Guide series

Exercise and Type 1 Diabetes

Making exercise predictable enough to manage — without pretending it can ever be perfectly controlled.

Exercise Insulin on board CGM

What this guide is for

For people living with type 1 diabetes, exercise can feel uniquely destabilising. Glucose drops that arrive fast and hard. Spikes that appear late and linger. Sessions that look identical on paper but behave very differently in real life.

The common conclusion is that exercise is “unpredictable”. That conclusion is understandable — and usually wrong.

Exercise does not introduce randomness. It introduces fast, powerful physiology into a system otherwise governed by slow, blunt insulin. When you understand the dominant drivers of glucose change during exercise, patterns start to appear — and with them, a clearer picture.

This guide is built around heuristics to experiment from. They are grounded in the evidence base and clinical practice, but they require individual tailoring. They are not rigid rules. They are starting points that survive contact with CGM and real life.

How to use this guide

Read the hub page (this one) first to understand the major drivers
Then explore the exercise-type page that matches your activity
Use CGM data as your feedback loop — outcomes inform the next attempt
Discuss adjustments with your diabetes care team

Major in the majors

If exercise has felt chaotic, start here. Across almost all forms of exercise, glucose behaviour is dominated by a small number of variables.

In order of importance:

Insulin on board — especially recent bolus insulin
Starting glucose value
CGM trend arrows — direction and speed of change

When these are prioritised first, exercise stops being mysterious and starts becoming manageable. Most exercise-related hypoglycaemia and volatility tends to be predictable from these starting conditions.

The core problem exercise creates

Exercise is hard to manage in type 1 diabetes not because it is dangerous, but because it amplifies insulin action.

Muscle contraction increases glucose uptake. Blood flow accelerates insulin delivery. Counter-regulatory hormones may push glucose out of the liver. These processes act quickly.

Injected or pumped insulin does not.

Exercise hypos are usually insulin problems

Large glucose drops during exercise are rarely caused by exercise itself. They are almost always caused by supercharged insulin action — exercise increasing blood flow and glucose uptake, amplifying the effect of insulin that is already present. When significant bolus insulin is still active, glucose can fall rapidly. This is physiology doing exactly what it should.

Diagram showing how exercise increases blood flow and amplifies insulin action in type 1 diabetes

The 90-minute window

When exercise occurs 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 hypoglycaemia. The mechanism here is straightforward — exercise and peak bolus action overlap.

Chart showing why bolus reductions tend to be needed when exercising within 90 minutes of eating

The Three-Hour Rule

One heuristic simplifies exercise management more than almost any other: when the last bolus was at least three hours before exercise, glucose behaviour is often far more predictable.

This is not a guarantee — it is an organising principle that tends to reduce hypos, corrections, and mental load. Many people find it worth building their exercise timing around where possible.

Diagram of the Three-Hour Rule showing how insulin on board decreases over time before exercise

CGM trace showing improved glucose predictability during prolonged exercise when the Three-Hour Rule is observed

Carbohydrate decisions during exercise

Carbohydrate decisions during exercise work best when they are based on both glucose value and trend direction. Numbers without direction are incomplete information.

As a starting heuristic, many people check CGM before exercise and reassess at least every 30 minutes during activity, using value plus trend to inform carbohydrate intake.

Personalised carbohydrate calculators are available for each CGM device in the individual exercise-type pages below.

CGM and AID during exercise

Exercise is one of the times when CGM data becomes less reliable due to sensor lag and movement artefacts. This is a context where over-trusting a single CGM reading warrants extra caution.

AID systems do not remove hypoglycaemia risk during exercise. Insulin on board, starting glucose, and trend direction still dominate the outcome. In practice, many people find manual mode more predictable during exercise because it removes continuous background adjustments.

Where exercise type fits

The type of exercise does matter — but it comes after insulin on board, glucose value, and trend direction. Exercise type explains the direction of glucose change; the three majors determine the risk.

Overview of exercise types and their typical glucose response directions in type 1 diabetes

Key mechanisms by exercise type:

Aerobic (endurance) — tends to lower glucose
Anaerobic (sprinting & lifting) — often raises glucose
Mixed / team sports — can rise or fall depending on intensity and insulin on board

Guide parts

Part 1 — Aerobic Exercise

Understand why aerobic exercise tends to lower glucose, how the 90-minute window and Three-Hour Rule apply, and how to build starting plans for running, cycling, rowing, swimming, and similar activities.

Read Part 1: Aerobic Exercise

Part 2 — Mixed and Team Sports

Explore why mixed exercise can raise or lower glucose in the same session, and how insulin on board determines whether aerobic phases or anaerobic spikes dominate — covering football, hockey, netball, basketball, and interval training.

Read Part 2: Mixed and Team Sports

Part 3 — Anaerobic Exercise

Understand why high-intensity short-burst exercise often raises glucose, how stress hormones drive this response, and how to build starting plans for sprinting, weight training, and hard intervals.

Read Part 3: Anaerobic Exercise