Review Article: The Impact Of Vitamin D₃ Supplementation On The Risk Of Respiratory Tract Infections In Working-Age Adults

Abstract

Purpose of the Research:

Respiratory tract infections (RTIs) are a major cause of morbidity among working-age adults, a population characterized by high occupational and community exposure to pathogens and widespread vitamin D insufficiency. Vitamin D₃ plays an essential role in immune modulation through effects on antimicrobial peptides, epithelial barrier integrity, and inflammatory regulation. This review aims to synthesize mechanistic, epidemiological, and clinical evidence assessing whether vitamin D₃ supplementation reduces the risk or severity of RTIs in adults aged 18–65 years.

Methods:

 A narrative, evidence-based synthesis was conducted using full-text randomized controlled trials, observational studies, mechanistic papers, and systematic reviews provided by the author. Studies published between 2003 and 2022 were included if they evaluated vitamin D₃ supplementation or serum 25(OH)D levels in relation to respiratory outcomes in working-age adults. Due to heterogeneity in study designs, dosing regimens, and outcome definitions, evidence was synthesized qualitatively.

Results:

 Mechanistic studies demonstrate that vitamin D enhances innate and adaptive immune responses by inducing antimicrobial peptides (LL-37, β-defensins), modulating cytokine production, supporting autophagy, and stabilizing epithelial defenses. Epidemiological evidence consistently links low 25(OH)D levels with increased susceptibility to RTIs, greater symptom severity, and higher respiratory mortality. Clinical data show that daily or weekly physiological supplementation (400–1000 IU/day) modestly reduces RTI incidence, particularly in individuals with severe deficiency (<25 nmol/L), whereas high-dose bolus regimens provide little benefit. Vitamin D₃ supplementation remains safe at recommended doses, with adverse effects being rare.

Conclusions:

Vitamin D₃ supplementation is a safe and biologically plausible preventive strategy for reducing RTI risk in working-age adults, especially those with low baseline vitamin D status. Regular, moderate dosing appears more effective than bolus administration. Although further targeted randomized trials are needed, maintaining adequate vitamin D levels may enhance respiratory immune resilience in this population.