Gut microbiota and ALS: cause, consequence or correlation? - a systematic review

Background: Gut microbiome disturbances have been proposed as contributors to amyotrophic lateral sclerosis (ALS), a multisystem neurodegenerative disorder characterised by motor neuron loss, extra-motor symptoms, and rapid progression. Mechanistic links between dysbiosis, epithelial and blood-brain barrier dysfunction, metabolic imbalance, and immune activation have been suggested, but causality remains unresolved. We conducted a systematic review to evaluate the evidence supporting microbiome involvement in ALS pathogenesis. Methods: We searched PubMed, Medline, Embase, Scopus, Semantic Scholar, and Google Scholar (Nov 23, 2025) for human and ALS-relevant animal studies assessing bacterial microbiota, gut or blood-brain barrier integrity, microbial metabolites, or immune pathways. No language or date restrictions were applied. Studies were screened according to predefined criteria, and quality was assessed using QUADAS-2. Owing to the heterogeneity of study designs and sequencing approaches, findings were synthesised narratively. Findings: 61 of 2,397 studies met inclusion criteria. Across human cohorts, ALS was consistently associated with reduced microbial diversity, shifts in key taxa, and disruption of microbial pathways regulating short-chain fatty acids, nicotinamide metabolism, and inflammatory signalling. Several mechanistic animal studies demonstrated that microbiota manipulation, through antibiotics, faecal microbiota transfer, or supplementation with protective taxa, modulated motor function, microglial activation, gut permeability, and survival, indicating that dysbiosis can influence disease trajectories. Conversely, longitudinal human data showed that dysbiosis often emerged alongside worsening physical function, gastrointestinal dysmotility, weight loss, and changes in dietary intake, suggesting secondary effects of disease progression. Integrative multi-omics studies linked microbial alterations with systemic cytokine profiles, metabolic stress pathways, and CNS immune phenotypes, reinforcing a bidirectional gut-brain axis. However, the predominance of cross-sectional designs and small sample sizes substantially limits causal inference. Interpretation: Current evidence supports a model in which gut dysbiosis interacts with ALS via barrier failure, metabolic disruption, and immune dysregulation, but does not establish dysbiosis as a primary cause of disease. Preclinical findings highlight microbiome-derived mechanisms with disease-modifying potential, yet human data largely indicate association rather than initiation. Clarifying temporal relationships will require longitudinal, multi-modal studies, integration with pre-symptomatic cohorts, and controlled interventional trials. Microbiome-targeted therapies remain a promising but unproven avenue for ALS.