Characterized by developmentally inappropriate levels of inattention, hyperactivity, and impulsivity, Attention-Deficit Hyperactivity Disorder (ADHD) is the most prevalent neurodevelopmental disorder, posing a significant public health concern. Its etiopathogenesis is considered multifactorial with complex determinism but remains unclear. Recent research highlights the gut microbiota and the gut-brain axis as promising avenues for understanding and potentially treating ADHD, with a growing number of studies exploring alterations in gut microbiota composition among affected individuals. This narrative review examines the current literature on the role of the gut microbiota in ADHD and focuses on key findings about bacterial composition, how it may be linked to ADHD symptomatology, and the possible mechanisms involved. While studies consistently report changes in microbial composition and diversity in individuals with ADHD, results remain heterogeneous across taxonomic levels. Some compelling evidence also suggests a link between gut microbial profiles and ADHD symptom severity. The involvement of microbiota in influencing neurodevelopment is proposed to be mediated through mechanisms related to SCFA production, immune modulation, and neurotransmitter synthesis. These findings pave the way for microbiota-targeted interventions as adjunct therapies for ADHD. This review evaluates areas of consensus and discrepancies between studies, while addressing the methodological limitations present in this field of research. Although the gut microbiota appears to play a meaningful role in the complex and multifactorial origins of ADHD, more rigorous and comprehensive studies are needed to confirm these findings and translate them into effective clinical applications. This could ultimately improve both our understanding and treatment of this heterogeneous disorder.
