Vitamin D and Microbiota in Cystic Fibrosis

50,000 IU of oral vitamin D3 once a week (the standard of care for repletion of vitamin D status by the Cystic fibrosis Foundation)

Participants will be asked to provide a stool sample in a collection container for analysis of stool microbiota. This will be done upon enrollment (baseline) and at 3 month follow-up.

"Participants will be asked to collect their sputum (the thick mucus or phlegm that is expelled from the lower respiratory tract through coughing) into a kit.~This will be done upon enrollment (baseline) and at 3 month follow-up."

A placebo capsule taken once a week (manufactured by the same company that makes the Vitamin D supplement).

Participants will be asked to provide 30 ml of blood collected via a blood draw to measure 25 (OH)D serum concentration and other nutrient markers related to vitamin D including Parathyroid hormone, fibroblast growth factor-23, vitamin D binding protein and markers of immune system/inflammation. This will be done at baseline and at 3 months follow up.