Same meal. Same carbs. Completely different insulin requirement depending on whether you trained today and what type of session it was. Exercise is the variable most bolus calculators ignore. This guide covers what actually changes and why.
Exercise increases insulin sensitivity. Your muscle cells become more receptive to glucose uptake, meaning the same amount of insulin has a stronger effect than it would at rest. This is generally a benefit for glucose management, but it also means your normal insulin doses can cause hypoglycemia when exercise is involved.
The effect is not uniform. It depends heavily on the type of exercise, the timing relative to your last bolus, the duration, and the intensity. Understanding these differences is what allows you to train hard without constantly fighting lows or unexpected highs.
Aerobic training — running, cycling, swimming, rowing, zone 2 cardio — is fuelled primarily by glucose and fat. Your muscles pull glucose directly from the bloodstream during the session, causing glucose to drop during and after exercise.
This effect continues for hours after you stop. Aerobic exercise increases insulin sensitivity for 24 to 48 hours post-session, meaning every bolus you give during that window will have a stronger effect than usual.
If you are eating within 90 minutes before an aerobic session, reduce your meal bolus by 25 to 50%. The exact reduction depends on your glucose at the time, the session intensity, and your individual response. Start with a 30% reduction and adjust based on your CGM data over time.
If you are eating more than 2 hours before a session, a smaller reduction of 10 to 20% is usually sufficient since most of the bolus will have cleared by the time you start moving.
The meal immediately after an aerobic session requires a meaningful bolus reduction. Your muscles are actively replenishing glycogen stores and will pull glucose from your bloodstream efficiently. A 30 to 35% reduction in your normal meal bolus is a reasonable starting point for a post-training meal.
Your CGM is your guide here. Check your trend arrow before eating. If you are flat or gently rising after a session, give a reduced bolus. If you are already dropping, eat first and give a very conservative bolus 20 minutes later.
Afternoon and evening aerobic sessions carry a specific risk of nocturnal hypoglycemia 6 to 15 hours later. Set a CGM alarm at 5.5 mmol/L before bed. If your pre-sleep glucose is below 8 mmol/L, eat a small protein and slow-carb snack before sleeping.
Resistance training — lifting weights, strength work, CrossFit — triggers a different hormonal response. The catecholamines (adrenaline and cortisol) released during intense effort cause the liver to release stored glucose, often raising blood glucose during and immediately after the session.
This acute rise is normal physiology and not something to correct aggressively during the session. It will typically self-resolve within 60 to 90 minutes of finishing.
Pre-session: a small reduction of 10% in your meal bolus is usually sufficient for resistance training. Unlike aerobic work, you do not need a large reduction because the acute glucose-raising effect of the session partially offsets the insulin-lowering effect of increased sensitivity.
Post-session: the story changes. Resistance training significantly increases insulin sensitivity for 24 to 48 hours after the session as your muscles repair and adapt. The post-training meal and every subsequent meal during that window will require less insulin than usual.
Both aerobic and resistance training increase insulin sensitivity well beyond the session itself. This is one of the most commonly overlooked factors in T1D glucose management.
If you train hard on Monday morning, your Tuesday boluses may still need to be lower than usual. If your glucose is running slightly low across the day after a training session, the session is the most likely explanation. Reduce your ICR slightly across the post-training period and monitor.
| Exercise type | During session | Pre-session meal | Post-session meal | 24h sensitivity |
|---|---|---|---|---|
| Aerobic (zone 2) | Glucose drops | Reduce 25 to 50% | Reduce 30 to 35% | Significantly elevated |
| Resistance/weights | Glucose rises | Reduce 10% | Reduce 15 to 20% | Significantly elevated |
| HIIT/intervals | Rises then drops | Reduce 15 to 25% | Reduce 25 to 30% | Significantly elevated |
| Short walk after meal | Glucose drops mildly | No change | No change | Mild improvement |
Before any session, check your CGM. If your glucose is below 5 mmol/L (90 mg/dL), treat with 15 to 20 grams of fast carbs and wait 15 minutes before starting. If your glucose is below 7 mmol/L (126 mg/dL) with a falling trend, take 15 grams of carbs and start cautiously. If you have significant IOB from a recent bolus, your risk of going low during aerobic exercise is substantially elevated.
Your CGM trend arrow matters more than your current number. Flat at 5.5 mmol/L is very different from falling fast at 5.5 mmol/L.
The numbers above are starting points. Your actual reductions will be personalised through data. Keep a simple log: session type, duration, pre-session glucose, bolus taken, post-session glucose, and how the following meals behaved. After 4 to 6 sessions of the same type, patterns will emerge that are specific to your physiology.
The goal is a protocol, not a formula. Once you know your aerobic reduction and your post-run meal behaviour, the decision becomes automatic rather than a calculation every time.
The Diabetic Wolf bolus calculator applies everything in this guide automatically. ICR by time of day, CGM trend adjustment, IOB correction, fat and protein split dosing, and exercise modifier.
Open the Bolus Calculator CGM Decision Tool