With improved survival, the clinical spectrum of cystic fibrosis (CF), a complex multi systemic disease, continue to evolve. A major emerging complication is CF related diabetes (CFRD) which is occurring in 40-50% of adults. Patients who develop CFRD are at increase risk of morbidity and mortality and they are also facing an increased medical burden with insulin therapy, the only recommended treatment. Accelerated decline in weight and/or pulmonary function start 2 to 4 years before CFRD onset and this period is characterized by frequent Post-Prandial Glycemic (PPG) excursions. Higher PPG excursions are associated with lower pulmonary function and it predicts future CFRD risk. To the improved nutritional status, lung function and survival of patients, the nutritional approach for patients with CF focuses on high-energy high-fat diet and a pancreatic enzyme supplementation. However, such diet also contributes to increased PPG excursions. Based on the beneficial effects of nutrition therapy to improve PPG in other forms of pre-diabetes and diabetes, extending such benefits to patients with CF is important. The investigators aim to test the feasibility and the effectiveness of a viscous fiber supplement to reduce PPG in adult patients with CF. Using a randomized crossover design, the investigators will study the impact of two doses of a viscous fiber supplement as compared to a placebo.

With improved survival, the clinical spectrum of cystic fibrosis (CF), a complex multi systemic disease, continue to evolve. A major emerging complication is CF related diabetes (CFRD). CFRD is a major co-morbidity occurring in 20% of adolescents and 40-50% of adults and is mainly due to reduced insulin secretion. Patients who develop CFRD are at increase risk of morbidity and mortality and they are also facing an increased medical burden with insulin therapy, the only recommended treatment. Because accelerated decline in weight and/or pulmonary function start 2 to 4 years before CFRD onset and that this period is characterized by frequent Post-Prandial Glycemic (PPG) excursions, the role of these PPG excursions in clinical deterioration have been investigated. The investigators have shown that higher PPG excursions are associated with lower pulmonary function and others have shown that PPG excursions predicts future CFRD risk. Food overconsumption is discouraged in other forms of diabetes to avoid overweight or obesity, but also to prevent an increase in insulin demand. In contrast, nutritional approach for patients with CF focuses on high-energy high-fat diet to meet increased energy requirements. In combination with an appropriate pancreatic enzyme supplementation, this approach contributes significantly to the improved nutritional status, lung function and survival of patients. However, such diet also contributes to increased PPG excursions. Based on the beneficial effects of nutrition therapy to improve PPG in other forms of pre-diabetes and diabetes, extending such benefits to patients with CF is important. In the context of high CF treatment burden, such approach should however be simple and not inducing weight loss. The investigators aim to test the feasibility and the effectiveness of a viscous fiber supplement to reduce PPG in adult patients with CF. Using a randomized crossover design, the investigators will study the impact of two doses of a viscous fiber supplement as compared to a placebo in controlled conditions, in 22 patients, over 8 hours, at the research center with 2 standardized mixed-meals: breakfast and lunch on PPG excursions.

Inclusion Criteria:

• Diagnosed with CF
• Aged above 18 years
• CF-Impaired glucose tolerance, CF-indeterminate glucose tolerance (INDET), de-novo diabetes not requiring immediate treatment and diabetes without pharmacological treatment, based on the oral glucose tolerance test (OGTT) performed within three months of the first visit

Exclusion Criteria:

• Treated or longstanding CFRD
• Subjects taking medication or affected by conditions that could interfere with glucose metabolism in the last 6 weeks: hemoptysis, fever, IV antibiotic treatment, pregnancy and oral steroids. If a patient presents any sign of infection confirmed by a trained CF pneumologist, OGTT testing is postponed to the next medical appointment 1 months later.
• Subjects with severe gastrointestinal disease (Crohn disease, coeliac disease and recent (<1 year) history of intestinal occlusion).
• Subjects with intestinal obstruction history.
• Allergy to an ingredient in the menu.
• Corrector or potentiator prescribed within the last 6 months prior to the first visit.