Efficacy of Probiotic Supplementation on Gastrointestinal and Behavioral Outcomes in Children With Autism Spectrum Disorder: A Meta-Analysis

Objective: This meta-analysis aimed to evaluate the efficacy of probiotic supplementation on both gastrointestinal (GI) and core behavioral symptoms in children with autism spectrum disorder (ASD). Methods: A systematic literature search was conducted in PubMed, Embase, Cochrane Central Register of Controlled Trials (CENTRAL), and Web of Science from inception to February 5, 2026. We included randomized controlled trials (RCTs) and randomized crossover trials comparing oral probiotics with placebo or control arms (including no treatment and standard care) in children with ASD (aged ≤18 years). To minimize carryover bias, only the first-period data from crossover trials were extracted and analyzed as parallel-group data. Primary outcomes were overall GI symptoms (assessed using continuous and dichotomous measures). Secondary outcomes were core behavioral symptoms assessed by standardized scales (e.g., Autism Diagnostic Observation Schedule-Calibrated Severity Score (ADOS-CSS), Social Responsiveness Scale-2 (SRS-2)). Risk of bias was assessed using the Cochrane Risk of Bias tool for randomized trials (RoB 2.0). Random-effects meta-analyses were performed using Review Manager (RevMan) 5.4. Results: Six studies (four RCTs and two crossover trials) involving a total of 348 participants (330 from parallel-group RCTs and 18 from crossover trials; note that for crossover trials the reported sample sizes reflect participants who completed the first period and were included in the analysis). For GI symptoms, probiotics did not demonstrate a significant benefit on continuous outcome scores (standardized mean difference (SMD) = –0.09, 95% confidence interval (CI): –0.46 to 0.29); however, probiotics significantly increased the likelihood of achieving clinically meaningful improvement compared with placebo (relative risk (RR) = 1.28, 95% CI: 1.02–1.60; p = 0.03). Regarding core behavioral symptoms, a measure-specific effect was observed. Probiotic supplementation was associated with a small but statistically significant improvement when symptoms were evaluated using the ADOS-CSS (SMD = –0.42, 95% CI: –0.80 to –0.04; p = 0.03), but not when assessed using the parent-reported SRS-2 (SMD = –0.24, 95% CI: –0.54 to 0.05; p = 0.11). Heterogeneity was low for most analyses. Conclusion: This meta-analysis provides suggestive evidence that probiotic supplementation may confer clinically meaningful GI benefits as indicated by responder analyses, and may also improve certain directly observed core behavioral symptoms in children with ASD. However, the robustness of these pooled estimates is limited by several factors: only 2–3 studies contributed data to each pooled outcome, with the exception of SRS-2, which was based on a single study; probiotic formulations and outcome definitions varied considerably across trials; the risk-of-bias assessment revealed that only one study was low risk, one was at high risk, and four raised some concerns. Consequently, these findings should be interpreted as preliminary. Definitive conclusions will require larger, well-designed RCTs that incorporate standardized and comprehensive outcome measures. This PROSPERO Registration: CRD420261307184 (https://www.crd.york.ac.uk/PROSPERO/view/CRD420261307184).