This study explores how **sugars in food** affect diabetes in teens and young adults with Cystic Fibrosis (CF). CF is a condition that causes mucus to build up in the lungs, leading to breathing problems. Some people with CF can develop a type of diabetes called Cystic Fibrosis-related Diabetes (CFRD). This study will look at how different meals, with high or low *glycemic index* (a measure of how quickly sugars in foods affect blood sugar levels), affect blood sugar after eating. Participants will eat a specific diet and have their blood tested before and after meals. This will help understand changes in blood sugar and insulin, a hormone that helps control blood sugar.

• The study involves eating controlled meals and having several blood tests.
• Participants must be English-speaking and already consume some sugary foods.
• Those with recent health changes or certain conditions cannot join.

By joining, participants help discover better ways to manage diet and diabetes in CF, but they must commit to following specific meal plans and undergoing blood tests. This research may help improve health guidelines for people with CF.