A 12-week double-blind randomised controlled trial investigating the effect of dietary supplementation with 125 μg/d vitamin D in adults with asthma.

Vitamin D deficiency has previously been linked to higher rates of exacerbation and reduced lung function in asthmatics. Previous randomised controlled trials investigating the effect of vitamin D supplementation have mainly focused on children with asthma. Trials involving adults have typically used bolus dosing regimens, and the main outcomes have been patient-focused without investigating underlying inflammation. The present study aimed to conduct a 12-week placebo-controlled randomised controlled trials administering a daily 125 µg vitamin D₃ supplement to adults with mild to moderate asthma. A total of 32 participants were randomised to receive either the 125 μg vitamin D₃ supplement or an identical matching placebo. The primary outcome of the study was lung function measured by the ratio of FEV₁:FVC (effect size 2·5) with secondary outcomes including asthma symptoms and inflammatory biomarkers. There was a small but statistically significant higher increase in the mean (±sd) ratio of FEV₁:FVC from baseline to post-intervention in the vitamin D group (+0·05 ± 0·06) compared with the placebo group (+0·006 ± 0·04, P = 0·04). There was no effect of the intervention on asthma control test scores, or the inflammatory biomarkers measured. There was a moderate, significant association between baseline plasma 25(OH)D concentration and baseline plasma IL-10 (r = 0·527, P = 0·005) and TNF-α (r = −0·498. P = 0·008) concentrations. A daily vitamin D₃ supplement led to slightly improved lung function in adult asthmatics and may be a useful adjunct to existing asthma control strategies, particularly for individuals with suboptimal vitamin D status.