The Microbiota–Gut–Brain Axis Across the Lifespan: From Neurodevelopment to Neurodegeneration

The microbiota-gut-brain axis (MGBA) is a complex bidirectional communication network integrating neural, endocrine, immune, and metabolic pathways linking intestinal microbiota to central nervous system function. Increasing evidence indicates that microbiota-derived signals are critical regulators of neurodevelopment and may contribute to vulnerability to neurodegenerative disorders across the lifespan. In this narrative review, we synthesize experimental and clinical evidence to define the key biological mechanisms underlying microbiota-brain interactions. Converging data indicate that immune activation, barrier dysfunction, and microbial metabolites, particularly short-chain fatty acids and tryptophan-derived compounds, represent central mediators linking gut dysbiosis to neuroinflammatory and neurodegenerative processes. Early-life microbial perturbations, driven by factors such as antibiotic exposure, diet, and psychosocial stress, appear to induce long-term immunometabolic programming that may increase susceptibility to neurological disorders later in life. Clinical studies consistently associate dysbiosis with neurodevelopmental conditions and major neurodegenerative diseases, including Alzheimer's disease and Parkinson's disease; however, causal relationships remain incompletely defined due to heterogeneity and the predominance of observational data. Overall, the available evidence supports a lifespan model in which microbiota-driven immune and metabolic dysregulation contributes to both early neurodevelopmental trajectories and late-life neurodegeneration. While microbiome-based biomarkers and therapeutic strategies show promise, their clinical translation requires validation in longitudinal and interventional studies.