Vitamin D and Respiratory Tract Infections: A Narrative Review of Deficiency, Supplementation Strategies, and Clinical Outcomes

Vitamin D is widely recognised for its role in calcium homeostasis and skeletal health, yet its biological relevance extends beyond the musculoskeletal system to immune regulation and host defence. Over the past decade, the relationship between vitamin D status and respiratory tract infections (RTIs) has attracted increasing scientific interest, particularly in the context of the widespread prevalence of vitamin D deficiency and the global impact of the COVID-19 pandemic. This narrative review synthesises evidence on vitamin D deficiency, supplementation strategies, and respiratory clinical outcomes, drawing on randomised controlled trials, systematic reviews, meta-analyses, observational studies, and selected mechanistic publications. The reviewed literature consistently suggests that low serum 25-hydroxyvitamin D [25(OH)D] concentrations are associated with increased susceptibility to RTIs and, in many settings, with poorer respiratory outcomes. However, evidence regarding the benefits of supplementation remains heterogeneous. Earlier meta-analyses suggested modest protective effects, particularly among individuals with lower baseline vitamin D status and in studies using daily or weekly standard-dose regimens, whereas more recent large randomised trials and updated meta-analyses have reported smaller, inconsistent, or statistically non-significant overall effects. These discrepancies likely reflect variation in study population, baseline deficiency, supplementation regimen, trial duration, and outcome definition. Current evidence does not support a universal preventive or therapeutic role of vitamin D supplementation for all RTIs, but it strongly supports the importance of identifying and correcting deficiency, particularly in high-risk groups. From a wider perspective, vitamin D deficiency should also be understood as a socially patterned and potentially modifiable determinant of health, with relevance to prevention, health equity, and public health planning.