The study shows that your airway’s natural antibacterial defense is stronger in summer when vitamin D levels are higher, and weaker in winter. Taking a modest vitamin D3 supplement (1000 IU daily for three months) during the colder months can boost this defense by increasing the antimicrobial peptide LL‑37, helping your lungs kill bacteria more effectively.

It is widely unknown why respiratory infections follow a seasonal pattern. Variations in ultraviolet B (UVB) light during seasons affects cutaneous synthesis of vitamin D₃. Serum vitamin D concentration influences the expression of airway surface liquid (ASL) antimicrobial peptides such as LL-37. We sought to determine the effect of seasons on serum vitamin D levels and ASL antimicrobial activity. Forty participants, 18-60 years old, were randomized 1:1 to receive 90 days of 1000 IU vitamin D₃ or placebo. We collected ASL via bronchoscopy and measured serum 25(OH) vitamin D from participants before and after intervention across seasons. We measured ASL antimicrobial activity by challenging samples with bioluminescent <i>Staphylococcus aureus</i> and measured relative light units (RLUs) after four minutes. We also investigated the role of LL-37 using a monoclonal neutralizing antibody. We found that participants, prior to any intervention, during summer-fall (<i>n</i> = 20) compared to winter-spring (<i>n</i> = 20) had (1) decreased live bacteria after challenge (5542 &#xb1; 175.2 vs. 6585 &#xb1; 279 RLU, <i>p</i> = 0.003) and (2) higher serum vitamin D (88.25 &#xb1; 24.25 vs. 67.5 &#xb1; 45.25 nmol/L, <i>p</i> = 0.026). Supplementation with vitamin D₃ increased vitamin D levels and restored ASL antimicrobial activity only during the winter-spring. The increased ASL antimicrobial activity seen during the summer-fall was abrogated by adding the LL-37 neutralizing antibody. ASL kills bacteria more effectively during the summer-fall compared to the winter-spring. Supplementation of vitamin D during winter-spring restores ASL antimicrobial activity by increasing the expression of antimicrobial peptides including LL-37.