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 25-hydroxyvitamin D level of less than 30 ng/mL was conducted across 9 academic US medical centers in the National Heart , Lung , and Blood Institute 's AsthmaNet network , with enrollment starting in April 2011 and follow-up complete by January 2014 .