High-intensity exercise may be defined as activities above the ‘lactate threshold’. This occurs when there is an exponential increase in lactate production caused by a greater reliance on anaerobic metabolism, occurring at approximately 60-70% of VO2max or 85-90% maximal heart rate. This threshold coincides with dramatic elevation in catecholamines that increase hepatic glucose release, FFA, and ketone levels, and impair glucose utilization by skeletal muscle (Fig. 1C). Even those individuals treated with intensive insulin therapy may have increases in blood glucose levels during and after high-intensity exercise, probably due to a failure in insulin release to offset the increases in counterregulatory hormones. This rise in glucose concentration is usually transient and tends to last only as long as there are elevations in counterregulatory hormones (i.e., 30-60 min).

Although some individuals can easily correct the elevations with an insulin bolus, particularly if they take rapid acting insulin analogs, others may be reluctant to take additional insulin after exercise, because there will be greater risk of late-onset postexercise hypoglycemia.

Competition Stress, Heat Stress and Hyperglycaemia

The psychological stress of competition is frequently associated with increases in blood glucose levels even though the pre-exercise glucose concentrations may be normal. Those pursuing vigorous aerobic exercise may find that on regular training or practice days they become hypoglycemic, but on the day of competition they develop hyperglycemia. Although empiric data do not exist for children with T1DM, excessive increases in glucose counterregulatory hormones probably occur just before exercise when anticipatory stress is high. It is also probable that the stress during competition can further increase blood glucose levels. Individuals may find that play or sporting activities in warm and humid environments also elevates blood glucose levels, probably because of excessive increases in circulating plasma catecholamines, glucagon, cortisol, and growth hormone (Fig. 1D).

Practical considerations for the clinical management of type 1 diabetes in athletic youth

The major challenge for active youth with T1DM is to balance food, insulin, and exercise to limit blood glucose excursions. Some of the factors affecting blood glucose levels during exercise are circulating plasma insulin levels, the intensity and duration of the exercise, the type of exercise performed (aerobic vs. anaerobic), and the prevailing concentrations of the glucose counterregulatory hormones. To a lesser extent, age, gender, level of metabolic control, and the level of aerobic fitness also contribute. Although the glycemic response varies greatly between children, blood glucose changes during exercise have some degree of reproducibility, as long as the exercise conditions and pre-exercise insulin and diet are consistent. Whereas no precise guidelines exist to limit fluctuations in glucose levels during exercise, some general strategies do exist (Table 1).