Insufficient incretin action has been associated with T2D. To study the possible link between insufficient incretin action and impaired insulin secretion in CFRD as in T2D, the present study will determine whether early intervention with incretin-based therapy using the DPP-4 inhibitor sitagliptin (Januvia®) to raise endogenous levels of the incretin hormones--i.e.--glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotrophic polypeptide (GIP) for a 6-month period will improve insulin secretion in CF patients with indeterminate glucose tolerance, impaired glucose tolerance or early CFRD.