17α-estradiol — The Non-Feminising Lifespan Extender
17α-estradiol is a non-feminising stereoisomer of the primary female sex hormone, estrogen. It has gained significant attention in longevity research after demonstrating robust, sex-specific lifespan extension and metabolic improvements in male mice, making it a leading candidate for geroprotective interventions in males.
Mechanism of Action
17α-estradiol (17α-E2) is a naturally occurring stereoisomer of 17β-estradiol that binds to estrogen receptor alpha (ERα) but exhibits significantly reduced feminising activity. In male mice, 17α-E2 signals through hepatic and hypothalamic ERα to improve metabolic homeostasis, enhance insulin sensitivity, and suppress hepatic gluconeogenesis. It beneficially modulates mTORC2 signalling and hepatic amino acid composition without inducing deleterious endocrine effects. The lifespan-extending effects appear to be dependent on these metabolic improvements and are strictly sex-specific, providing benefits only to males.
Human Trial Evidence
No published human longevity trials. Animal/in-vitro evidence only. The Interventions Testing Program (ITP) demonstrated that 17α-estradiol extends median lifespan in male mice by up to 19%, but it has no effect on female lifespan. Human data are currently limited to historical safety studies for other indications, such as hair loss.
Dosing Protocol
Unestablished in humans. In preclinical murine models, 17α-estradiol is typically administered via diet at 4.8 to 14.4 ppm, which corresponds to approximately 0.2 to 0.6 mg/kg/day. Recent pilot studies in non-human primates (marmosets) have evaluated oral doses of 0.37–0.72 mg/kg/day. No human dosing protocols for longevity exist.
Safety & Contraindications
Safety in humans for longevity purposes is unknown. While 17α-estradiol has reduced affinity for classical estrogen receptors compared to 17β-estradiol, potential risks include mild feminising effects at high doses, endocrine disruption, and unknown long-term impacts on hormone-sensitive tissues. It should be avoided by women, particularly those who are pregnant or breastfeeding, and men with a history of hormone-sensitive cancers.