Selenium (selenomethionine vs selenite)
Summary
Selenium supplements come as selenomethionine, selenium yeast, selenite or selenate, each absorbed up to about 90%. Large long-term prevention trials found no protective effect, and one trial linked 200 mcg/day to a higher chance of developing high blood sugar. Most North Americans already exceed the RDA of 55 mcg; the adult UL is 400 mcg/day.
Key facts
- Selenomethionine, selenium-enriched yeast (mostly selenomethionine), sodium selenite, sodium selenate[1]
- ODS reports the body absorbs up to about 90% from all four forms; no head-to-head outcome trials in our sources[1]
- Long-term prevention end points ↔ (no-effect, 10 trials); blood sugar risk ↑ in one trial (C); total cholesterol ↓ modestly at 100–200 mcg/day (C); thyroid function in older adults ↔ (C)[2][3][4][5][6]
- RDA 55 mcg; 60 mcg in pregnancy, 70 mcg while breastfeeding. UL 400 mcg/day (EFSA set 255 mcg/day in 2023)[1]
- Intakes in North America, even in low-selenium regions, are well above the RDA. One Brazil nut can contain 68–91 mcg[1]
- Cisplatin can lower selenium levels; clinical significance unknown[1]
- 5 graded human outcomes on this page
- October 8, 2026
How it works
EstablishedSelenium is incorporated as selenocysteine into 25 human selenoproteins, including glutathione peroxidases, thioredoxin reductases and selenoprotein P. These enzymes take part in thyroid hormone metabolism (converting T4 to active T3), DNA synthesis, reproduction and protection from oxidative damage and infection (established).[1]
What the human research says
Human trials only, one row per outcome. The form column shows what each trial actually tested. Grades follow our evidence-grading policy.
At a glance: strongest evidence per outcome
| Outcome | Grade | Effect | Strongest evidence | Rows |
|---|---|---|---|---|
| Metabolic | Grade C | Improved | RCT · n = 1,202 · 2007 | 2 rows |
| Heart | Grade C | Worsened | RCT · n = 394 · 2011 | 1 row |
| Healthy aging | No effect | No effect | RCT · n = 35,533 · 2009 | 2 rows |
Every graded row
| Outcome | Effect | Grade | Best evidence | PMID |
|---|---|---|---|---|
| Developing high blood sugar over 7.7 years | Improved | Grade C | RCT · n = 1,202 · 2007 Adults from dermatology clinics in low-selenium areas of the eastern US, without type 2 diabetes at baseline Form: selenium (form not specified in abstract) · Dose: 200 microg/day · Duration: Average follow-up 7.7 years Type 2 diabetes developed in 58 people on selenium vs 39 on placebo (HR 1.55; 95% CI 1.03 to 2.33), with the highest risk in those with the highest baseline selenium (HR 2.70). | 17620655 (opens PubMed)Source [4] |
| Total and non-HDL cholesterol | Worsened | Grade C | RCT · n = 394 · 2011 UK volunteers aged 60 to 74 with relatively low selenium status Form: high-selenium yeast · Dose: 100, 200 or 300 mcg/day · Duration: 6 months Total cholesterol fell vs placebo by 8.5 mg/dL at 100 mcg/day and 9.7 mg/dL at 200 mcg/day, but not significantly at 300 mcg/day; non-HDL cholesterol fell similarly. The authors call the clinical significance unclear. | 21576533 (opens PubMed)Source [5] |
| Thyroid hormone levels in healthy older adults | No effect | Grade C | RCT · n = 368 · 2008 Euthyroid UK volunteers aged 60-74 (baseline plasma selenium 91 microg/L) Form: high-selenium yeast · Dose: 100, 200 or 300 microg/day · Duration: 6 months No effect on thyroid function (TSH, T3, T4) despite significant increases in plasma selenium. | 18258627 (opens PubMed)Source [6] |
| SELECT trial primary end point (men 50+) | No effect | No effect | RCT · n = 35,533 · 2009 Relatively healthy men aged 50+ (African American) or 55+ in the US, Canada and Puerto Rico Form: L-selenomethionine · Dose: 200 microg/day, alone or with vitamin E · Duration: Median follow-up 5.46 years Selenium did not lower prostate cancer risk (HR 1.04; 99% CI 0.87-1.24) or any other prespecified cancer end point. Type 2 diabetes was non-significantly more common with selenium (RR 1.07; 99% CI 0.94-1.22). | 19066370 (opens PubMed)Source [3] |
| Long-term prevention end points (10 placebo-controlled trials) | No effect | No effect | Systematic review · n = 27,232 · 2018 Participants in 10 randomized trials of selenium vs placebo for cancer prevention Form: selenium supplements (forms varied) · Dose: Varied across trials · Duration: Varied across trials In low-risk-of-bias trials, selenium did not change overall cancer incidence (RR 1.01; 95% CI 0.93 to 1.10; high certainty) or cancer deaths, nor colorectal, lung, bladder or prostate cancer. Low-bias trials suggested a higher melanoma risk. | 29376219 (opens PubMed)Source [2] |
Forms
| Form | Elemental % | Absorption (human data) | GI tolerance | Studied for |
|---|---|---|---|---|
| Selenomethionine (L-selenomethionine) | Varies[1] | Up to about 90% absorbed (ODS); no head-to-head human outcome data in our sources.[1] | No form-specific tolerance data in our sources.[1] | SELECT prevention trial (200 mcg/day) |
| Selenium-enriched yeast | Varies[1] | Up to about 90% absorbed (ODS).[1] | No form-specific tolerance data in our sources.[1] | UK trials on cholesterol and thyroid function in older adults; the Nutritional Prevention of Cancer trial (per ODS) |
| Sodium selenite and sodium selenate | Varies[1] | Up to about 90% absorbed (ODS); no head-to-head human data in our sources.[1] | No form-specific tolerance data in our sources; ODS notes chronic excess of organic and inorganic forms has similar effects.[1] | Some thyroid-antibody trials (selenite, per ODS) |
Studied doses
Doses studied in human trials ranged from 100 to 300 mcg/day of selenium, for 6 months to about 7.7 years. The adult UL is 400 mcg/day, and most people in North America already get more than the RDA from food.[5][6][3][4][1]
Intake reference: Adult RDA: 55 mcg/day; 60 mcg in pregnancy and 70 mcg while breastfeeding. Adult UL: 400 mcg/day from food and supplements. EFSA set a lower adult upper limit of 255 mcg/day in 2023.[1] The UL is based on hair and nail brittleness and loss (selenosis). It does not apply to people taking selenium under a physician's care. NIH ODS fact sheet (updated September 4, 2025).
