Efficacy and safety of vitamin D in the treatment of asthma: an overview of systematic reviews and meta-analyses

Background Asthma is a common chronic airway disease that can cause recurrent symptoms, acute exacerbations, impaired quality of life, and substantial healthcare burden. Vitamin D has been proposed as a potential add-on therapy because of its roles in immune regulation and respiratory defense, but its clinical benefits in asthma remain uncertain. Objective This umbrella review aimed to evaluate the efficacy and safety of vitamin D supplementation as an adjunctive treatment for asthma and to assess the credibility of the existing evidence. Methods We systematically searched PubMed, Cochrane Library, Web of Science, Embase, CNKI, WANFANG, VIP, and CBM from inception to November 26, 2025. Published systematic reviews and meta-analyses evaluating vitamin D supplementation for asthma were included. We assessed methodological quality, risk of bias, reporting quality, certainty of evidence, and overlap among primary studies using standard appraisal tools. Results 21 systematic reviews and meta-analyses were included. Overlap among primary studies was substantial. Vitamin D supplementation was associated with a lower risk of asthma exacerbations, but its effects on asthma control, lung function, and inflammatory outcomes were limited or inconsistent. No significant increase in adverse events was observed. The certainty of evidence varied across outcomes, and many findings were supported by low or very low certainty evidence. Conclusion Vitamin D supplementation may reduce asthma exacerbations and appears to be safe as an adjunctive treatment, but current evidence does not consistently support improvements in daily symptom control, lung function, or inflammatory outcomes. These findings should be interpreted cautiously because of heterogeneity, evidence uncertainty, and overlap among primary studies. Future studies should identify patients most likely to benefit and clarify optimal dose, duration, and biomarker-guided supplementation strategies. Systematic review registration https://www.crd.york.ac.uk/PROSPERO/view/CRD420251239612 , identifier: CRD420251239612.