Effects of probiotics, prebiotics, and synbiotics on lung function and symptoms in adults with bronchial asthma: a systematic review and meta-analysis.

BACKGROUND: Probiotics, prebiotics, and synbiotics may exert beneficial effects on asthma by modulating the gut microbiota and the "gut-lung axis". However, no systematic review or meta-analysis has specifically focused on adult asthma patients. OBJECTIVE: To systematically evaluate the effects of probiotics, prebiotics, and synbiotics on lung function, symptom control, and inflammatory markers in adults with bronchial asthma. METHODS: Five electronic databases were systematically searched for relevant randomized controlled trials (RCTs) up to May 2026 (PROSPERO registration: CRD420261412024). Standardized mean differences (SMDs) were pooled using a random-effects model. Outcomes for which quantitative data could not be extracted were summarized narratively. The Cochrane RoB 2.0 tool and GRADE framework were used to evaluate risk of bias and grade evidence quality for primary endpoints, respectively. RESULTS: 13 RCTs were included, 7 of which provided extractable continuous quantitative data. Pooled results demonstrated significant elevations in absolute FEV1 (SMD=0.71, 95%CI 0.29-1.13, P=0.020) and FEV1% predicted (SMD=0.51, 95%CI 0.06-0.96, P=0.026). Subgroup analyses identified numerically larger lung function gains for multi-strain formulations and 4-week interventions; no significant between-subgroup difference was observed for intervention type (P=0.462), while a significant difference was found between 4-week and 8-week treatment durations (P=0.002). The pooled change in ACT scores was not statistically significant, and this analysis was accompanied by high inter-study heterogeneity. A reduction in FeNO was observed in only a single, small trial that combined synbiotics with allergen immunotherapy, and no consistent alterations of IL-5 and IgE were detected across included studies. No severe adverse events were reported, and GRADE evidence certainty ranged from low to moderate. CONCLUSION: Moderate-quality evidence supports a mild beneficial effect of multi-strain Bifidobacterium/Lactobacillus probiotics or synbiotics on lung function in adult asthma; however, the current evidence does not justify their routine clinical use. Large long-term RCTs are required to confirm efficacy and establish optimal regimens. SYSTEMATIC REVIEW REGISTRATION: https://www.crd.york.ac.uk/PROSPERO/, identifier CRD420261412024.