Effects of Probiotic Supplementation on Core Symptoms of Autism Spectrum Disorder in Children

Background/Objectives: Increasing evidence implicates the microbiome-gut-brain axis in Autism Spectrum Disorder (ASD) pathophysiology, prompting interest in probiotics as a therapeutic strategy, although findings remain inconsistent. This systematic review aimed to evaluate the clinical efficacy of probiotic supplementation on core ASD symptoms, examine the outcome measures used, and provide insights into optimal probiotic interventions. Methods: This review was conducted in accordance with PRISMA 2020 guidelines. PubMed, Scopus, Web of Science, Ovid, ProQuest, and Wiley Online Library were searched for studies published between January 2015 and July 2025. Randomized, non-randomized, and open-label clinical studies evaluating oral probiotic supplementation in children and adolescents with ASD were included. Outcomes assessed core symptom domains using validated instruments. Study selection, data extraction, and risk-of-bias assessment (RoB 2 and ROBINS-I) were performed independently by multiple reviewers. Due to methodological heterogeneity, the findings were synthesized narratively. Results: Fourteen studies involving 924 children and adolescents with ASD across seven countries or regions were included, of which ten were randomized controlled trials. Eight studies reported significant improvement in core ASD symptoms, predominantly within the social and communication domain. The most frequently used assessment tools were the Social Responsiveness Scale (SRS), Autism Treatment Evaluation Checklist (ATEC), and Autism Diagnostic Observation Schedule (ADOS). Lactobacillus reuteri supplementation for at least three months was consistently associated with improvement in social behavior. Conclusions: L. reuteri supplementation possibly improves social and communication function in children with ASD. However, there is limited high-quality evidence on this. Evidence for other core domains remains limited and inconsistent, highlighting the need for well-designed, multicenter trials using standardized outcome measures and strain-specific hypotheses.