Exercise Guide — Part 3
Anaerobic Exercise and Type 1 Diabetes
Anaerobic high-intensity exercise often raises glucose — and insulin on board still determines the size and direction of the response.
Overview
This page explores how to build reasonable starting plans for anaerobic (high-intensity) exercise — short bursts of all-out effort such as sprinting, weight lifting, field events, hard intervals (for example 400–1500m running), a hard spin class, or a hard 1000m row.
Anaerobic exercise often raises glucose during or shortly after the effort. The mechanism here is that the size and direction of that response is still dominated by starting conditions — especially insulin on board.
This page follows the GNL approach: 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.
A quick recap from the Exercise Guide introduction: 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 delayed drop. Too little can make the rise larger and longer.
Many people find that observing the Three-Hour Rule makes it easier to identify what the exercise itself is doing, because it removes bolus insulin as a confounding variable.
Major 2: Starting glucose value
Starting glucose matters because anaerobic exercise often pushes glucose up. Starting higher may mean there is less tolerance for a rise; starting lower may mean the session can proceed without a corrective concern. This is why a pre-exercise CGM check tends to be informative.
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, many people check the CGM before exercise and re-check at regular intervals during activity — commonly every 20–30 minutes. Combining glucose value with trend arrow helps inform whether carbohydrate is relevant.
Why does glucose tend to rise during all-out exercise?
During anaerobic or very intense exercise, the body’s stress response releases catecholamines and other counter-regulatory hormones. These signal the liver to release glucose into the bloodstream rapidly — often faster than insulin can counteract it.
This is the mechanism that tends to raise glucose during sprinting, heavy lifting, and hard intervals — even when the session is relatively brief.
After exercise: the delayed risk
Anaerobic activities can deplete muscle and liver glycogen when repeated over 45 minutes or more — for example during a hard HIIT session. In this case, the same delayed hypoglycaemia risk that applies after aerobic exercise can occur in the hours following the session. This is worth keeping in mind when planning the next meal or bolus.
Building a starting plan
The algorithm below gives a starting plan and two adjustment rows. The approach is to begin with the “Starting plan”, then use the outcome as data. If glucose ran high the first time, use the “Went high” row next time. If glucose ran low, use the “Went low” row.
The glucose never lies — use the outcome as feedback, then adjust.
Generic worked example
Meal at midday, intense session late afternoon for 45 minutes, then evening meal afterwards. Building a plan using the algorithm and reviewing the outcome:
- Before exercise: follow the starting plan row for bolus and/or basal based on timing and insulin on board
- During exercise: check CGM before and every 20–30 minutes; use glucose value plus trend arrow to inform carbohydrate intake using the chart for body weight
- After exercise: be alert for delayed hypoglycaemia after repeated high-intensity work over 45 minutes; consider whether bolus and/or basal warrants reducing
- Review: repeat the starting plan if it worked; if glucose ran high or low, use the corresponding adjustment row next time
When glucose consistently rises during the session
Many people find that when glucose rises consistently during anaerobic sessions — for example moving from 7.5 to 12.0 mmol/L — a small bolus of fast-acting insulin before the same session next time may be worth exploring. The principle here is:
Start low and go slow.
This requires careful, step-by-step exploration, ideally with your diabetes care team. The aim is to find the smallest amount that prevents the rise without creating a risk of hypoglycaemia during the session.
Carbohydrate charts and calculators
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
Carbohydrate charts covering body weights from 10–60+ kg — available in both mmol/L and mg/dL:
mmol/L device — carbohydrate charts for 20 minutes of anaerobic exercise
mg/dL device — carbohydrate charts for 20 minutes of anaerobic exercise
Optional deep-dive videos
The first video shows a 1000m row with a live commentary on glucose and lactate responses. The second provides additional context on anaerobic physiology. Both go into more depth — skip them if you just want the practical steps above.
Watch: 1000m row — glucose and lactate during anaerobic exercise (YouTube)
Watch: additional anaerobic exercise physiology and type 1 diabetes (YouTube)
Key principles for anaerobic exercise
- Anaerobic exercise tends to raise glucose — stress hormones drive hepatic glucose release during high-intensity effort
- Insulin on board still dominates: too much can turn a typical rise into a crash; too little can extend the rise
- The Three-Hour Rule helps isolate what the exercise itself is doing by reducing bolus insulin as a variable
- Delayed hypoglycaemia risk can occur after longer high-intensity sessions as glycogen stores are replenished
- When glucose consistently rises, exploring a small pre-exercise bolus on the next occasion — starting low and going slow — is worth discussing with your care team
- Use CGM outcomes as feedback to refine the next plan — the glucose never lies
This content is for educational exploration only. It describes average responses and general principles. It is not medical advice and cannot replace individual clinical guidance from your diabetes care team.
