The Microbiota-Gut-Brain Axis in Autism: Associations, Causal Inference, and Interventions-A Narrative Review

Autism spectrum disorder is markedly heterogeneous and frequently accompanied by gastrointestinal symptoms that often correlate with behavioral phenotypes. Emerging evidence suggests that the microbiota-gut-brain axis may contribute to these associations through multiple bidirectional communication routes-including neural, immune, and endocrine pathways, as well as microbial metabolites such as short-chain fatty acids and tryptophan-kynurenine intermediates. This narrative review synthesizes clinical, mechanistic, and interventional evidence published between January 2010 and July 2025, clarifies the extent to which current data support association versus causation, evaluates key confounding factors, summarizes evidence for interventions such as probiotics, prebiotics, and fecal microbiota transplantation, and outlines future directions for precision research and targeted interventions based on functional pathways and stratified subgroups.