Spermidine in Alzheimer's Disease: Evidence from Animal Models and Human Studies.

Spermidine is a naturally occurring polyamine involved in multiple cellular processes, including growth regulation, protein translation, and autophagy. Increasing attention has been devoted to its potential neuroprotective effects, particularly in Alzheimer's disease (AD), a neurodegenerative disorder characterized by β-amyloid and phosphorylated tau accumulation, synaptic dysfunction, and progressive neuronal loss. In this narrative review, we examine potential mechanisms through which spermidine may influence AD pathophysiology and summarize available preclinical and clinical evidence. Preclinical studies indicate that spermidine induces autophagy, a key cellular clearance pathway responsible for removing damaged organelles and aggregated proteins. Because impaired neuronal autophagy contributes to the accumulation of β-amyloid and tau in AD, increasing intracellular spermidine levels may enhance the degradation of these toxic species. In addition, spermidine exhibits anti-inflammatory and antioxidant properties, attenuates microglial activation, and supports mitochondrial function. In animal models of AD and brain aging, spermidine administration has been associated with improvements in cognitive performance and synaptic function. However, human clinical evidence remains limited and largely inconclusive. Observational studies suggest associations between higher dietary spermidine intake and better cognitive outcomes, but do not establish causality. Randomized clinical trials to date are few, include small and heterogeneous populations, and have not demonstrated consistent effects on primary cognitive endpoints. Overall, spermidine represents a biologically plausible modulator of pathways relevant to neurodegeneration, but translation of preclinical findings into clinical benefit remains uncertain. Current evidence is insufficient to support its use as a therapeutic or preventive intervention in AD, and further well-designed clinical studies are required to clarify its efficacy and mechanisms of action.