Glucose Response to Intense Aerobic Exercise in Type 1 Diabetes: Maintenance of near euglycemia despite a drastic decrease in insulin dose

In patients with type 1 diabetes, hypoglycemia or degradation in blood glucose control may occur during and after physical exercise (1–3). This may be avoided in patients with good glycemic control by decreasing insulin doses and/or ingesting carbohydrate. Although most sport activities involve intense muscle exercise, current recommendations are based on studies on moderate exercise (4). We therefore investigated the effect of a drastic reduction in insulin dose before a 60-min high-intensity cycle exercise in 12 subjects with uncomplicated type 1 diabetes (age 32 ± 7 years, BMI 23 ± 7 kg/m², HbA_1c 7.2 ± 3.8%, and Vo_2max 40 ± 27 ml · min⁻¹ · kg⁻¹ [mean ± SD]). Six patients were treated with three daily injections (regular insulin in the morning and at noon and mixed regular NPH insulin before dinner). The six remaining patients were treated with two daily injections (30% regular/70% NPH insulin). After determination of Vo_2max, patients reported to the laboratory on two separate occasions, 1 week apart, in randomized order 90 min after breakfast (60 g carbohydrates, 10 g lipids, and 8 g proteins) to perform a 60-min exercise session on an ergocycle at 70% Vo_2max. On one occasion, exercise was performed with the usual morning insulin dose. On the other, morning insulin dose was reduced by 90% for patients treated with three daily injections and by 50% for patients treated with two injections per day. Power output was monitored and strictly maintained during the tests to correspond to ∼70% Vo_2max.

Changes in plasma glucose (PG) levels were analyzed by a mixed-model ANCOVA, with a random subject effect and random coefficients for time within each subject, using the SAS version 6.12 software package (SAS Institute, Cary, NC).

At the beginning of exercise and over the test period, PG levels were higher in the experimental condition with insulin dose reduction (P < 0.0001) (Fig. 1). Changes in PG levels were similar in both conditions during exercise and recovery (P = 0.99). No difference was observed when considering insulin regimen. During exercise, the mean decrease in PG concentrations was −0.085 ± 0.012 mmol · l⁻¹ · min⁻¹ (mean ± SE) (P < 0.0001), whereas no significant variation was observed during the recovery period. When exercise was performed without reducing insulin doses, eight patients (66%) had hypoglycemia and were given oral sucrose (22 ± 3 g). Changes in plasma lactate, growth hormone, cortisol, glucagon, and norepinephrine were not statistically different. The peak of plasma epinephrine concentration was higher in the test without insulin reduction: 2.3 ± 1.5 vs. 1.1 ± 0.7 nmol/l (P < 0.04, Student’s t test).

In previous studies, decreasing insulin dose before moderate exercise (∼55% Vo_2max) was not associated with degradation in blood glucose control during and after exercise (1,4,5). However, most sport activities, whether individual (running, biking, hiking, and swimming) or team (basketball, football, and handball) involve intense muscle exercise. We show here that a 90% reduction of morning regular insulin before intense exercise allows the maintenance of near normal blood glucose levels without occurrence of hypoglycemia. When insulin was not reduced, two-thirds of patients experienced hypoglycemia.

This study emphasizes the importance of insulin-independent contraction-induced glucose uptake by muscle, previously demonstrated in healthy men (6) and mice lacking muscle insulin receptor (rev. in 7).

In conclusion, we demonstrate that type 1 diabetic patients can perform intense muscle exercise after a 50–90% reduction in insulin dose, depending on their insulin regimen. This decrease prevents hypoglycemia without worsening metabolic control. Such advice could be given to young type 1 diabetic patients engaged in sports activities.