In the present study, we demonstrated that stopping the basal insulin infusion at the start of a prolonged period of moderate aerobic exercise in the late afternoon was an effective strategy for reducing the risk of hypoglycemia during the exercise period. While this maneuver did not completely eliminate the risk of hypoglycemia, a fall in glucose that required treatment was infrequently observed if the pre-exercise plasma glucose level was >130 mg/dL. Moreover, the response to treatment of hypoglycemia with oral carbohydrate was more effective under basal-stopped conditions, since none of the subjects required more than one treatment with carbohydrate snacks compared with approximately one-third of the subjects during the basal-continued visit. Discontinuation of basal infusion was associated with a modest increased risk of hyperglycemia (12% vs. 4%, P=0.11) during exercise, but blood ketone levels remained suppressed.
Since children and adolescents with T1D may have periods of exercise and rest that extend beyond 75 minutes (e.g., an afternoon at the beach), we monitored the subjects for 45 minutes following exercise on both study days. During the basal-continued visit, 4 subjects who had not been hypoglycemic during exercise became hypoglycemic and one subject became hyperglycemic compared with no subjects becoming hypoglycemic and 7 becoming hyperglycemic during the basal-stopped visit. At the end of the study (approximately 120 minutes from the start of the study), a much greater rise in plasma glucose was observed on the basal-stopped day than on the basal-continued day, with hyperglycemia being present in 27% and 4% of subjects, respectively. The effect of delaying the restarting of insulin after exercise (as was done in the study) versus restarting it immediately following exercise warrants further study.
The duration and intensity of exercise in this study reflect the current national recommendation of at least 60 minutes of daily moderate to vigorous activity for children (11
). We chose to study the effects of exercise at approximately 4 p.m., since this is when children and adolescents often engage in after school physical activities. Recent work using an objective monitoring system suggests that youth are most active from 3–7 p.m. (12
). However, we recognize that a structured exercise program such as the one utilized in this study is not the same as real life exercise performed by children. We plan to conduct future studies with the use of accelerometry to better address the effect of “real life exercise” on glucose levels.
Since the last pre-meal bolus dose of insulin was given approximately 4 hours earlier, the subcutaneous depot of rapid-acting insulin analog was likely to be quite small in our subjects at baseline (13
). While discontinuation of basal infusion was very effective in preventing episodes of exercise-induced hypoglycemia under these conditions, alterations in pre-meal bolus doses might be a more effective strategy to reduce the risk of hypoglycemia during bouts of exercise that occur shortly after a meal. This question was addressed by Schiffrin et al. in a study that pre-dated the introduction of insulin analogs (14
). These investigators examined the effect of altering pre-meal insulin doses before a 45-minute exercise session in 7 adolescents on insulin pumps and 6 on multiple daily injections. Subjects were tested one time resting and four times during exercise after administering varied proportions of their usual insulin doses (0%, 50%, 67% and 100%). Under these conditions a 50% reduction in the pre-meal dose provided an effective means to reduce the risk of hypoglycemia. A recent study comparing 50% basal-insulin vs. basal-stopped during morning exercise in 10 adolescents with T1D found no difference in acute hypoglycemia or fall in BG (15
). Hypoglycemia developed during 2 of the 10 basal-stopped sessions and during 2 of the 10 basal-continued sessions. The authors concluded that discontinuing the basal rate did not prevent hypoglycemia. A small sample size, shorter exercise session (40–45 minutes), timing of the exercise two hours after the breakfast meal and pre-meal bolus dose reduction in first study and 50% reduction in basal-insulin on the control visit in the latter study may explain why those results differ from the present study.
Because of its complexity, trial and error remains the principal method of regulating plasma glucose levels during exercise. However, the results of the present study can be used to guide recommendations for managing youth receiving insulin pump treatment during similar late afternoon exercise. The plasma glucose should be checked prior to exercise and 15–30 gms of carbohydrate should be taken if the glucose is <130 mg/dL or a small correction bolus should be given if the glucose is >200 mg/dL. While in most patients the pump can then be safely suspended or disconnected for up to 2 hours, glucose levels should be measured every 60–90 minutes during and after exercise and insulin administered when needed. The child and parents can also be informed that if hypoglycemia develops during exercise, it will be easier and more consistently treated with 15–30 gms of carbohydrate if the basal insulin infusion has been temporarily interrupted. The ability to suspend or reduce basal insulin during increased physical activity is another example of the flexibility of insulin pump therapy that distinguishes it from multiple daily injection regimens that utilize long-acting insulin analogs for basal insulin replacement.