There is an inverse relationship between 25(OH)D and asthma severity and a positive relationship between
25(OH)D and both lung function and asthma control. Lower 25(OH)D levels are associated with worse lung function, higher levels of exhaled NO, sputum eosinophilia, oral corticosteroid use and higher BMI. The frequency of vitamin D insufficiency was highest in patients with severe, uncontrolled asthma, in particular in patients with a sputum eosinophil count ≥3% despite treatment with inhaled and oral corticosteroids.