Vitamin D3 increased ASL antimicrobial activity in non-smokers. Vitamin D3 increased ASL antimicrobial activity in non-smokers. ASL antimicrobial activity assay. (A) Non-smokers ASL antimicrobial activity at baseline and after vitamin D3. Non-smokers significantly increased their baseline antimicrobial activity (baseline vs vitamin D3 mean±SD, 6186±1205 vs 5344±874.3; 95% CI of the difference −1502 to −181.9; *p=0.0159 by paired t-test). (B) Smokers ASL antimicrobial activity at baseline and after vitamin D3. Smokers did not increase their ASL antimicrobial activity (baseline vs vitamin D3 mean±SD, 5321±1029 vs 5575±863; 95% CI of the difference −235.1 to 743.2; p=0.2227 by paired t-test). ASL, airway surface liquid. Luis G Vargas Buonfiglio et al. BMJ Open Resp Res 2017;4:e000211 ©2017 by British Thoracic Society