Gut Microbiota in Autism Spectrum Disorder: A Scoping Review of Clinically Relevant Evidence and Future Research Directions

Autism spectrum disorder (ASD) is a heterogeneous neurodevelopmental condition frequently accompanied by gastrointestinal manifestations. Growing interest has focused on the gut microbiota and its possible relationship with ASD, although the clinical significance of reported microbiota alterations remains uncertain. This scoping review aimed to map and synthesise human evidence on the relationship between gut microbiota and ASD, with particular attention to gastrointestinal manifestations, microbial function, microbiota-derived metabolites and microbiota-targeted interventions. The review followed Joanna Briggs Institute methodology and was reported according to the PRISMA Extension for Scoping Reviews (PRISMA-ScR). We searched PubMed/MEDLINE, Web of Science Core Collection and CINAHL Complete from inception to 28 July 2026 using database-specific strategies. Forty-five publications met the eligibility criteria. Differences in gut microbiota composition were frequently reported in individuals with ASD, but findings varied considerably across studies, and no reproducible ASD-specific microbial signature emerged. Studies of microbial function and metabolites also produced heterogeneous findings. Microbiota-targeted interventions showed variable gastrointestinal, behavioural, and microbiological outcomes, but controlled studies provided insufficient evidence to establish consistent clinical benefits. Critical appraisal using design-appropriate JBI tools identified recurring methodological limitations, including inadequate control of confounding, small sample sizes, incomplete reporting, and, in some intervention studies, limitations in blinding or follow-up. Overall, microbiome findings should be interpreted alongside gastrointestinal symptoms, diet, medication exposure and other relevant clinical characteristics. Further research would benefit from well-characterised cohorts, standardised methodologies, and longitudinal studies that integrate functional and multi-omics data.