Glucose Metabolism in Cystic Fibrosis Related Diabetes (CFRD)

The study aims to test whether use of CFTR modulators (ETI) improves fasting and post prandial glycemia by enhancing disposition index (DI) in individuals with CFRD with CFTR +ve mutation (at least one copy of F508del) on CFTR modulator (ETI) therapy (CFRDF508del+ETI). CFRD with a mutation that is not eligible for modulator therapy (CFRD-ETI) will be the control group.

Inclusion Criteria: 1. Diagnosis of Cystic Fibrosis Related Diabetes (CFRD) 2. Age 21-75 years at time of consent 3. BMI 19-50 kg/m2 (In Asians BMI is \~ 2 points lower for comparison so it is 17-48 kg/m2) 4. Creatinine ≤ 1.4 mg/dl in women and ≤ 1.5 mg/dl in men 5. HbA1c ≤ 11% lifestyle treatment or mono/combination therapy with oral hypoglycemic agents (e.g. metformin or sulphonylurea or SGLT2i) and preferably on insulin pump therapy (i.e., SAP, HCL) with or without CGM will be recruited. However, use of MDI (Multiple Daily Injections) of insulin, i.e., on basal-bolus insulin therapy will also be included. Most of our patients are on insulin therapy with very few on oral antidiabetes medications but we would like to offer them the choice of participation. 6. Subjects on FDA approved/recommended full doses of ETI will be offered participation. If dose adjustments of ETI are made after enrollment a discussion will be done with the primary care provider and HCSTOC as required. 7. Willing to be at a stable weight for duration of the study. 8. An understanding of and willingness to follow the protocol and sign the informed consent Subjects meeting any of the exclusion criteria at baseline will be excluded from study participation: Exclusion Criteria: 1. Debilitating chronic disease 2. Anemia \