Selenium is a trace mineral essential for producing glutathione peroxidase — a key enzyme in the body's antioxidant defense system. It works with vitamin E to protect cell membranes from free radical damage. The body uses selenium to synthesize 25 human selenoproteins involved in thyroid function, immune response, and DNA synthesis [1].
Selenium deficiency is uncommon in the U.S. but more prevalent in regions with selenium-depleted soil, including Northern Europe and certain areas of China. Importantly, while deficiency is associated with health risks, supplementation in people with already adequate levels may cause harm.
Table of Contents
- Overview
- Forms and Bioavailability
- Evidence for Benefits
- Recommended Dosing
- Safety and Side Effects
- Drug Interactions
- Dietary Sources
- References
Overview
Selenium is a trace mineral essential for producing glutathione peroxidase — a key enzyme in the body's antioxidant defense system. It works with vitamin E to protect cell membranes from free radical damage [1]. Selenium deficiency is uncommon in the U.S. but is more prevalent in regions with selenium-depleted soil, including Northern and Central Europe and certain areas of China [2].
Deficiency may contribute to weakened immunity, hypothyroidism, and Keshan disease (a childhood heart condition). However, supplementation in people with already adequate levels may increase diabetes and prostate cancer risk.
Forms and Bioavailability
Selenium compounds are generally very efficiently absorbed, with the body absorbing up to 90% from most supplement forms [3]:
- Selenomethionine — absorption >90%. The predominant form in selenium-enriched yeast and many clinical trials.
- Selenite (sodium selenite) — absorption >80%. An inorganic form.
- Selenate (sodium selenate) — absorption >90%. Another inorganic form.
- Selenium glycinate — promoted as better absorbed, but not demonstrated in humans [4].
- Se-methyl-L-selenocysteine — naturally occurring in cruciferous vegetables; less studied as a supplement [5].
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Evidence for Benefits
Cancer
An early landmark study showed a 50% reduction in overall cancer deaths with selenium supplementation, though benefits were largely in participants with lower baseline selenium [7][8]. A study of African-American women found 20+ mcg/day supplemental selenium was associated with approximately 30% lower ovarian cancer risk [9].
However, the SELECT trial found selenium failed to prevent prostate cancer and actually increased high-grade prostate cancer risk by 91% in men with already-high selenium [10]. In men with existing prostate cancer, 140+ mcg/day was associated with 160% greater prostate cancer mortality [11]. A Cochrane review found no benefit for skin or colorectal cancer prevention [12].
Diabetes Risk
Selenium supplementation at 200 mcg/day made participants with adequate baseline selenium 2.7 times more likely to develop type 2 diabetes [13]. An Italian study found the highest dietary selenium intakes were associated with 64% greater diabetes risk [14].
Cognitive Function
A major placebo-controlled study found that 200 mcg/day of selenium, vitamin E, or both did not significantly affect dementia risk over 13 years [15].
Other Conditions
Low selenium has been associated with rheumatoid arthritis development, though supplements do not help once developed [16]. Greater bone mineral density has been associated with higher selenium in postmenopausal women [17]. Selenium deficiency was common in COVID-19 patients and more prevalent in those who died [18]. Selenium does not appear helpful for preventing heart disease [19].
Recommended Dosing
The RDA for selenium is 55 mcg/day for adults 14+ (60 mcg during pregnancy, 70 mcg while nursing) [3]. Most U.S. adults already consume adequate selenium from diet (average blood level: 137 mcg/L).
Supplementation primarily benefits those with documented low levels (serum selenium below 122 mcg/L). Total selenium intake (diet + supplements) should remain well below the Tolerable Upper Intake Level of 400 mcg/day.
Safety and Side Effects
- Hair/nail brittleness: Can occur above 400 mcg/day total intake [20].
- Selenosis: Begins at ~900 mcg/day — depression, nervousness, nausea, vomiting.
- Diabetes risk: 200 mcg/day increased diabetes risk 2.7-fold in selenium-adequate people [13].
- Prostate cancer risk: 200 mcg/day increased high-grade prostate cancer 91% in men with high selenium [10].
- Increased mortality: 300 mcg/day for 5 years associated with 11% greater all-cause mortality [21].
People with adequate-to-high selenium (serum ≥122 mcg/L) should generally avoid selenium supplements [23].
Drug Interactions
- Acid-reducing medications: PPIs and H2 blockers may reduce selenium absorption.
- Cisplatin and other chemotherapy: May reduce selenium levels; consult oncologist.
- Fluoroquinolone antibiotics: Prophylactic selenium with vitamin E may reduce Achilles tendinopathy severity.
Dietary Sources
| Food | Selenium (mcg) | Serving |
|---|---|---|
| Brazil nuts, dried | 543.5 | 1 oz (6–8 nuts) |
| Rockfish, cooked | 113.5 | 1 fillet |
| Tuna, yellowfin | 92 | 3 oz |
| Halibut, cooked | 88.1 | ½ fillet |
| Salmon, sockeye | 82 | 1 fillet |
| Chicken liver | 75 | 3 oz |
| Whole wheat flour | 74.2 | 1 cup |
| Eggs, hardboiled | 41.9 | 1 cup |
| Turkey | 36 | 3 oz |
| Shiitake mushrooms | 36 | 1 cup |
Selenium content in plants varies considerably depending on soil levels. Selenium in plant-based foods is more bioavailable than that in animal products [1].
Is Your Selenium Intake in the Right Range?
Too little selenium increases risk, but too much can cause harm. Get personalized mineral recommendations based on your diet and health profile with Health Roadmap.
Get Your Personalized Health PlanReferences
1. Levander OA et al., Annu Rev Nutr, 1987.
2. Gibbs J et al., Dietetics, 2024.
3. NIH Office of Dietary Supplements, Selenium Fact Sheet, 2024.
4. Geraert PA et al., AFMA Matrix, 2017.
5. European Food Safety Authority, EFSA Scientific Opinion, 2009.
6. Clark LC et al., JAMA, 1996.
7. Clark LC et al., JAMA, 1996.
8. Reid ME et al., Canc Epidem Biomark Prev, 2002.
9. Terry P et al., J Nutr, 2017.
10. Kristal AR et al., JNCI, 2014.
11. Kenfield SA et al., J Natl Canc Inst, 2015.
12. Filippini T et al., Cochrane Database Syst Rev, 2018.
13. Stranges S et al., Ann Intern Med, 2007.
14. Vinceti M et al., Nutr Metab Cardiovasc Dis, 2021.
15. Kryscio RJ et al., JAMA, 2017.
16. Tarp U et al., Analyst, 1995.
17. Hoeg A et al., J Clin Endocrinol Metab, 2012.
18. Moghaddam A et al., Nutrients, 2020.
19. Rayman MP et al., Biol Psychiatry, 2006.
20. Civas E et al., J Cosmet Dermatol, 2023.
21. Rayman MP et al., Free Radic Biol Med, 2018.
22. Lippman SM et al., JAMA, 2009.
23. Rayman MP, Lancet, 2012.
