Cystic Fibrosis-Related Diabetes and Abnormal Glucose Tolerance: Overview and Medical Nutrition Therapy

Abstract

Cystic fibrosis (CF) is an autosomal recessively inherited defect in the cystic fibrosis transmembrane receptor, a cell membrane chloride channel. The deficiency or absence of the channel results in thick, sticky secretions in many organs, including lung, liver, gastrointestinal tract, and pancreas. Eighty-five percent of CF patients have exocrine pancreatic insufficiency. The obstructive nature of the tenacious mucus predisposes to infection, particularly in the respiratory tract.1 CF was once considered a disease of childhood. However, with improvements in medical care during the past few decades, patients with CF are now living well into their third, fourth, or fifth decade of life and have a median life expectancy of 32.2 years.2 As survival has increased, CF-related diabetes (CFRD) has become the leading co-morbidity in this patient population,3–5 occurring in ∼13% of all patients with CF.2 This figure is widely believed to be an underestimate because of the lack of routine screening for diabetes in the CF population.6 CFRD is most commonly diagnosed in patients who are between 18 and 21 years of age.4,7 The 1998 CFRD Consensus Committee identified four glucose tolerance categories for clinical management and research purposes (Table 1).8 The four categories are based on a standard glucose tolerance test: normal glucose tolerance (NGT), impaired glucose tolerance (IGT), CFRD without fasting hyperglycemia (FH), and CFRD with FH. There are important differences in medical nutrition therapy (MNT) for the various categories of glucose intolerance and other circumstances in patients with CFRD. Glycated hemoglobin (A1C) is not useful for diagnosing CFRD because increased red blood cell turnover in CF patients may falsely lower A1C levels. However, it can be useful in monitoring overall blood glucose control in established CFRD patients.8 The American Diabetes Association (ADA) classifies CFRD under …