous system problems. Organic and inorganic forms have similar effects (NIH ODS).
Acute poisoning from a misformulated supplement
In 2008, a US outbreak was traced to a liquid supplement that contained 200 times its labeled selenium concentration. Investigators identified 201 cases in 10 states. The median estimated dose was 41,749 mcg/day. Common symptoms were diarrhea (78%), fatigue (75%), hair loss (72%), joint pain (70%), nail discoloration or brittleness (61%) and nausea (58%). After 90 days or more, 52% still had nail problems and 29% still had hair loss (MacFarquhar et al., Arch Intern Med 2010). Severe acute toxicity can also cause breathing failure, heart attack, kidney failure and, rarely, death (NIH ODS).
Stacking
Because many people already get 100–150 mcg/day from food, adding a 200 mcg “testosterone” product on top of a multivitamin and regular Brazil nuts can approach or pass the European UL of 255 mcg/day. SELECT’s 200 mcg/day dose, given to well-nourished men, is the dose linked to the high-grade prostate cancer and diabetes signals described above.
Interactions
The ODS lists cisplatin, a chemotherapy drug, which can lower selenium levels in hair and serum. Whether that matters clinically is not known, and a Cochrane review found insufficient evidence that selenium reduces chemotherapy side effects (NIH ODS).
Legal status and use in sport
In the US, selenium is sold as a dietary supplement and is not approved by the FDA to treat any condition. In 2003 the FDA allowed qualified health claims about selenium and cancer, and in 2009 it denied claims for most cancers (NIH ODS). Selenium is not named on the WADA 2026 Prohibited List. Like any multi-ingredient supplement, a selenium product can still be contaminated with prohibited substances, so tested athletes rely on third-party certification.
For related minerals often stacked with selenium, see Zinc, Magnesium and Boron. For thyroid hormone drugs, see T3 (liothyronine) and T4 (thyroxine).
Key takeaways
- Essential, but needed in tiny amounts. The adult RDA is 55 mcg/day, and most people in the US eat about twice that from food.
- U-shaped effects. Correcting low status can help, but extra selenium in replete people has shown no benefit and some signals of harm.
- SELECT was negative. 200 mcg/day did not prevent prostate cancer in 35,533 men and raised high-grade prostate cancer risk in men who started with high selenium status.
- Diabetes concern. The NPC trial found 55% more type 2 diabetes with 200 mcg/day; SELECT and a meta-analysis showed smaller, nonsignificant increases.
- Thyroid data are mixed. Selenium lowers thyroid antibodies in Hashimoto thyroiditis and helped mild Graves’ eye disease in one trial, but it is not a routine treatment.
- No testosterone effect shown. Trials in healthy men found no change in serum androgens, and sperm benefits are mainly seen in low-selenium populations.
- Watch the upper limit. The US UL is 400 mcg/day and the EU UL is 255 mcg/day; hair and nail loss are early signs of excess, and Brazil nuts are very concentrated.
Sources
- National Institutes of Health Office of Dietary Supplements. Selenium: fact sheet for health professionals. Updated September 4, 2025. NIH ODS.
- Institute of Medicine. Dietary Reference Intakes for Vitamin C, Vitamin E, Selenium, and Carotenoids. Washington, DC: National Academies Press; 2000. National Academies Press.
- EFSA Panel on Dietetic Products, Nutrition and Allergies (NDA). Scientific opinion on dietary reference values for selenium. EFSA J. 2014;12(10):3846. PubMed.
- EFSA Panel on Nutrition, Novel Foods and Food Allergens (NDA), Turck D, Bohn T, et al. Scientific opinion on the tolerable upper intake level for selenium. EFSA J. 2023;21(1):e07704. PubMed.
- Rayman MP. Selenium and human health. Lancet. 2012;379(9822):1256–1268. PubMed.
- Lippman SM, Klein EA, Goodman PJ, et al. Effect of selenium and vitamin E on risk of prostate cancer and other cancers: the Selenium and Vitamin E Cancer Prevention Trial (SELECT). JAMA. 2009;301(1):39–51. PubMed.
- Klein EA, Thompson IM, Tangen CM, et al. Vitamin E and the risk of prostate cancer: the Selenium and Vitamin E Cancer Prevention Trial (SELECT). JAMA. 2011;306(14):1549–1556. PubMed.
- Kristal AR, Darke AK, Morris JS, et al. Baseline selenium status and effects of selenium and vitamin E supplementation on prostate cancer risk. J Natl Cancer Inst. 2014;106(3):djt456. PubMed.
- Vinceti M, Filippini T, Del Giovane C, et al. Selenium for preventing cancer. Cochrane Database Syst Rev. 2018;1(1):CD005195. PubMed.
- Stranges S, Marshall JR, Natarajan R, et al. Effects of long-term selenium supplementation on the incidence of type 2 diabetes: a randomized trial. Ann Intern Med. 2007;147(4):217–223. PubMed.
- Mao S, Zhang A, Huang S. Selenium supplementation and the risk of type 2 diabetes mellitus: a meta-analysis of randomized controlled trials. Endocrine. 2014;47(3):758–763. PubMed.
- Stranges S, Rayman MP, Winther KH, et al. Effect of selenium supplementation on changes in HbA1c: results from a multiple-dose, randomized controlled trial. Diabetes Obes Metab. 2019;21(3):541–549. PubMed.
- Huwiler VV, Maissen-Abgottspon S, Stanga Z, et al. Selenium supplementation in patients with Hashimoto thyroiditis: a systematic review and meta-analysis of randomized clinical trials. Thyroid. 2024;34(3):295–313. PubMed.
- Marcocci C, Kahaly GJ, Krassas GE, et al. Selenium and the course of mild Graves’ orbitopathy. N Engl J Med. 2011;364(20):1920–1931. PubMed.
- Scott R, MacPherson A, Yates RW, et al. The effect of oral selenium supplementation on human sperm motility. Br J Urol. 1998;82(1):76–80. PubMed.
- Hawkes WC, Turek PJ. Effects of dietary selenium on sperm motility in healthy men. J Androl. 2001;22(5):764–772. PubMed.
- Hawkes WC, Alkan Z, Wong K. Selenium supplementation does not affect testicular selenium status or semen quality in North American men. J Androl. 2009;30(5):525–533. PubMed.
- Steiner AZ, Hansen KR, Barnhart KT, et al. The effect of antioxidants on male factor infertility: the Males, Antioxidants, and Infertility (MOXI) randomized clinical trial. Fertil Steril. 2020;113(3):552–560.e3. PubMed.
- Michaelsen MP, Poulsen M, Bjerregaard AA, et al. The effect of dietary supplements on male infertility in terms of pregnancy, live birth, and sperm parameters: a systematic review and meta-analysis. Nutrients. 2025;17(10):1710. PubMed.
- MacFarquhar JK, Broussard DL, Melstrom P, et al. Acute selenium toxicity associated with a dietary supplement. Arch Intern Med. 2010;170(3):256–261. PubMed.
- World Anti-Doping Agency. The 2026 Prohibited List. WADA 2026 Prohibited List.