# Free radicals

Free radicals are atoms or molecules with one or more unpaired electrons. That makes them highly reactive. They come from your normal metabolism, your immune activity, and outside sources like UV light, pollution, and tobacco smoke. By stealing electrons from nearby molecules, free radicals damage your membranes, enzymes, and DNA. The free-radical theory of aging says this cumulative damage drives functional decline and age-related disease. The theory was hugely influential. But it is now seen as incomplete. Antioxidant supplements have not reliably extended lifespan, in animal trials or human RCTs. And reactive oxygen species turn out to be signaling molecules too, not just damaging agents.

## Sources

- Harman. (1956). Aging: a theory based on free radical and radiation chemistry. Journal of Gerontology. https://doi.org/10.1093/geronj/11.3.298
- Finkel & Holbrook. (2000). Oxidants, oxidative stress and the biology of ageing. Nature. https://doi.org/10.1038/35041687

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