IMPACT OF VITAMIN D DEFICIENCY ON IMMUNE FUNCTION AND RECURRENT RESPIRATORY TRACT INFECTIONS IN PEDIATRIC PATIENTS: AN EVIDENCE-BASED CLINICAL REVIEW

Background Vitamin D is traditionally recognized for its essential role in calcium metabolism and skeletal development; however, increasing evidence indicates that it also plays an important role in immune regulation. Vitamin D receptors are expressed in several immune cell populations, suggesting that vitamin D status may influence host defense against infectious diseases. Respiratory tract infections are among the most common causes of illness in children, and recurrent infections may result in substantial morbidity and healthcare utilization. Objective This review aimed to examine the relationship between vitamin D deficiency, immune function, and recurrent respiratory tract infections in pediatric patients and to evaluate the current evidence regarding the potential preventive role of vitamin D supplementation. Methods Relevant scientific literature concerning vitamin D status, immune function, and respiratory tract infections in children was reviewed using evidence indexed in PubMed and other major biomedical databases. Particular attention was given to systematic reviews, meta-analyses, randomized controlled trials, and pediatric clinical studies. Recent evidence published through 2026 was considered. Results Available evidence suggests that vitamin D participates in both innate and adaptive immune responses. Vitamin D may enhance antimicrobial defense through the regulation of antimicrobial peptides and may influence inflammatory and cellular immune responses. Several observational studies have reported an association between low serum 25-hydroxyvitamin D [25(OH)D] concentrations and an increased susceptibility to respiratory infections in children. However, interventional studies have produced inconsistent results. A recent meta-analysis of 17 randomized controlled trials involving 18,372 pediatric participants found no statistically significant overall reduction in acute respiratory infection incidence with vitamin D supplementation, although a subgroup receiving daily low-dose supplementation of ≤1000 IU/day demonstrated a lower risk of infection. Conclusion Vitamin D deficiency may contribute to impaired immune regulation and may be associated with increased susceptibility to respiratory tract infections in some pediatric populations. Nevertheless, current evidence does not support universal vitamin D supplementation solely for the prevention or treatment of respiratory infections. Assessment of vitamin D status and individualized management may be particularly relevant in children at increased risk of deficiency.