Overview
This page teaches you how to make reasonable starting plans for aerobic (endurance) exercise — exercise at a consistent pace such as running, cycling, fast walking, swimming, rowing, and stepping.
Aerobic exercise usually lowers glucose in type 1 diabetes. The key point is that the size and speed of that drop is 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. Aerobic physiology comes after — because it explains the direction, not the risk.
The detail
A quick recap from the exercise introduction page: aerobic exercise usually drops glucose, and that drop is usually an insulin-on-board issue.
Major 1: Insulin on board (the main driver)
Most large drops during aerobic exercise are not caused by the exercise itself. They are caused by aerobic exercise supercharging insulin action when bolus insulin is still active.
If you exercise soon after eating and bolusing, the same session can behave very differently — because insulin on board is different.
The importance of planning the meal before exercise.
The 90-minute window: if exercise happens within roughly 90 minutes of a meal bolus, insulin action is often near its peak. In this window, bolus reduction is commonly required to avoid predictable hypos.
If possible, observe the Three-hour rule.
The Three-Hour Rule is a powerful organising heuristic: when the last bolus was at least three hours before aerobic exercise, glucose behaviour is often far more predictable. It’s not a guarantee — it’s a reliable starting point.
Major 2: Starting glucose value
Your starting glucose matters because aerobic exercise usually pulls glucose down. Starting higher may buy time; starting lower reduces your buffer. This is why pre-exercise checks matter.
Major 3: Trend arrows (direction and speed)
Carbohydrate decisions during aerobic exercise work best when they are based on both the 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 drop during aerobic exercise?
Aerobic exercise opens an alternative “side door” into muscle cells, allowing glucose uptake with less reliance on insulin.
This is powerful — and it becomes risky when bolus insulin is still active. That’s why the first part of this page focuses on insulin on board and timing.
After exercise: the delayed risk
Be careful after aerobic exercise if you have been at it for 45 minutes or longer. Hypoglycaemia risk can remain elevated for hours afterwards.
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 08:00, exercise at 09:00 for 60 minutes, then lunch afterwards. Build a plan using the table and review the outcome:
- Before exercise: reduce meal bolus (start with a sensible reduction based on timing and past experience)
- 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: consider whether the next bolus needs reducing; be alert for delayed hypos after longer sessions
- Review: repeat the “Starting plan” if it worked; if glucose ran high or low, use the corresponding adjustment row next time
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 the carb charts that cover 10–60+ kg and both mmol/L and mg/dL (if you cannot use the 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: Anaerobic (Sprinting & Lifting) 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 an optional deep dive video showing how to build plans and then apply them during a 40-minute rowing session. Skip it if you just want the practical steps.
