Core Guide
Exercise and Type 1 Diabetes
Making exercise predictable enough to manage – without pretending it can ever be perfectly controlled. From ten minutes after a meal to training for an endurance event: the same physiology, the same three variables.
TL;DR
- Insulin on board is the dominant driver of exercise hypo risk; the 90-minute window and Three-Hour Rule are how you manage it.
- Activity and exercise are not the same thing. Ten to twenty minutes of everyday movement after meals is a distinct, lower-risk lever for smoothing glucose that most people underuse.
- Three variables decide almost everything: insulin on board, starting glucose, trend arrows. Address these first before adjusting anything else.
- AID changes the levers but not the physics. Announcing exercise and pre-emptive reductions matter more, not less, on an AID system.
- Read the Activity Snacking guide if you want to understand how twenty minutes of moderate post-meal activity can blunt a postprandial rise without triggering a later hypo.
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.
Activity versus exercise
Not all movement requires a full plan
Exercise and everyday activity are not the same thing, and the distinction matters for how you manage glucose.
Activity is everyday movement: walking, light cycling, housework, gardening, playing with children. It is not training. The point is not fitness. The point is glucose leverage. Activity works best as an add-on to normal life – you do not need to be an exercise person to use it.
The contrast matters because the management demands are different:
- Activity: lower hypoglycaemia risk, easier to repeat, well suited to glucose smoothing and post-meal peaks.
- Exercise: higher potential benefit for fitness and performance, but typically requires more deliberate glucose management – before, during, and after.
Starting with activity snacking (ten to twenty minutes after the meal that produces your biggest spike) is often the most accessible entry point, particularly for families and children who find formal exercise management daunting.
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.
This shows the research-predicted order (which of these best predicts an exercise low). The list above shows what is worth acting on first day to day; insulin on board is what you can most directly change, even where glucose and trend are the stronger predictors.
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.
Why exercise hypos are usually insulin problems, not exercise 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.
The implication is practical: reducing IOB before exercise (via timing, bolus reduction, or basal adjustment on a pump) addresses the actual cause, not just the symptom.
The 90-minute window explained
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.
This is also where the activity-versus-exercise distinction matters: a ten-minute walk after a meal sits in this window deliberately and at low enough intensity that hypo risk is lower than with structured exercise.
The reduction protocols: 50/50/20 and T25/T25
Two population-average starting frameworks, one for injections or a pump, one for a hybrid closed loop. Both are teaching simplifications of the graded ISPAD/Rabasa-Lhoret reduction scale, not fixed doses; any insulin change is a care-team decision.
Working out carbs for 30 minutes of exercise, in simple terms
The same three majors (insulin on board, starting glucose, trend arrow) drive how many grams of carbohydrate a 30-minute session needs, plus body weight. Two verified worked examples from the GNL Carbs for 30 Minutes Exercise calculator, both starting glucose 6.5 mmol/L, steady trend, aerobic exercise, insulin on board from a bolus 90 minutes earlier:
- Adult, 70 kg, 7 units on board: around 30 g of carbohydrate (25 to 35 g range).
- Child, 40 kg, 4 units on board: around 17 g of carbohydrate (15 to 20 g range).
Two ceilings sit behind every figure the calculator returns. Body weight above 60 kg is capped at 60 kg in the calculation (the same population-average ceiling used across every GNL carb calculator); and the maximum carbohydrate figure for any 30-minute window is capped at 30 g, since the gut absorbs glucose at roughly 1 gram per minute regardless of how much is eaten. A lower starting glucose, a falling trend, or more insulin on board all push the figure up within that ceiling; a higher starting glucose or a rising trend pull it down.
The Three-Hour Rule: what it is and when to use it
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.
The honest trade: you can have exercise that is more predictable, or you can have exercise that fits life as it actually arrives. The Three-Hour Rule and the 90-minute window are levers, not gates. Most days you will choose flexibility; on the days the stakes are higher (long ride, race, mountain), the rule is there to lean on.
Activity snacking
Ten minutes after meals: its own guide now
Everyday post-meal movement, ten to twenty minutes after the meal that produces your biggest spike, is a distinct, lower-risk lever from structured exercise, and it has its own guide with the full detail: the 20 by 2 and 20 by 40 protocols, the fast-mover/slow-mover distinction, and what to look for on CGM.
Explorers and tools
Three GNL explorers are built directly on the evidence and algorithms in this guide.
- 10, 20, 30 Minutes Walking to Lower Highs, the activity-snacking tool that pairs with the Activity Snacking guide
- Carbs for 30 Minutes Exercise, the exercise IOB calculator behind the worked examples above
- Planning for Before, During and After Exercise – the full planning explorer that pulls all five Parts together
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.
Further listening and related content
- FAQ – Activity, Exercise and Type 1 Diabetes
- Podcast with Prof Othmar Moser on exercise and T1D
- Episode 33 – Exercise, hormones and T1D in females
- The Menstrual Cycle and Type 1 Diabetes, three-part guide: cycle physiology, AID across the cycle, contraception, PCOS, pregnancy, and menopause
- Foundations – Activity and Movement
- CGM and exercise: bolus reduction and carbohydrate timing
- Fast and slow movers, understanding your individual activity response
- Activity Snacking (20 by 2)
- Hypoglycaemia in T1D – understanding and preventing lows
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.
For competitive athletes, a bespoke service
If you (or your child) are training for an Ironman, marathon, rowing championship, cycling stage race or similar endurance event, and want a personalised plan that combines paediatric T1D clinical expertise, sports nutrition, pump-data interpretation and AI-augmented in-the-moment support, John runs a bespoke Athlete Performance service through Via Negativa Health. Free 60-minute sound-out call before any engagement.
Keep reading
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.