Gut Microbiota Composition and Diversity in Attention-Deficit/Hyperactivity Disorder: A Systematic Review

Attention-deficit/hyperactivity disorder (ADHD) is a common neurodevelopmental condition. Growing evidence suggests that the gut–brain axis may contribute to its pathophysiology. However, findings regarding gut microbiota alterations in ADHD remain inconsistent. This systematic review aimed to synthesize the current evidence on the gut microbiota composition and microbial diversity in individuals with ADHD. A systematic search of PubMed, Scopus, and Web of Science was conducted up to 31 December 2025 following PRISMA guidelines, yielding 562 studies. Twenty-three studies published between 2015 and 2025 were included. Most studies reported no significant differences in alpha-diversity in ADHD and control groups. More consistently, beta-diversity analysis reported significant differences in microbial composition between ADHD and control groups. ADHD was often associated with a reduced abundance of Alistipes and butyrate producers such as Faecalibacterium and increased abundance of Roseburia and Agathobacter. Some longitudinal studies suggested that distinct early-life microbial patterns may precede the ADHD diagnosis. ADHD appears to be associated with alterations in the gut microbiota, particularly in taxa involved in short-chain fatty acid production and immune regulation. However, findings remain inconsistent due to methodological heterogeneity and potential confounding factors. Future research should prioritize longitudinal multi-omics approaches to clarify causal mechanisms and refine microbiota-targeted interventions.