Gut Microbial Metabolism as a Dynamic Interface in Neurodegenerative Diseases

Gut microbial metabolism links intestinal ecology with systemic physiology and neural pathology, but its effects vary across disease stage, tissue compartment, and host background. This review uses Alzheimer's disease (AD) as the principal model and compares selected features with Parkinson's disease (PD) and amyotrophic lateral sclerosis (ALS). Across the AD continuum, fermentation-related changes appear in prodromal cohorts, whereas broader alterations in amino acid products, host-microbial co-metabolites, bile acids, and lipids accompany mild cognitive impairment and dementia. These group-level patterns do not constitute a fixed patient trajectory. Microbial production, intestinal absorption, hepatic conversion, renal clearance, barrier integrity, and tissue-specific receptors jointly determine biological exposure. Experimental studies connect short-chain fatty acids and indole derivatives with epithelial and neuroimmune homeostasis, while imidazole propionate, trimethylamine N-oxide, selected kynurenine products, and remodeled bile acid pools engage vascular, inflammatory, amyloid, or tau-related pathways. Cerebral pathology can also remodel the intestinal ecosystem, creating reciprocal feedback. Apolipoprotein E4 modifies lipid handling, vascular permeability, and immune responses, helping to explain why comparable metabolic profiles may carry different consequences among individuals. Translation therefore requires more than a change in community composition. Trials must verify microbial function, metabolite target engagement, AD biomarker response, and clinical benefit in appropriately stratified participants. Shared pathways in PD and ALS provide comparison points, but disease-specific cells, proteinopathies, and treatment exposures constrain direct transfer of AD-derived targets.