Vitamin D and Asthma
Vitamin D deficiency is more common in adults and children with asthma than in controls, and it correlates with worse lung function and more frequent exacerbations in observational cohorts. Trial evidence for supplementation is mixed but positive overall for reducing severe attacks in deficient patients.
The Cochrane evidence
A 2016 Cochrane systematic review (nine trials, ~1,100 patients) concluded vitamin D supplementation reduced the rate of asthma exacerbations requiring systemic corticosteroids by roughly 30%, and cut ED-visit or hospitalisation-triggering attacks. The 2023 update found the effect largely confined to adults with baseline deficiency (25(OH)D < 25 nmol/L / < 10 ng/mL).
Key individual trials
- VIDA (2014) — 408 adults with mild-moderate asthma and low vitamin D. 100,000 IU bolus + 4,000 IU/day did not improve time-to-treatment-failure primary endpoint, but reduced overall exacerbation rate.
- VDAART (2016) — maternal 4,400 IU/day in pregnancy vs 400 IU/day; 20% relative reduction in offspring asthma/recurrent wheeze by age 3, though not statistically significant at the pre-specified threshold.
- Copenhagen COPSAC (2016) — similar pregnancy design; 24% reduction in persistent wheeze by age 3.
Mechanisms
Vitamin D modulates T-helper cell balance (favouring Treg over Th2/Th17 responses), induces the antimicrobial peptide cathelicidin (which may reduce viral triggers of asthma), and influences airway smooth muscle proliferation. Deficiency is associated with steroid resistance in some asthma phenotypes.
Practical guidance
- Test 25(OH)D in adults with poorly controlled asthma.
- If deficient (< 20 ng/mL), correct with 2,000–4,000 IU/day for 8–12 weeks, then re-test.
- If already sufficient, adding more is unlikely to reduce exacerbations.
- Vitamin D supplementation is an adjunct to, not a replacement for, controller inhalers.