Exercise of short duration (<10 minutes) is managed very differently than sports of longer duration. Many athletes in short sprints do not worry about elevated glucose levels before events because there is little concern about the onset of hyperglycemia. Athletes participating in longer duration events, such as 10 km runs, marathons, basketball, or soccer do not want to start an event in a hypoinsulinemic state with elevated glucose because their glucose levels can climb to dangerous levels during the event. Athletes participating in both short and long duration events do not want to start an event in a low glucose or hyperinsulinemic state because it will produce very low glucose levels and dangerous consequences. For sprint events, elevated serum glucose without ketones is acceptable. For longer duration events, serum glucose more than 250 mg/dL with ketones or any serum glucose measurement more than 300 mg/dL pre-exercise should not be initiated until glucose is under better control.

The intensity and duration of exercise will determine specific modifications in the insulin regimen. These include: eating a low glycemic index meal 3 hours before exercise; having a small snack 1 hour before exercise; exercising after the peak action of subcutaneous insulin injection; and, delaying exercise until glucose and ketones are under control (4). Before exercise, depending on the predicted intensity, athletes should modify insulin dosage accordingly. Typically, this reduction ranges from 20% to 50% (5). Taking into account the peak action of each insulin type (see Table 20A.2), it is advised to decrease the dose of the specific insulin that would peak during an upcoming sporting event. For example, for a morning workout, the morning
dose of short-acting insulin (regular insulin—onset: 1–2 hours, peak: 2–4 hours) should be reduced whereas an afternoon activity requires reduction of the morning dose of intermediate-acting insulin (NPH or Lente insulin—onset: 1–3 hours, peak: 4–10 hours). The physician must understand that some trial and error occurs to meet each athlete’s insulin and carbohydrate requirements (4).

When administering exogenous insulin, the insulin level may remain elevated during exercise leading to inhibition of both glycogenolysis and gluconeogenesis. To avoid the resultant hypoglycemia, adequate calorie intake and blood glucose monitoring is critical. The athlete needs to replace fluid losses adequately. The athlete with diabetes, if appropriately managed, will initially experience a drop
in glucose levels. As insulin levels begin to decrease after cessation of the insulin pump or from properly timed subcutaneous injection, frequent monitoring is necessary to assess subsequent glucose elevation. Because it is important for the athlete to replenish glycogen stores during and immediately after exercise, they should ingest 40 g of carbohydrate for every half hour of intensive exercise.

Following exercise, the athlete with diabetes should anticipate late-onset hypoglycemia. If the exercise intensity was unusually high, blood glucose monitoring should occur frequently in the hours following activity, even throughout the night. Any sense of exhaustion, or weakness, or increase in appetite hours after exercise may warn the athlete of possible hypoglycemia. Prevention requires frequent blood glucose checks, upward adjustments of caloric intake, and lowering of longer acting insulin dosage that typically would peak overnight.

For the athlete with DM using multiple injections, the site of injection and rate of absorption is important. Insulin absorption is more rapid and less predictable when injected into the leg before exercise (4). Care should be taken to avoid accidental intramuscular injection. The most common site used by athletes for injection is the abdomen, given its ease of access during meals and more predictable insulin absorption time (6).

The insulin pump or continuous subcutaneous insulin injection (CSII) system allows more flexibility for skipped meals, sleeping late, and spontaneous exercise. The subcutaneous insulin delivery with the pump allows for precise dosing of insulin. The CSII delivers a continuous basal rate and then boluses can be given by the user for meals. In order to reduce hypoglycemia, the basal rate of the pump is reduced by 50% approximately 1 hour before activity (7). For exercise of lower intensity or shorter duration, the standard basal rate can be maintained and a simple reduction in the premeal bolus is sufficient.

Insulin pumps can malfunction during exercise. The athlete must be mindful of displacement of the infusion set which can lead to a hypoinsulinemic state and diabetic ketoacidosis (DKA) in a short period. Continuing to exercise, unaware of a displaced catheter results in lower insulin levels than expected and can easily quicken the progression to DKA.

Sweating can displace the pump; liquid skin preparations can be used to prevent displacement. The use of antiperspirants around the infusion site has helped reduce sweating around the infusion set. Insulin is heat sensitive and overheating can occur when exercising in the heat with the pump next to the body. The environment (cold and heat) can affect the overall effectiveness of an insulin pump. If unexplainable hyperglycemia occurs, the infusion catheter and insulin cartridge should be replaced. Proper care and monitoring of the equipment is essential for successful use of the pump during exercise.

If removal of the pump is needed during contact and collision sports, it should be stopped 30 minutes before short duration exercise (<1 hour) due to the persistent action of insulin after pump removal. Care should be taken in order to ensure protection of the catheter that will remain in the athlete. Small boluses during exercise may be needed for longer activities (>1 hour) to prevent hypoinsulinemic states during prolonged activities such as marathons and soccer. These boluses should be given hourly and the amount of insulin given should represent approximately 50% of the usual hourly basal rate (8).