CFTR and vitamin D deficiency: CFBD is a multi-factorial disease stemming from nutritional deficiencies including vitamin D deficiency due to exocrine pancreas insufficiency, chronic inflammation, glucocorticoid therapy, hypogonadism, physical inactivity [3, 4], and the presence of the disease-causing cystic fibrosis transmembrane conductance regulator (CFTR) mutation expressed on osteoblasts and osteoclasts [5].