Interactions
Published interactions only: each row cites a source. No row means none was found in our sources, not that a combination is safe.
Medicines that can lower levels
- CisplatinNIH fact sheetCan lower selenium levels in hair and serum; whether this matters clinically is unknown. A Cochrane review found insufficient evidence that selenium eases chemotherapy side effects.[1]
Who should be careful
- High intakes
- Chronic excess causes selenosis: hair loss and brittle nails, garlic breath, a metallic taste, skin rash, nausea, diarrhea, fatigue and nerve problems. Brazil nuts (68–91 mcg each) can supply toxic amounts if eaten regularly, and misformulated products have caused severe poisoning.[1]
- Blood sugar and prostate findings
- One long-term trial found more type 2 diabetes with 200 mcg/day, and ODS reports that SELECT investigators advised men over 55 against selenium above recommended intakes after finding more high-grade prostate cancer in men with high baseline selenium.[4][1]
- Thyroid autoimmunity in pregnancy
- ODS reports that the American Thyroid Association recommends against selenium supplements for pregnant women with thyroid autoimmunity.[1]
- Before you start
- Talk to your pharmacist or clinician before adding this supplement, especially if you take prescription medicines, are pregnant or breastfeeding, or have a chronic health condition.[1]
Talk to your pharmacist or clinician before starting a supplement, especially if you take prescription medicines.
Articles about Selenium (selenomethionine vs selenite)
Plain-English answers to common questions, built on the evidence above.
Frequently asked questions
Selenomethionine or sodium selenite: which is better?
ODS reports the body absorbs up to about 90% of selenium from selenomethionine, selenium yeast, selenite and selenate. Our sources contain no head-to-head outcome trials between forms; the largest prevention trial used selenomethionine.[1][3]
Does selenium lower cancer risk?
Not in randomized trials. A 2018 Cochrane review of 10 trials (27,232 participants) found no effect on overall cancer incidence, and the 35,533-man SELECT trial found no effect on prostate cancer.[2][3]
Does selenium help thyroid antibodies?
A 2016 meta-analysis of 16 trials in adults with chronic autoimmune thyroiditis found lower TPO antibody levels with selenium, but evidence quality was low, side effects were reported more often, and links to clinically relevant outcomes were not shown. In healthy older adults, selenium did not change thyroid function.[7][6]
How many Brazil nuts is too many?
Each Brazil nut can contain 68–91 mcg of selenium, and an ounce averages 544 mcg, above the 400 mcg adult UL. ODS warns regular intake could cause toxicity.[1]
Sources
- [1]Selenium: Fact Sheet for Health Professionals. NIH Office of Dietary Supplements, 2025. fact-sheet
- [2]Selenium for preventing cancer.. Cochrane Database Syst Rev, 2018. systematic-review
- [3]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. RCT
- [4]Effects of long-term selenium supplementation on the incidence of type 2 diabetes: a randomized trial.. Ann Intern Med, 2007. RCT
- [5]Effect of supplementation with high-selenium yeast on plasma lipids: a randomized trial.. Ann Intern Med, 2011. RCT
- [6]Randomized controlled trial of the effect of selenium supplementation on thyroid function in the elderly in the United Kingdom.. Am J Clin Nutr, 2008. RCT
- [7]Selenium Supplementation Significantly Reduces Thyroid Autoantibody Levels in Patients with Chronic Autoimmune Thyroiditis: A Systematic Review and Meta-Analysis.. Thyroid, 2016. meta-analysis
What changed
- First draft. Every evidence-row PMID checked with NCBI E-utilities (esummary + efetch abstract) and re-read in an independent adversarial pass.