How should a diabetic patient on insulin or sulfonylureas manage blood glucose before, during, and after exercise to prevent hypoglycemia?

Managing Exercise-Related Hypoglycemia in Diabetic Patients on Insulin or Sulfonylureas

For patients on insulin or sulfonylureas, check blood glucose 15-30 minutes before exercise and ingest 15-30 grams of carbohydrate if pre-exercise glucose is below 90 mg/dL (5.0 mmol/L), while reducing rapid-acting insulin by 30-50% before planned exercise to prevent both immediate and delayed hypoglycemia. 1, 2

Pre-Exercise Blood Glucose Assessment

Check blood glucose 15-30 minutes before starting exercise with a target range of 90-250 mg/dL. 2

• Do not exercise if glucose is below 90 mg/dL without first consuming carbohydrates, as this significantly increases acute hypoglycemia risk during the workout. 1, 2
• If glucose is below 90 mg/dL, consume 15-30 grams of carbohydrate and recheck glucose before starting exercise. 1, 2
• Avoid exercise if glucose exceeds 250 mg/dL with ketones present, as intense activity can paradoxically raise glucose levels further. 1

Insulin Dose Adjustment Strategy (Primary Prevention)

Reduce rapid-acting insulin (such as lispro, aspart, or glulisine) by 30-50% before meals preceding planned exercise. 2

• This insulin reduction is the primary strategy because exercise during peak insulin times creates a "double effect" on glucose uptake—elevated circulating insulin combined with exercise-enhanced muscle glucose uptake substantially amplifies hypoglycemia risk. 2
• For exercise occurring 1-3 hours after lunch, reduce the pre-lunch rapid-acting insulin dose by 30-50% initially, then adjust based on individual glucose response patterns. 2
• Avoid injecting rapid-acting insulin into muscles that will be exercised, as this accelerates insulin absorption and increases hypoglycemia risk; inject into the abdomen instead. 2
• Do not completely omit rapid-acting insulin without carbohydrate coverage, as this risks meal-related hyperglycemia. 2

Carbohydrate Supplementation (Adjunctive Strategy)

Consume 15-30 grams of carbohydrate before exercise if pre-exercise glucose is trending lower or if insulin dose reduction is insufficient. 2

• Carbohydrate intake should supplement—not replace—insulin dose reduction, as relying solely on carbohydrates without insulin adjustment leads to cycles of hyperglycemia followed by hypoglycemia. 2
• For exercise lasting longer than 60 minutes, consume 10-15 grams of carbohydrates every 30-60 minutes during the activity. 1, 2
• The exact carbohydrate requirement depends on insulin doses, exercise duration and intensity, and time of day. 1

During-Exercise Monitoring

Monitor blood glucose or continuous glucose monitor (CGM) values during exercise and watch for hypoglycemia symptoms. 1

• Symptoms include tremor, sweating, confusion, weakness, or unusual fatigue. 2
• Longer exercise duration increases hypoglycemia risk—sessions lasting 60-120 minutes show significantly greater glucose drops than 15-30 minute sessions. 3
• Aerobic activities cause the greatest glucose decline, followed by mixed activities, then anaerobic/resistance exercise. 3

Post-Exercise Management (Critical for Delayed Hypoglycemia)

Consume 5-30 grams of carbohydrate within 30 minutes after completing exercise to prevent delayed hypoglycemia. 2

• Blood glucose can continue to drop for 24-48 hours after exercise due to increased insulin sensitivity and muscle glycogen replenishment. 1, 2
• This delayed hypoglycemia risk is particularly high after glycogen-depleting workouts (longer duration or higher intensity exercise). 2
• Consider reducing evening basal insulin (such as glargine or detemir) by 10-20% on workout days to prevent nocturnal hypoglycemia. 2
• Check blood glucose before bed and consider a bedtime snack if glucose is trending downward. 1

Special Considerations for Patients on Sulfonylureas

Patients on sulfonylureas face similar hypoglycemia risks as those on insulin but cannot adjust medication doses acutely. 1

• Sulfonylureas stimulate endogenous insulin secretion continuously and cannot be "turned down" for exercise like injectable insulin can be adjusted. 1
• These patients must rely more heavily on carbohydrate supplementation before, during, and after exercise. 1
• Pre-exercise carbohydrate intake of 15-30 grams is essential if glucose is below 90 mg/dL. 1
• Hypoglycemia is less common in patients not treated with insulin or insulin secretagogues, and routine preventive measures are usually not needed in those populations. 1

Common Pitfalls to Avoid

• Never exercise with glucose below 90 mg/dL without first consuming carbohydrates, as 90% of exercise sessions starting below 90 mg/dL result in hypoglycemia. 2, 3
• Do not skip insulin dose reduction and rely only on extra carbohydrates, as this approach leads to poor overall glycemic control. 2
• Remember that higher insulin on board before exercise increases hypoglycemia risk, even when carbohydrates are consumed. 3
• Males experience larger glucose drops during exercise than females, requiring potentially more aggressive preventive measures. 3
• Declining glucose rates before starting exercise significantly increase hypoglycemia risk during the activity. 3