The Impact of Morning Versus Afternoon Aerobic Exercise on Carbohydrate and Lipid Metabolism in Type 1 Diabetes

Aim. To determine whether the need for exogenous carbohydrate supplementation during exercise in type 1 diabetes is lower during morning exercise compared to afternoon exercise, and whether this is associated with greater lipid oxidation in the morning.

Rationale. Carbohydrate intake requirements to prevent hypoglycemia may vary throughout the day. Determining whether increased carbohydrate requirements are needed in the afternoon will help patients avoid hypoglycemia.

Design and methods Twelve subjects with type 1 diabetes [age 18-65, male/female, >2 years of diabetes duration, multiple daily injections (MDI) or continuous subcutaneous insulin infusion (CSII))] will participate in a randomized crossover study. Subjects will perform moderate intensity at 60% of VO2max for 1 hour during one morning (9 AM) and one afternoon (4 PM). For both morning and afternoon exercise bouts, subjects will be asked to refrain from exercise for 48hours prior to the test. For the morning exercise bout, the night before subjects will consume a standardized meal at 7 pm, followed by an overnight fast. The afternoon exercise test will be performed 5 hours after an early standardized lunch with full dose of rapid-acting insulin. Basal insulin will be continued in the usual dosage, and starting blood glucose value will be standardized to target 7.0 mmol/l with acceptable range 5.0-9.0 mmol/l prior to initiation of the exercise session. Glucose monitoring will occur every 10 minutes during exercise, and every 5 minutes if blood glucose drops below 3.9 mmol/l. If blood glucose drops below 3.9 mmol/l, an unlabeled glucose infusion will be administered at a rate that keeps blood glucose just above 5 mmol/l.

Glucose and palmitate tracers will be infused to determine differences in diurnal glucose and lipid metabolism during exercise. Indirect calorimetry will be used to measure glucose and lipid oxidation rates. Blood samples will be gathered to measure tracer levels, growth hormone, cortisol, and free fatty acids (FFA) pre-exercise, during exercise, and post-exercise. Continuous glucose monitoring (CGM) and accelerometer activity and sleep data will be collected 24 hours before exercise and for 24 hours after the exercise bout.

Endpoints. The primary endpoint will be the volume of intravenous glucose (g of glucose) needed to maintain glucose above 5 mmol/l during the two exercise bouts. The secondary endpoints will be Lipid- and carbohydrate oxidation rates, time to hypoglycemia, delta blood glucose levels, lipolysis rate, and endogenous glucose production.