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.