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Extended Exercise Simulator

Model blood glucose across a 4-hour workout: pre-exercise preparation, in-exercise fueling, and post-workout recovery.

Long workouts lasting four or more hours deplete glycogen stores, increase insulin sensitivity, and require active fueling strategies to maintain blood glucose. The effects extend well beyond the exercise window itself.

This chart is designed to help you visualize how pre-exercise fueling, in-exercise nutrition, pump temp basals, and post-exercise recovery all interact across the full timeline of a long workout.

Pay attention to:

For educational purposes only. This simulator is a visual learning tool — not a medical device or dosing calculator.

What's happening here?

Extended exercise changes how both insulin and carbohydrates behave in your body. Muscles run primarily on glycogen — a form of glucose stored in muscle tissue and the liver. At sustained effort, your body burns through that glycogen steadily, and stores typically last around 90-120 minutes before they start running low. Once glycogen is depleted, performance drops sharply and blood sugar becomes harder to maintain.

This is why fueling during long workouts isn't optional — it's not just about managing blood sugar, it's about keeping your muscles supplied with the energy they need to keep going. The good news: during aerobic exercise, working muscles can pull glucose directly from the blood without needing insulin to do it. That means carbs you eat during the workout go largely straight to the muscles, so you need far less insulin to cover in-exercise fueling than you would for the same amount of carbs eaten at rest.

Important — insulin does more than lower blood sugar: Most people only think about insulin when blood sugar is high, but your body needs insulin continuously — even when blood glucose is normal. Every cell in your body uses glucose as fuel, but glucose can't get into cells without insulin acting as a "key" to unlock the door. Without it, cells are forced to burn fat for energy instead, producing acidic byproducts called ketones. If ketones build up faster than the body can clear them, the blood becomes dangerously acidic — this is diabetic ketoacidosis (DKA). DKA is caused by a lack of insulin, not by high blood sugar, and it can develop even when blood glucose looks normal. For pump users especially, suspending insulin for more than a few hours removes all background coverage and creates real DKA risk.
Phase 1
Before Exercise

Fueling before a long workout tops off glycogen stores. Carbohydrates are stored as glycogen in muscles and the liver — once those stores run out, performance drops and blood sugar becomes harder to control.

Phase 2
During Exercise

Working muscles absorb glucose directly without needing insulin. This means less insulin is required to cover in-exercise carbs, and any insulin already on board has a much stronger effect than usual.

Phase 3
After Exercise

Muscles and liver continue pulling glucose to replenish depleted glycogen. Increased insulin sensitivity can last many hours — sometimes up to 24 hours.

During exercise (4 hours)

  • The same amount of insulin lowers blood sugar more than usual.
  • If fueling is too low, blood sugar may drop steadily throughout the workout.
  • If fueling matches muscle demand and insulin is appropriately adjusted, blood sugar may stay stable.

After exercise (post-workout)

  • Blood sugars trending lower in the hours after finishing.
  • Reduced insulin needs for meals eaten after exercise.
  • Increased risk of delayed lows, especially overnight.

The day after

  • Muscles and liver continue restocking glycogen for 24-48 hours, pulling glucose out of the bloodstream even when you're at rest.
  • Insulin sensitivity may remain elevated the next morning — breakfast and correction doses can hit harder than usual.
  • Overnight lows are most common after a long afternoon or evening workout when dinner insulin and the post-exercise effect overlap.
  • A small bedtime snack or reduced overnight basal can help bridge the gap between the post-exercise effect and waking up in range.

MDI vs Pump

MDI users rely on long-acting basal insulin, which cannot be adjusted quickly. Pump users can adjust basal rates, but changes take time due to absorption. This is why temp basal reductions often need to start 90-120 minutes before exercise begins.

Applying this to your own life

Extended workouts require planning in three phases:

  • Before: Evaluate insulin on board, adjust pump basal early if needed, fuel to support glycogen stores.
  • During: Fuel regularly, monitor trends, remember less insulin is needed but not zero insulin.
  • After: Expect increased insulin sensitivity, consider reduced bolus doses, watch for delayed lows especially overnight.