The VIDA ( Vitamin D Add-on Therapy Enhances Corticosteroid Responsiveness in Asthma ) randomized , double-blind , parallel , placebo-controlled trial studying adult patients with symptomatic asthma and a serum @-hydroxyvitamin D level of less than @ ng/mL was conducted across @ academic US medical centers in the National Heart , Lung , and Blood Institute 's AsthmaNet network , with enrollment starting in April @ and follow-up complete by January @ .