The variable that runs the show. Almost nobody fully understands it.
The Insulin On Board Guide for T1D
DAFNE classroom, week five. The AIT slide goes up. Half the room set theirs to two hours. The other half are at four or five. Both groups think they are doing it right. Both are. One number is being asked to do two completely different jobs.
Ask Grace
Want to ask about IOB, AIT settings, or exercise risk in your own situation? Ask Grace.
The Glucose Never Lies® Explorers
The Explorers are designed to make insulin exposure visible and usable in real-world decision making. They are built for people who use shorter Active Insulin Time settings to optimise glycaemic control, but still want a more realistic understanding of what is happening physiologically. There are two complementary tools.
Exercise IOB Hypoglycaemia Risk Explorer
Enter your current glucose, recent bolus doses, and activity type. The Explorer estimates your exercise-related insulin exposure risk and a suggested carbohydrate intake for 30 minutes of activity. Designed to support safer exercise decisions when insulin exposure may not be fully reflected on the device screen.
Activity to Lower Hyperglycaemia Explorer
Using the same underlying principles, this Explorer estimates how much glucose is likely to fall with activity and how long it may take to reach Time in Range or tighter targets. Allows activity to be used intentionally as a tool to bring glucose down, rather than relying solely on correction insulin.
For education and discussion, not medical instruction. Always discuss changes with your diabetes care team.
TL;DR
The IOB number shown by devices is not the same as the physiological insulin exposure occurring in the body. Insulin on Board is the active insulin from your previous doses that the bolus calculator counts; the goal is to avoid stacking insulin and causing hypoglycaemia. The problem is that one number is currently being asked to do two different jobs:
- Correction safety: preventing too much insulin when correcting high glucose.
- Exercise safety: estimating how much insulin is still physiologically active when you start activity.
The duration mismatch. Insulin action in the body typically persists for 4 to 6 hours and can extend to 8 hours or longer in some individuals. Most bolus calculators use a shorter IOB duration, often 2 to 4 hours, so that corrections are not blocked. A deliberate design choice for correction behaviour, not an accurate reflection of how long insulin remains physiologically active.
Short versus long AIT: the trade-off
Short AIT (2 to 3 hours)
- Easier corrections
- More freedom to treat highs
- Less visibility of insulin still active during exercise
Long AIT (4 to 6 hours)
- Better visibility of insulin exposure
- Better exercise hypoglycaemia risk awareness
- Corrections may feel slower or more restricted
Time-based heuristics for exercise risk
If you use a shorter AIT duration, these approximate guides can help you think about exercise risk. These are rough indicators, not clinical rules. Individual insulin responses vary significantly.
- Less than 3 hours since last bolus: higher exercise hypoglycaemia risk
- 3 to 4 hours since last bolus: moderate insulin exposure
- More than 4 hours since last bolus: lower exercise hypoglycaemia risk
If exercise safety is a priority, setting AIT to around 4 to 6 hours gives a more realistic picture of circulating insulin. This may restrict correction doses, meaning extra insulin sometimes needs to be given manually, and that itself carries a hypoglycaemia risk. The GNL Exercise IOB Explorer is built for finer-grained planning.
The full guide is seven parts; work through them in sequence or jump around as your needs dictate.
How to use this guide
Ideally, work through the guide in sequence, as each section builds on the previous one. If you are already familiar with IOB concepts, feel free to jump between sections and explore the parts most relevant to you.
Hub: The Insulin On Board Guide for T1D
Overview, the Explorers, the TL;DR. This page.
Part 1: The Insulin On Board to Physiology Mismatch
Why the IOB number on your device is not the same as the insulin circulating in your body, and why that gap matters.
Part 2: Different Models For Calculating Insulin On Board
How different devices and calculators model IOB, and what the differences mean for real-world decision making.
Part 3: Choosing Device-Specific IOB Settings: What Are You Optimising For?
A framework for making deliberate choices about your AIT/DIA setting, depending on whether your priority is correction freedom or exercise safety.
Part 4: The Future of Calculating Insulin On Board
How combining correction behaviour modelling with exercise hypoglycaemia risk could change the way devices present insulin exposure.
Part 5: Exercise IOB Hypoglycaemia Risk Explorer
The GNL Exercise Insulin on Board Calculator for Type 1 Diabetes. Estimate exercise-related hypoglycaemia risk and carbohydrate strategy based on your current insulin exposure.
Part 6: Activity to Lower Hyperglycaemia Explorer
The GNL Physical Activity with IOB to Lower Hyperglycaemia Explorer. Estimate how short periods of activity may reduce glucose and help bring levels back into range.
Part 7: Recommended Reading and Resources
The research, references, and further reading that underpin this guide.
Related article: The IOB Trade-Off
How four AID systems make different bets on insulin, real-world data from 190,000+ users, the dose-dependent exercise risk table, and system-by-system guidance.
Acknowledgements
A special thank you to Simon Helleputte, MSc, PhD (Ghent University) and Joseph Henske, MD (University of Arkansas) for their generous feedback during the development of this guide.
Deep gratitude also to Professor Michael Riddell (York University, Canada) for his ongoing support, patience with very lengthy emails, and his ability to continually steer the discussion back to the most important question: “So what?” His perspective has been instrumental in helping refine the practical implications of insulin-on-board modelling and why it matters for people living with Type 1 diabetes.