What Is Selenium and How Does It Work?
Selenium is an essential trace element that humans must obtain from diet or supplementation because the body cannot synthesise it. It functions primarily as a cofactor in selenoproteins — a family of over 25 enzymes with roles in antioxidant defence (glutathione peroxidases), thyroid hormone metabolism (deiodinases), and immune regulation (thioredoxin reductases).
Selenium status varies considerably across Europe due to soil depletion, and deficiency is genuinely common in several Northern and Central European countries including Finland and parts of the Baltic region. This geographic reality makes selenium one of the micronutrients where supplementation has a clear rationale for people in low-soil-selenium areas.
What the RCT and Meta-Analysis Evidence Shows
The evidence for selenium spans several distinct domains:
Thyroid function: A Cochrane systematic review of selenium supplementation in Hashimoto's thyroiditis found that selenomethionine reduced thyroid peroxidase antibody concentrations compared to placebo (van Zuuren et al., 2013). The clinical significance of this antibody reduction, however, remains debated, as not all trials showed improvements in thyroid hormone levels or symptoms.
Antioxidant defence: Selenium unambiguously increases glutathione peroxidase activity when given to deficient individuals. This is a well-replicated biochemical finding, though translating enzyme activity into clinical outcomes is more complex.
Cancer prevention: The SELECT trial — a large, well-powered RCT — found no reduction in prostate cancer risk with selenium supplementation (Lippman et al., 2009). Earlier epidemiological associations between high selenium status and lower cancer risk have not been consistently reproduced in intervention studies.
Immune function: Selenium supplementation in deficient populations supports immune cell function, consistent with its role in selenoprotein-mediated immune signalling.
Effect Sizes and Who Benefits Most
Selenium's benefits are most pronounced in people who are genuinely deficient. In selenium-replete individuals, supplementation adds little biochemical benefit and — critically — carries the risk of excess. The safe intake window for selenium is narrower than for most minerals.
People most likely to benefit include those living in low-selenium regions, individuals with thyroid autoimmune conditions (under medical guidance), and those following highly restrictive diets that exclude selenium-rich foods such as Brazil nuts, organ meats, and seafood.
EFSA-Approved Claims
EFSA has authorised the following health claims for selenium:
- Contributes to the normal function of the immune system
- Contributes to normal thyroid function
- Contributes to the maintenance of normal hair and nails
- Contributes to normal spermatogenesis
- Contributes to the protection of cells from oxidative stress
These are the only claims that can lawfully appear on selenium supplement labels in the EU. Claims about cancer prevention or disease treatment are not authorised.
Honest Verdict
Selenium has some of the strongest authorised claim support of any micronutrient, reflecting its genuine essentiality. The evidence is clearest at the level of correcting deficiency — the immunological, thyroid, and antioxidant benefits are biologically real when intake is insufficient. For people already meeting adequate intake from diet, supplementation offers diminishing returns and carries a real toxicity risk at doses not dramatically above the recommended level. More is not better with selenium: get tested before supplementing, or choose a low-dose formulation aligned with the EU reference intake.
Selenium supplements are available at maxfit.ee.
References
van Zuuren, E. J., Albusta, A. Y., Fedorowicz, Z., Carter, B., & Pijl, H. (2013). Selenium supplementation for Hashimoto's thyroiditis. Cochrane Database of Systematic Reviews, (6), CD010223. PMID: 23744563
Lippman, S. M., Klein, E. A., Goodman, P. J., Lucia, M. S., Thompson, I. M., Ford, L. G., … Coltman, C. A. (2009). Effect of selenium and vitamin E on risk of prostate cancer and other cancers: the Selenium and Vitamin E Cancer Prevention Trial (SELECT). JAMA, 301(1), 39–51. PMID: 19066370 https://pubmed.ncbi.nlm.nih.gov/19066370/
Rayman, M. P. (2012). Selenium and human health. The Lancet, 379(9822), 1256–1268. PMID: 22381456 https://pubmed.ncbi.nlm.nih.gov/22381456/
FAQ
How much selenium do I need per day?
The EU Nutrient Reference Value for selenium is 55 µg/day for adults. The tolerable upper intake level set by EFSA is 300 µg/day from all sources combined. Most supplements provide 50–200 µg per serving.
Can you take too much selenium?
Yes. Selenosis — selenium toxicity — causes hair loss, nail brittleness, garlic breath, and neurological symptoms. It can occur at intakes chronically above the tolerable upper limit. This makes selenium unusual among supplements: the margin between adequate and toxic is relatively narrow, especially when combining supplements with selenium-rich foods like Brazil nuts.
What is the best form of selenium in supplements?
Selenoorganic forms — selenomethionine and selenium-enriched yeast — have higher bioavailability than inorganic selenate or selenite. Most quality supplements use selenomethionine for this reason.
