Giving a big dose of vitamin D3 (250,000‑500,000 IU over five days) raises the amount of free vitamin D in the blood, and this rise is linked to higher activity of the gene that makes the antimicrobial peptide LL‑37. However, the actual blood levels of LL‑37 and another peptide, hBD‑2, didn't go up, though higher LL‑37 was associated with better immune cell (macrophage) function in the lungs. The study was done in very sick, ventilated patients, so the findings may not directly apply to healthy people, but they hint that high‑dose vitamin D could influence the body’s innate immunity.

High-dose vitamin D₃ increases plasma total 25-hydroxyvitamin D [25(OH)D] in critically ill, ventilated patients; however, to our knowledge, the effect on plasma levels of free (nonprotein-bound) 25(OH)D has not been investigated in critical illness. Moreover, the relationship of free 25(OH)D and the regulation of endogenous antimicrobial peptides (AMPs) remains unknown. The aims of this study were to determine in critically ill adults with respiratory failure the effect of previous high-dose regimens of vitamin D₃ on free 25(OH)D concentrations, the relationship of free 25(OH)D with circulating cathelicidin (LL-37) and human beta-defensin-2 (hBD-2), and the associations between plasma levels of free 25(OH)D and these AMPs to alveolar macrophage phagocytosis function. In a double blind, randomized controlled trial, critically ill ventilator-dependent adults (N = 30) received enteral vitamin D₃ (250,000 or 500,000 IU total over 5 d) or placebo. Plasma was obtained serially for concentrations of free 25(OH)D, LL-37, hBD-2, and expression of peripheral blood mononuclear cell human cationic antimicrobial protein (hCAP18) mRNA. Total 25(OH)D and LL-37 concentrations and alveolar macrophage phagocytosis were determined in bronchoalveolar lavage fluid. Plasma concentrations of free 25(OH)D over time were correlated with total 25(OH)D levels (r= 0.82; P < 0.001). The increase in free 25(OH)D was greater with the 500 000 IU vitamin D₃ dose than with the lower dose. The percent change in mRNA expression of hCAP18 was positively associated with percent change in free 25(OH)D at days 7 and 14 (ρ = 0.48; P = 0.04 and ρ = 0.59; P = 0.03, respectively). Additionally, plasma LL-37 levels correlated with the percentage of alveolar macrophages exhibiting phagocytosis (ρ = 0.51; P = 0.04). The present study found a dose-related increase in plasma free-25(OH)D levels, which was associated with increasing circulating mRNA expression of hCAP18 over time. There were no correlations between changes in total and free 25(OH)D against plasma LL-37 and hBD-2 concentrations. Larger studies appear warranted to determine the impact of high-dose vitamin D₃ administration on endogenous AMPs.