Cystic fibrosis (CF) is a genetic disease with an autosomal recessive, chronic and progressive character. About 10 to 25% of patients develop CF-related diabetes (DRFC), whose pathogenesis is associated with pancreatic fibrosis and destruction. Until now, there is no evidence to support the use of low glycemic index diet to improve glycemic response in pre-diabetic and CF patients. The objective of this study is to evaluate the glycemic improvement after nutritional orientation in patients with cystic fibrosis in the pre-diabetic phase. Methods: Randomized clinical trial in patients with CF aged over 10 years without hospitalization in the last 4 weeks. Anthropometric measurements, blood pressure, 24 hour recall and food frequency questionnaire will be evaluated. Oral glucose tolerance and glycated hemoglobin test will be dosed at baseline and during 12 weeks of follow-up. The intervention group will receive a nutritional guidance at the beginning of the research.