Gut Dysbiosis and Microbiota-Derived Metabolites in Neurodegenerative Diseases: Molecular and Biochemical Mechanisms Along the Gut–Brain Axis

Neurodegenerative disorders such as Alzheimer's disease (AD), Parkinson's disease (PD), and amyotrophic lateral sclerosis (ALS) share key molecular features, including neuroinflammation, oxidative stress, mitochondrial dysfunction, and progressive neuronal loss. Increasing evidence indicates that gut dysbiosis and alterations in microbiota-derived metabolites are involved in these processes through multiple pathways along the gut-brain axis. However, while broad compositional changes are well-documented, a critical knowledge gap remains regarding the specific biochemical signal transduction pathways translating dysbiosis into pathology. This narrative review addresses this gap by synthesizing current human and…