The microbiota–gut–brain axis as a modulator of symptom expression in autism spectrum disorder, with exploratory insights into ADHD: evidence from a structured narrative review on paediatric population

Autism spectrum disorder (ASD) and attention-deficit/hyperactivity disorder (ADHD) are increasingly conceptualized as multisystem neurodevelopmental conditions involving behavioral, gastrointestinal, immune, and metabolic alterations. The microbiota-gut-brain axis (MGBA) has been proposed as a key framework linking these systems, although evidence from human studies remains heterogeneous and inconclusive. We conducted a structured narrative review of pediatric human studies investigating the microbiota-gut-brain axis (MGBA) in ASD and ADHD, excluding preclinical and animal studies. PRISMA 2020 principles were used as a transparency framework to support reporting of study identification, screening, and eligibility. Observational, cohort, and interventional studies were synthesized with particular attention to study design, methodology, associations with standardized clinical scales, dietary factors, and therapeutic implications. Across ASD (n = 90), most studies were observational, predominantly cross-sectional or case-control in designs, reporting consistent associations between ASD symptom severity and gastrointestinal symptoms, intestinal permeability, immune activation, and dietary selectivity. Evidence in ADHD (n = 21) was substantially more limited and largely observational. Diet emerged as a major modulating and confounding factor in microbiota-behavior associations. Interventional studies involving probiotics, microbiota transfer therapy, and other gut-directed approaches showed more consistent improved gastrointestinal outcomes but showed modest and heterogeneous effects on core ASD symptoms, often limited to subgroups with prominent gastrointestinal comorbidities. To strengthen interpretation of interventional findings, randomized trial reports and non-randomized interventional studies were appraised according to study design, whereas the predominantly heterogeneous observational literature was interpreted narratively. Overall, this review suggests that MGBA alterations may be better conceptualized as modifiers of symptom expression, particularly in ASD and with only emerging relevance in ADHD, rather than as disorder-specific causal mechanisms. This interpretation underscores the need for stratified, longitudinal, and clinically informed research frameworks, in line with the focus of the present research topic.