# Selenium

Selenium is built into proteins as the amino acid selenocysteine, often called the 21st amino acid. That forms a class of enzymes called selenoproteins, with 25 encoded in the human genome. Key members include the glutathione peroxidases (GPx1 to GPx4), which neutralize reactive oxygen species in your cells, and the thioredoxin reductases (TxnRD1 to 3). Three iodothyronine deiodinases (DIO1 to 3) control the conversion of T4 to the active thyroid hormone T3. The functional sign of having enough is full saturation of selenoprotein P (SELENOP), at a plasma level around 110 µg/L. That is the threshold the Nordic Nutrition Recommendations 2023 use to set optimal intake (75 to 90 µg a day). The dose-response is U-shaped. Deficiency (serum selenium below 90 µg/L) is tied to faster epigenetic aging on the DunedinPACE and GrimAge clocks, in the Berlin Aging Study II (Vetter et al. 2025, n=1,568). But excess carries its own risk: a case-control study (Le et al. 2024) found a safe window of 111 to 124 µg a day, with higher cancer risk at both extremes. The SELECT trial (Klein et al. 2011, n=35,533) tested 200 µg a day of L-selenomethionine in men who already had enough selenium. Neither selenium nor vitamin E cut prostate cancer. That shows supplementing a well-nourished population does not behave like correcting a deficiency.

## Sources

- Vetter VM, Demircan K, Homann J, et al.. (2025). Low blood levels of selenium, selenoprotein P and GPx3 are associated with accelerated biological aging: results from the Berlin Aging Study II (BASE-II). Clinical Epigenetics. https://doi.org/10.1186/s13148-025-01863-7
- Klein EA, Thompson IM, Tangen CM, et al.. (2011). Vitamin E and the Risk of Prostate Cancer: The Selenium and Vitamin E Cancer Prevention Trial (SELECT). JAMA. https://doi.org/10.1001/jama.2011.1437
- Alexander J, Olsen AK. (2023). Selenium – a scoping review for Nordic Nutrition Recommendations 2023. Food & Nutrition Research. https://doi.org/10.29219/fnr.v67.10320
- Le NT, Pham YTH, Le CTK, et al.. (2024). A U-shaped association between selenium intake and cancer risk. Scientific Reports. https://doi.org/10.1038/s41598-024-66553-5

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_Canonical: https://longevity-switzerland.com/en/glossary/selenium · Part of Longevity Cities · Updated 2026-06-22_