The Microbiota–Gut–Brain Axis and Nutritional Interventions in Amyotrophic Lateral Sclerosis: Pathophysiological Mechanisms, Neuroinflammation, and Non-Motor Manifestations—Scoping Review

Amyotrophic Lateral Sclerosis (ALS) is a progressive neurodegenerative disorder in which systemic pathophysiological alterations significantly contribute to disease progression and non-motor manifestations, such as depression and anxiety. The microbiota–gut–brain axis represents a critical bidirectional pathway in which intestinal dysbiosis and epithelial barrier disruption catalyze central neuroinflammation. This scoping review synthesizes evidence from 43 empirical and analytical studies across 28 countries and maps the findings under the WHO International Classification of Functioning (ICF) framework. Pathophysiological data reveal a profound taxonomic shift in patients with ALS, characterized by severe depletion of neuroprotective, butyrate-producing genera (Akkermansia and Prevotella) and enrichment of pro-inflammatory Enterobacteriaceae. This dysbiotic state leads to structural damage to the intestinal mucosa, alteration of Paneth cells, and downregulation of tight junction proteins (zonulin), triggering a “leaky gut” phenomenon. Subsequent systemic translocation of lipopolysaccharides (LPS) induces TLR4-mediated endotoxemia, microglial hyperactivation, and accelerated motor neuron apoptosis. Conversely, therapeutic modulation via Fecal Microbiota Transplantation (FMT), psychobiotics, and metabolic interventions (ketogenic or Mediterranean diets) has demonstrated significant efficacy in restoring epithelial integrity, mitigating mitochondrial hypermetabolism, and reducing emotional distress. This review identifies a critical research gap in the microstructural characterization of the enteric nervous system in ALS. Incorporating microbiome-targeted biomarkers into clinical protocols is crucial for implementing a stratified, multi-systemic therapeutic strategy aimed at enhancing patient prognosis and psychological well-being.