Glucose tolerance abnormalities are common in CF, reported Antoinette Moran, MD, assistant professor, Department of Pediatrics, University of Minnesota at the Ninth North American Cystic Fibrosis Conference held in October, 1995 in Dallas, Texas. There are four categories of glucose tolerance in CF, based on the response to a standard oral glucose tolerance test. The first is normal, with normal fasting blood sugar and the challenged glucose level never above 200 mg/dL. The second is impaired glucose tolerance, in which the fasting blood sugar is normal, and the challenge glucose level exceeds 200 mg/dL but returns to normal after 2 hours. The third category is diabetic glucose tolerance, with normal fasting blood sugar and challenge glucose level that exceeds 200 mg/dL and stays elevated for at least 2 hours. The most severe type of abnormal glucose tolerance is overt diabetes mellitus, with fasting blood sugar above 140 mg/dL.
At present, only patients with overt diabetes usually receive treatment. In these patients, the pancreas is unable to produce an adequate supply of insulin-the hormone responsible for glucose absorption. Of 400 persons with CF treated at the University of Minnesota, 42% of children, 66% of adolescents, and 72% of adults have abnormal glucose tolerance. Of these adults, 39% have impaired glucose tolerance, 25% have a diabetic glucose tolerance response, and 8% have overt diabetes.
Dr. Moran explained that the degree of glucose intolerance depends, in part, on the severity of CF-related damage to insulin-producing beta cells in the pancreas and in part on the insulin sensitivity of the individual.
“I believe that there is a spectrum of beta cell function in CF, with normal glucose tolerance on one end and diabetes on the other. The majority of persons with CF fall in the middle-in the glucose intolerant range. Where a patient is depends in part on how much beta cell damage has occurred, and that may be slowly progressive over time. But the other important factor is insulin sensitivity. During stress, such as infection, pregnancy, or steroid treatment, patients become insulin-resistant. For example, when a person with CF who is diabetic becomes ill, the insulin requirement may triple or quadruple while the exacerbation lasts,” Dr. Moran said.
Treatment goals for overt diabetes are the same as for diabetes that is unrelated to CF. Insulin therapy is a mainstay, and dietary recommendations are similar to those for the CF population in general (except that Dr. Moran advises patients to avoid soda containing sugar). There are no standard treatment guidelines for CF patients with less serious glucose intolerance.
“The question is whether the insulin-deficient, glucose intolerant state is harmful in CF. I think it might be. Insulin, in addition to controlling blood sugar, controls protein synthesis and lipid metabolism. Insulin deficiency might have adverse effects on these metabolic functions and affect body weight and muscle mass. The data are inconclusive, however, and until more information is available there are no specific recommendations for the treatment of nondiabetic, glucose intolerant CF patients,” Dr. Moran concluded.