In recent years, diabetes has emerged as one of the most significant co-diseases that many Cystic Fibrosis (CF) patients develop. Type 1 (T1D) and Type 2 (T2D) diabetes results when either the body does not make enough insulin or the body does not respond correctly to this insulin, respectively. Insulin is a hormone which is made by cells in the pancreas and helps carry glucose (sugar) from the food we eat to the cells of the body for energy. While cystic fibrosis related diabetes (CFRD) has many features similar to both T1D and T2D, patients with CF may not have the same symptoms as either T1D or T2D patients. Currently, there is little understanding of CFRD and the best options for treatment remain unclear.

The purpose of this research study is to examine and understand the various mechanisms that contribute to CFRD and gain a better understanding of potential means to treat CFRD. In particular, we plan to study the effects of incretin hormones that can enhance insulin production in CF patients.

Enrollment is complete for the protocol as initially written. In order to further study the role of the incretin hormone on Cystic Fibrosis Transmembrane Conductance Regulator (CFTR) function , we have received approval to extend our investigation to include the following study groups:

• Cystic Fibrosis participants with normal glucose tolerance
• Non-Cystic Fibrosis controls

Previously, cystic fibrosis related diabetes (CFRD) was considered to be a consequence of damage to the pancreas therefore the cells contained in the pancreas--i.e.--islets that house beta cells, which make and release insulin (similar to T1D). Recent evidence suggests that other factors may also be associated that are similar to those with T2D. For example, patients with T2D, have decreased secretion of incretins, hormones released by the small intestine in response to nutrients from food which act, among other things, to increase insulin secretion from Beta cells of the pancreas. When patients with T2D are treated with incretin hormones, their pancreatic Beta cells release more insulin (measured as 'second phase insulin secretion'). Currently, we do not know if patients with CFRD have decreased incretin secretion like T2D or if treating CFRD patients with incretin hormones will improve their insulin levels. This study will measure insulin release from the Beta cells from CFRD patients (second phase insulin secretion) that are being given incretin hormones in the veins. This will be compared with insulin release when the same patients are given a placebo (salt containing solution). The patients and the research team will not know what is being given until all the results are collected. The results will provide unbiased evidence if incretins will help improve insulin release in CFRD patients.

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.

Upon completion of enrollment, once our data has been analyzed.

Upon completion of enrollment, once our data has been analyzed. Informed Consent has already been shared.

Will upload on clinical trials.gov and attach to results section. Informed Consent has already been shared on ct.gov.