Comparative effectiveness of oral nutritional supplements in preventing respiratory tract infections among adults: a systematic review and network meta-analysis.

Background: Different nutritional supplements may prevent respiratory tract infections (RTIs) in adults, but their comparative effectiveness remains unclear. We aimed to evaluate the effectiveness of oral nutritional supplements in preventing RTIs and reducing their symptom duration and severity. Methods: In this systematic review and network meta-analysis, we searched for randomized controlled trials (RCTs) assessing oral micronutrients, flavonoids, probiotics, or synbiotics in adults for the prevention of RTIs in PubMed, Embase, Cochrane Central Register, and ClinicalTrials.gov from inception to February 25, 2025. Studies with participants under 18 years, immunodeficiencies, non-oral administration, or a therapeutic rather than preventive focus were excluded. We performed network meta-analyses (NMA) using frequentist random-effects models to compare interventions through direct and indirect evidence. We used the CINeMA framework to rate the overall certainty of evidence. The primary outcome was the incidence of RTIs. The secondary outcomes were upper respiratory tract infections (URTIs) or common cold, COVID-19 or influenza, RTI symptom duration, RTI symptom severity, and adverse events. The study is registered with PROSPERO, CRD420250653276. Findings: We identified 120 eligible trials, among which 107 trials involving 101,751 adults were included in the network meta-analysis. Compared with placebo, catechin (RR = 0.79, 95% CI: 0.66, 0.95; n = 5; high certainty), Interpretation: Based on the current evidence, catechin, Funding: This work was supported by the Ningbo Top Medical and Health Research Program and the Zhejiang Provincial Disease Prevention and Control Science and Technology Plan.