The investigators predict that a powder pill form of vitamin D will be more effectively absorbed than an oil form of vitamin D in people diagnosed with Cystic Fibrosis.

The purpose of this study is to see how well two different forms of vitamin D work in Cystic Fibrosis (CF). Patients with Cystic Fibrosis have a hard time absorbing certain foods and often have low vitamin D levels. Studies show that vitamin D may help fight infections common in Cystic Fibrosis. The investigators would like to see if a powder pill form of vitamin D will work better than a pill mixed with oil. Each person will randomly receive either a powder pill or an oil based pill. Blood will be tested while during a 3-4 day hospital stay. The investigators plan to enroll no more than 24 patients with CF from Emory University Hospital for this study.

Inclusion Criteria:

• Adult CF patients (age >18 years
• Able to tolerate oral medication
• Expected to survive the duration of the study

Exclusion Criteria:

• Inability to obtain or declined informed consent from the subject and/or legally authorized representative
• History of disorders associated with hypercalcemia
• Current hypercalcemia (albumin-corrected serum calcium >10.8 mg/dL or ionized calcium >5.2 mg/dL)
• Chronic kidney disease worse than stage III (<60 ml/min), 7) Forced Expiratory Volume (FEV1)% predicted <20%
• Current significant hepatic dysfunction total bilirubin > 2.5 mg/dL with direct bilirubin > 1.0 mg/dL
• Current use of cytotoxic or immunosuppressive drugs
• History of AIDS or illicit drug abuse
• too ill to participate in study based on investigator's or study team's opinion
• current enrollment in another intervention trial