Spermidine

Spermidine is a naturally occurring polyamine found throughout human cells and in concentrated amounts in wheat germ, aged cheese, soy products, and mushrooms. Its longevity credentials center on autophagy induction — the cellular self-cleaning process that degrades damaged proteins, dysfunctional organelles, and intracellular pathogens. Autophagy declines with age, and its declining activity is now recognized as a primary driver of the proteostasis failure and mitochondrial dysfunction that characterize aged tissues. Spermidine's autophagy-inducing mechanism is distinct from caloric restriction and mTOR inhibition: it works by inhibiting acetyltransferases that would otherwise suppress the autophagy regulatory network, and by stabilizing microtubule-associated protein 1 light chain 3 (LC3), a central component of autophagosome formation. This mechanistic independence from mTOR/AMPK pathways means spermidine may complement metformin, rapamycin, or AKG in a stack without significant overlap. The human evidence base took a meaningful step forward with the SmartAge trial (PMC9136623) — a 12-month double-blind RCT testing 0.9mg/day of spermidine from wheat germ extract in older adults with subjective cognitive decline. Results: the intervention was safe and well-tolerated with no significant adverse events. Cognitive outcomes showed a non-significant trend toward improvement versus placebo, but the trial was not powered to detect small effects. An earlier safety study (PMC5807086) confirmed safety in older adults at comparable doses. Longevity researchers note that the SmartAge dose (0.9mg/day) may be subtherapeutic for maximal autophagy induction. Epidemiological data correlating higher dietary spermidine intake with reduced mortality has driven interest in higher-dose supplementation protocols, with some practitioners using 5–15mg/day of purified spermidine — doses significantly higher than the SmartAge trial. The wheat germ source used in SmartAge provides a food-derived, well-tolerated delivery vehicle. Spermidine levels in blood decline with age, supporting a depletion-and-restoration rationale analogous to taurine.