A Pilot Study of a Low Glycemic Load Diet in Adults With Cystic Fibrosis

Non-pulmonary complications of cystic fibrosis (CF) are becoming increasingly prevalent with the changing landscape of CF care. CF related diabetes mellitus (CFRD) and CF related gastrointestinal (GI) complications have significant effects on morbidity and mortality. Treatment options are limited to insulin therapy for CFRD and symptom control for most GI complications. BMI is a well-established marker of morbidity and mortality in patients with CF. Many patients consume a high carbohydrate intake to meet their increase caloric needs, potentially leading to complications including post-prandial hyperglycemia, increased inflammation, and abnormal GI motility. Dietary recommendations for children and adults with CF are limited and based entirely on consensus and expert opinion. As patients with CF live longer with highly effective modulator therapy, it is important to understand the effects of dietary composition on short and long-term endocrine, GI, and pulmonary outcomes. The investigators will conduct a prospective, open-label pilot study in adults with CF and impaired glucose tolerance or indeterminate glycemia to establish the safety and tolerability of a low glycemic load (LGL) diet. Subjects will initially follow their standard diet for a 2-week run-in period, then transition to a LGL diet provided by a food delivery service for the remaining 8 weeks. The investigators will also investigate potential short-term outcomes of dietary carbohydrate manipulation, including glycemic variability measured by continuous glucose monitor (CGM), body composition via DXA, GI symptoms, and quality of life measures. The investigators hypothesize that a diet lower in carbohydrate content will be safe, tolerable, and associated with weight maintenance or gain, and that a LGL diet will result in decreased glycemic variability via CGM, improved GI symptoms, increased lean to fat mass ratio, and improved quality of life measures over an 8-week period.