Guide series · Five parts

Exercise and Type 1 Diabetes

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

Exercise Insulin on board CGM and AID

GNL Grace

Not sure where to start? Tell Grace what kind of exercise is giving you trouble and she will point you to the right Part.

What this guide is for

Exercise is not random. It is fast physiology meeting slow insulin.

For people living with type 1 diabetes, exercise can feel uniquely destabilising – drops that arrive fast and hard, spikes that 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, patterns appear – and with them, a clearer picture.

This guide is built around heuristics to experiment from. Grounded in the evidence base (EASD/ISPAD 2025 consensus, ISPAD exercise chapter, GNL’s own causal modelling), tailored by individual response. They are not rigid rules. They are starting points that survive contact with CGM and real life.

Major in the majors

Three variables decide almost everything

If exercise has felt chaotic, start here. Across almost all forms of exercise, glucose behaviour is dominated by three variables, in order of importance.

1 · Most important

Insulin on board

Recent bolus insulin is the dominant driver of exercise hypo risk.

2

Starting glucose

Where you start shapes where you land, especially for aerobic work.

3

Trend arrows

Direction and speed of change. Numbers without direction are incomplete.

When these are prioritised first, exercise stops being mysterious and starts becoming manageable. Most exercise-related hypoglycaemia and volatility is 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. This is physiology doing exactly what it should.

Diagram showing how exercise increases blood flow and amplifies insulin action in type 1 diabetes - The Glucose Never Lies®

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. Exercise and peak bolus action overlap.

Chart showing why bolus reductions tend to be needed when exercising within 90 minutes of eating - The Glucose Never Lies®

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.

Not a guarantee. An organising principle that tends to reduce hypos, corrections, and mental load. Worth building exercise timing around where possible.

Diagram of the Three-Hour Rule showing how insulin on board decreases over time before exercise - The Glucose Never Lies®

What GNL research shows

The 20-minute paradigm

Pemberton et al (2024, 2025) identified that twenty minutes of moderate activity, timed into the post-meal window, is often enough to blunt a postprandial rise without triggering a later hypo – provided insulin on board and trend direction are accounted for. This reframes activity from an all-or-nothing risk to a precision tool. Developed further in Pemberton & Russon (2025) causal modelling work. Covered in depth in Part 3.

The five parts

Read in order for the full picture, or jump to the Part closest to your question. Every Part opens with the same “major in the majors” framing, so you can skip between them without losing your bearings.

Part 1

How exercise moves glucose

Mechanism, exercise types, the three majors, the 90-minute window, the Three-Hour Rule. The physiology that sits underneath every other Part.

Read Part 1

Part 2

The two levers – insulin and carbs

Bolus reductions (the 25/50/75% framework), carb timing and amounts, fasted versus fed, pump disconnection, and the evidence from Rabasa-Lhoret, Moser, West, Campbell, and Riddle.

Read Part 2

Part 3

Activity between meals – the 20-minute paradigm

John’s research on postprandial activity: when, why, and how twenty minutes can shift a day. Pre-meal versus post-meal walking. Sitting breaks. The evidence for activity snacking in both children and adults.

Read Part 3

Part 4

AID and exercise

What AID can and cannot do during exercise. Announce strategies, 30-minute activity features, manual mode trade-offs, pre-emptive carbs and reductions. Based on the Moser/Zaharieva EASD/ISPAD 2025 consensus.

Read Part 4

Part 5

Mastering Exercise – Top 10

Ten tactics that survive contact with real-world exercise and real CGM data. CGM accuracy during exercise, sex differences, fear of hypo, altitude, barriers and facilitators, T1DeXi and Syno/MIMIC insights.

Read Part 5

Explorers and tools

Three GNL explorers are built directly on the evidence and algorithms in this guide.

Survive and Thrive – Exercise resources

Three one-page A4 resources built for the first weeks of getting exercise predictable. Pick the one that matches your therapy.

AID PDF Pump PDF MDI PDF

Further listening and related content

Pairs with the CGM and AID guides

Exercise, CGM, and AID are one decision system. If you are planning for all three, start with the guide you have not yet read.

CGM Guide AID Guide

This guide is educational. It describes average responses and general principles. It is not medical advice and cannot replace individual clinical guidance from your diabetes care team.