A Pilot and Feasibility Study to Evaluate High vs Low Glycemic Index Mixed Meal Tolerance Test in Adolescents and Young Adults With Cystic Fibrosis

Who May Qualify: - English speaking - Diagnosis of CF based on the presence of two known CF causing mutations and/or positive sweat test - Pancreatic insufficiency - Baseline dietary consumption of \>10% total kcal from added sugars and self-reported consumption of \>/= sugar-sweetened beverages per week Who Should NOT Join This Trial: - Current use or anticipated use of medication that is known to raise or lower blood glucose in the past 4 weeks. - Oral or IV glucocorticoid current or previous use in the past 4 weeks will prohibit enrollment in the study. - Recent pulmonary exacerbation within 3 weeks of enrollment and/or an acute illness requiring a change in antibiotics will also exclude participants. - BMI below the 5th percentile or greater than the 95th percentile for age and sex - FEV1 \<40% or awaiting a lung transplant; - Prior lung or liver transplant or kidney or liver dysfunction. - Use of CFTR modulators is not an exclusion criterion. Rather, for patients recently started on CFTR modulators, we will wait to enroll in the study until on CFTR modulator for at least 2 months. - Diagnosis of CF liver disease. - Uncontrolled exocrine pancreatic insufficiency/malabsorption - Diagnosis of CFRD - G-tube feeds (bolus and/or continuous) - Current enrollment in another intervention study - Changes in diet to lose or gain weight - Gluten allergy or intolerance - Current pregnancy or lactation or plans to become pregnant during study period - History of drug or alcohol abuse - Restrictive dietary patterns (e.g, vegan, ketogenic, intermittent fasting) for more than one month within the last two months prior to screening. - More than 5% body weight change within 2 months of screening visit or Day 1 of mixed meal tolerance test Always talk to your doctor about whether this trial is right for you.