Intensive Insulin Therapy and Exercise in Type 1 Diabetes
A 24-year-old man with T1D for 10 years, using an insulin pump (CSII) with rapid-acting insulin, plans to start a high-intensity interval training (HIIT) program 5 days a week, 45 minutes per session, in the evening. His current basal rate is 1.0 units/hr, and his insulin-to-carb ratio is 1:10. His HbA1c is 7.0%, TIR 75%, but he has frequent post-exercise hyperglycemia followed by nocturnal hypoglycemia. He uses a CGM. He wants to improve his fitness and avoid glucose fluctuations.
1. Explain the physiological effects of high-intensity interval training on glucose metabolism in T1D and how they contribute to his pattern of post-exercise hyperglycemia and nocturnal hypoglycemia.
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HIIT combines anaerobic and aerobic phases. During high-intensity bursts, catecholamines and growth hormone increase, causing hepatic glucose production and insulin resistance, leading to transient hyperglycemia. After exercise, muscle glycogen replenishment and increased insulin sensitivity can cause delayed hypoglycemia, especially overnight (up to 12–24 hours). In T1D, the lack of endogenous insulin regulation exacerbates these swings. The post-exercise hyperglycemia may be due to insufficient insulin at the time of exercise (e.g., if he reduces basal too much or disconnects pump) or due to stress hormones. Nocturnal hypoglycemia results from increased insulin sensitivity and inadequate compensation.
2. What specific adjustments to his insulin pump settings and carbohydrate intake would you recommend to minimize these fluctuations? Include strategies for pre-exercise, during, and post-exercise, and for nocturnal prevention.
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Pre-exercise: check glucose 30–60 min before; if <100 mg/dL, consume 15–20 g carbs; if >180 mg/dL with ketones, delay. Consider a temporary basal reduction of 30–50% starting 30–60 min before exercise if on pump, but for HIIT, may need to maintain or slightly increase basal to counter hyperglycemia. During: monitor CGM; for HIIT, may not need carb intake unless glucose trending down. Post-exercise: if glucose >250 mg/dL with ketones, correct with insulin; otherwise, consider a small bolus for post-exercise meal with reduced bolus (e.g., 50% of usual) due to increased sensitivity. For nocturnal prevention: reduce basal rate by 20–30% for 6–8 hours after exercise, or reduce bedtime bolus if applicable; consume a bedtime snack with protein and carbs if glucose <120 mg/dL; set CGM low alerts. Also consider exercise timing: morning may reduce nocturnal risk. Educate on ketone monitoring. Adjust insulin-to-carb ratio for post-exercise meals (e.g., 1:15). Review with diabetes team.