Inclusion Criteria:

• 1. Confirmed diagnosis of cystic fibrosis, defined by positive sweat test or CFTR mutation analysis according to CFF diagnostic criteria,
• 2. Age greater than or equal to 18y on date of consent
• 3. Pancreatic insufficiency
• 4. Recent OGTT consistent with Indeterminate-GT, IGT, CFRD w/o fasting hyperglycemia, or an established diagnosis of CFRD without fasting hyperglycemia
• 5. For female subjects, negative urine pregnancy test at enrollment.
• Control Subjects:
• 1. No history of cystic fibrosis.
• 2. Age ≥ 18y on date of consent.
• 3. Recent OGTT consistent with NGT.
• 4. For female subjects, negative urine pregnancy test at enrollment.

Exclusion Criteria:

• 1. Established diagnosis of non-CF diabetes (i.e. T1D) or CFRD with fasting hyperglycemia (fasting glucose greater than126 mg/dL)
• 2. History of clinically symptomatic pancreatitis within last year
• 3. Prior lung or liver transplant
• 4. Severe CF liver disease, as defined by portal hypertension
• 5. Fundoplication-related dumping syndrome
• 6. Medical co-morbidities that are not CF-related or are unstable per investigator opinion (i.e. history of bleeding disorders, immunodeficiency)
• 7. Acute illness or changes in therapy (including antibiotics) within 6 weeks prior to study procedures
• 8. Treatment with oral or intravenous corticosteroids within 6 weeks of study
• 9. Hemoglobin less than10g/dL, within 90 days of Visit 1 or at Screening
• 10. Abnormal renal function, within 90 days of Visit 1 or at Screening; defined as Creatinine greater than 2x upper limit of normal (ULN) or potassium greater than 5.5mEq/L on non-hemolyzed specimen
• 11. Inability to perform study specific procedures (MMTT, GPA)
• 12. Subjects, who in study team opinion, may be non-compliant with study procedures.
• Control Subjects who will be exposed to GIP only:
• 1. History of clinically symptomatic pancreatitis.
• 2. History of liver disease.
• 3. History of any illness or condition that, in the opinion of the investigator might confound the results of the study or pose an additional risk to the subject.
• 4. Hemoglobin \<10g/dL, within 90 days of GPA test or at Screening.
• 5. Abnormal renal function, within 90 days of GPA test or at Screening; defined as creatinine \> 2x upper limit of normal (ULN) or potassium \> 5.5mEq/L on non-hemolyzed specimen.
• 6. Inability to perform study specific procedures (MMTT, GPA).
• 7. subjects, who in study team opinion, may be non-compliant with study procedures.
• 8. elevation of serum amylase or lipase \> 1.5x ULN within 90 days of GPA test.