Cystic Fibrosis is a lethal inherent disorder largely prevalent in young Canadians. In addition to respiratory infections, responsible for the premature death of these patients before the age of 50, gastrointestinal complications occur early in the course of the disease and cause significant nutritional deficiencies. Among those deficiencies, vitamin D insufficiency is the most prevalent and can strike up to 90% of children and adult with Cystic Fibrosis despite intake of daily and large doses of supplements. Vitamin D is the liposoluble vitamin that has gained the most attention in the scientific and lay community over the past few years. Its high profile is attributed to its wide involvement in the regulation of multiple biological functions. Among other roles, vitamin D is involved in bone mineralization, muscular strength, immune function and protection against diabetes and colon cancer. Thus, vitamin D insufficiency can have major consequences on multiple body systems and contribute to or accentuate the severity of Cystic Fibrosis complications. Since vitamin D supplementation regimens are ineffective in correcting the vitamin D status of Cystic Fibrosis patients, we postulate that vitamin D insufficiency results from alterations in its absorption and metabolism. We will therefore carry out studies, using cellular and animal models of Cystic Fibrosis, to investigate vitamin D intestinal absorption and metabolism. These studies will broaden our knowledge on vitamin D and will be a significant contribution, not only for Cystic Fibrosis, but also for the general Canadian population characterized by high rates of vitamin D insufficiency.