Overview
This page teaches you how to make safe starting plans for anaerobic (high-intensity) exercise — short bursts of all-out effort such as sprinting, weight lifting, field events, hard intervals (e.g. 400–1500m running), a hard spin class, or a hard 1000m row.
Anaerobic exercise often raises glucose during or shortly after the effort. The key point is that the size and direction of that response is still dominated by your starting conditions — especially insulin on board.
This page follows the GNL rule: major in the majors. Start with insulin on board, then starting glucose, then trend arrows. Anaerobic physiology comes after — because it explains the direction, not the risk.
The detail
A quick recap from the exercise introduction page: anaerobic exercise often raises glucose, but insulin on board and starting conditions still dominate what happens next.
Major 1: Insulin on board (the main driver)
Even in anaerobic exercise — where glucose often rises — insulin on board still matters most. Too much insulin on board can turn a “typical rise” into a flat response or a crash later. Too little can make the rise larger and longer.
It is best to obey the Three-hour rule to make sure it’s the exercise and not another variable causing glucose issues.
Major 2: Starting glucose value
Your starting glucose matters because anaerobic exercise often pushes glucose up. Starting higher may mean you tolerate less of a rise; starting lower may mean you can allow the session without corrective insulin. This is why pre-exercise checks matter.
Major 3: Trend arrows (direction and speed)
Carbohydrate decisions during anaerobic exercise still work best when they are based on both glucose value and the trend arrow. Numbers without direction are incomplete information.
As a starting heuristic, check the CGM before exercise and then re-check regularly during activity (many people use 20–30 minute intervals). Use value + trend to decide whether carbohydrate is needed.
Why does glucose usually rise during all-out exercise?
Why does glucose usually rise during all-out exercise?
After exercise: the delayed risk
Anaerobic activities can cause muscle and liver glucose depletion if repeated over 45 minutes or more (for example, a hard HIIT session). Therefore, the same risk of going low later applies.
Practical
The table below gives a starting plan and two adjustment rows. Always start with the “Starting plan”. If glucose runs high the first time, use the “Went high first time” row next time. If glucose runs low the first time, use the “Went low first time” row next time.
The Glucose Never Lies — use the outcome as feedback, then adjust.
Generic worked example (template): Meal at midday, intense session late afternoon for 45 minutes, then evening meal afterwards. Build a plan using the table and review the outcome.
- Before exercise: follow the table starting plan for bolus and/or basal based on your timing and insulin on board
- During exercise: check CGM before and every 20–30 minutes; use glucose value + trend arrow to guide carbohydrate intake using the chart for body weight
- After exercise: be alert for delayed hypos after repeated high-intensity work over 45 minutes; consider whether bolus and/or basal needs reducing
- Review: repeat the “Starting plan” if it worked; if glucose ran high or low, use the corresponding adjustment row next time
If glucose consistently increases during the session (for example, rising from 7.5 mmol/L to 12.0 mmol/L), a small bolus before the same session next time may be needed.
Here is the rule for giving small boluses of fast-acting insulin before exercise that consistently increases glucose:
Start low and go slow
This requires safe trial and error.
Personalised PDF calculators (choose the device you use):
- Dexcom G6 – Exercise Carbohydrate Calculator
- Libre 1 & 2 – Exercise Carbohydrate Calculator
- Medtronic – Exercise Carbohydrate Calculator
- Dexcom G6 with pump – Exercise Carbohydrate Calculator
Here are all the carb charts that cover from 10–60+ kg and both mmol/L and mg/dL (just in case you cannot use PDFs):
Carbohydrate charts for 20 minutes of exercise with a mmol/L CGM device
Carbohydrate charts for 20 minutes of exercise with a mg/dL CGM device
What’s next
Next step: Mixed / Team Sports Exercise
- Aerobic (endurance) exercise
- Anaerobic (sprinting & lifting) exercise
- Mixed / team sports
- AID systems and exercise
- Exercise carbohydrate calculators
- FAQ: Exercise and Type 1 diabetes
- Podcast episode: Prof. Othmar Moser on exercise and Type 1 diabetes
This is a video I shot for a Masters module doing a 1000m row and explaining what’s happening to glucose and lactate. It gets a bit deep so feel free to skip.
