The triangular relationship between vitamin D, the respiratory microbiome, and pneumonia

Abstract Pneumonia remains a leading cause of morbidity and mortality worldwide, particularly in populations with a high prevalence of vitamin D deficiency. Emerging evidence indicates a complex triangular relationship between vitamin D, the respiratory microbiome, and pneumonia, suggesting that vitamin D influences respiratory health not only through immune regulation but also by shaping airway microbial composition. Vitamin D enhances epithelial barrier integrity, stimulates the production of antimicrobial peptides such as cathelicidin and defensins, and modulates innate and adaptive immune responses. These effects support a balanced mucosal environment and microbial stability. In contrast, vitamin D deficiency is associated with dysbiosis, reduced colonization resistance, and increased infection risk. Alterations in the respiratory microbiome, characterized by loss of beneficial commensals and expansion of pathogens, are linked to increased pneumonia severity. Additionally, interactions between vitamin D signaling and microbial metabolites may influence host immunity and microbial dynamics. However, current evidence is limited by the predominance of observational studies and heterogeneity among randomized controlled trials, leading to inconsistent clinical findings. While adequate vitamin D status may reduce the risk of respiratory infections, particularly in deficient populations, the effects are not uniform across different settings. This review synthesizes established mechanisms and key uncertainties, highlighting the need for well-designed longitudinal and interventional studies. Improved understanding of these interactions may inform targeted, personalized strategies that integrate vitamin D optimization and microbiome-based interventions to reduce the burden of pneumonia.