<qa>  What is the name of the study that demonstrates vitamin D supplementation?The vast majority of Cystic Fibrosis CF patients worldwide are vitamin D insufficient. There is no evidence of benefit of vitamin D supplementation for CF patients yet. However, descriptive cross-sectional studies suggest that vitamin D might be beneficial with respect to bone health, as well as to the newly described non-classical functions of vitamin D such as the potential anti-diabetic and immunomodulatory effects. To prove causation, and to determine which serum vitamin D concentration is optimal for CF patients, vitamin D supplementation interventional studies are needed, such as our trial. 