The variable that runs the show — and almost nobody fully understands

The Insulin On Board Guide for T1D

A mechanism-first framework for understanding what insulin on board actually means biologically, why the number your device shows can be both useful and misleading, and why it matters most when you exercise.

Type 1 Diabetes
Insulin Action

What this guide is about

In Type 1 diabetes, insulin exposure is the dominant variable driving post-meal control, correction effectiveness, exercise response, and glucose volatility. If it is misunderstood, the whole system becomes harder to interpret. People start doing things that look strange from the outside, not because they are reckless, but because the system in front of them does not match what their body is actually doing.

This guide majors in the majors. It covers the handful of concepts that explain most of the chaos — not every edge case, not every pump feature, just the core physiology that makes the rest make sense.

The distinction at the centre of this guide

Actual insulin action in the body typically persists for 4 to 6 hours and can extend to 8 hours or longer in some individuals and contexts. The Active Insulin Time (AIT) or Duration of Insulin Action (DIA) setting on a device is rarely set this high, because it exists primarily to govern correction behaviour, not to represent physiological insulin duration. One number is being asked to do two completely different jobs. Understanding that tension is what this entire guide is built on.

This is written for well-informed people living with Type 1 diabetes and the clinicians who support them. It is deliberately more technical and conceptual than most IOB explanations, because insulin exposure is not a settings problem. It is a physiology problem: absorption, dose size, clearance, liver versus muscle signalling, and the way exercise amplifies insulin action.

Diagram illustrating the trade-off between short and long Active Insulin Time settings in Type 1 diabetes: correction freedom versus physiological insulin exposure visibility

This graphic and the TL;DR that follows are a taster of what the full guide offers. They cover the core concepts and the central trade-off that every person using insulin faces when choosing their AIT setting.

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.

Open the Explorer

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.

Open the Explorer

The principle behind both tools

Insulin exposure defines both the risk of going low and the ability to bring glucose down using activity. One Explorer focuses on safety. The other focuses on utility. Together, they show how insulin exposure can be used to support better decisions without changing insulin delivery.

Looking ahead, approaches like this could be integrated into diabetes devices, allowing people with Type 1 diabetes to benefit from both flexible correction behaviour and realistic visibility of insulin exposure.

These tools are for education and discussion, not as medical instruction. Always discuss changes to your approach with your diabetes care team.

TL;DR

The key idea of this guide is simple: the IOB number shown by devices is not the same as the physiological insulin exposure occurring in the body.

What is IOB?

Insulin on Board is the amount of insulin from previous doses that is still active in your body.
It reduces how much additional insulin a bolus calculator recommends.
The underlying idea is simple: avoid stacking insulin and causing hypoglycaemia.

One number is currently trying 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. This is a deliberate design choice for correction behaviour. It is not an accurate reflection of how long insulin remains physiologically active in the body.

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

The three IOB models you will see

No IOB deduction from corrections. Full corrections are always delivered. IOB was displayed but ignored by the calculator.
Correction IOB only. Only correction insulin blocks future corrections. Meal insulin does not.
Total IOB (meal plus correction). All insulin blocks further corrections. Most protective against stacking.

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 the AIT duration to around 4 to 6 hours gives a more realistic picture of circulating insulin. Be aware that this may restrict correction doses, meaning extra insulin may sometimes need to be given manually — and that itself carries a hypoglycaemia risk. If you prefer greater precision, the Glucose Never Lies® Exercise IOB Explorer is built for this.

The full guide covers

Why insulin action depends on units per kilogram
Why IOB models use simplified decay curves
How exercise dramatically amplifies insulin effects
Why future systems may separate correction safety from physiological insulin exposure

It is made up of seven parts and is intended to be worked through in sequence, though you are welcome to jump around as your needs dictate.

Four common IOB situations in daily life

Almost everyone living with Type 1 diabetes has experienced at least one version of the following situations. At the time, they feel random, frustrating, or unpredictable. By the end of this guide, you will view each of them differently and approach them more deliberately. Each example illustrates a different limitation of how current devices display IOB compared with what is actually happening physiologically.

Example 1 — “0.0 IOB” and still crashing during exercise

Your AIT/DIA is set to 2 hours. Two hours ago, you ate and delivered 10 units of insulin. Your glucose is 11.1 mmol/L (200 mg/dL) with a steady arrow. Your pump shows 0.0 units IOB. You exercise for 30 minutes. No carbohydrate — why would you? Within 15 minutes, your glucose falls rapidly and you are treating a significant hypo.

The conclusion many people draw is that exercise is unpredictable or dangerous. In reality, the physiology is not unpredictable. The device displaying “0.0 IOB” is substantially under-representing the insulin still circulating. Exercise simply amplified its effect.

Example 2 — High IOB blocks a correction

Your AIT/DIA is set at 5 hours. You take 10 units for 100 g of carbohydrate. Two hours later, your glucose is 13.9 mmol/L (250 mg/dL). Your correction factor is 1 unit per 2.0 mmol/L (36 mg/dL). You calculate that you need roughly 4 units to correct. The device says you still have 4.4 units IOB and offers no correction. You know you are still rising. So you override the calculator, ghost-carb, or inject using a pen. You may be told later that you should trust the technology.

You may think: Foxtrot Oscar.

What is often missed is that much of that 4.4 units is still covering carbohydrate digestion — roughly 44 g of carbohydrate is still to be absorbed. The IOB model does not distinguish between meal insulin and correction insulin. The result: the system may block a correction you expect physiologically, while providing a more realistic picture of insulin exposure for exercise risk. These are the two jobs in conflict.

Example 3 — Short AIT and repeated corrections

Your AIT/DIA is set to 2 hours. Two hours ago you ate 100 g of carbohydrate, gave 10 units for the meal, and added a 4-unit correction. Total insulin delivered: 14 units. Two hours later, glucose is 12.5 mmol/L (220 mg/dL) and the pump shows 0.0 IOB. The pump offers another 3-unit correction. You take it. Ninety minutes later, you are hypoglycaemic.

The device’s IOB model had reached zero. Your physiology had not. Roughly 2 units were still physiologically active. The system could not warn you about stacking because the model cannot separate meal insulin from correction insulin, and the displayed duration had already expired.

Over time, this creates a difficult situation. You may begin to stop trusting the calculator, or stop trusting your own instincts. Neither outcome serves you.

Example 4 — The same exercise gives three different results

You take the dog for the same 20-minute walk on three different mornings using an automated insulin delivery system. The walk is identical each day. No breakfast. No bolus.

Day 1, 0.0 units IOB: glucose remains stable.
Day 2, negative IOB: glucose rises during activity.
Day 3, 1.1 units IOB: glucose falls rapidly.

The behaviour is identical. The activity is identical. The glucose response is completely different. The natural conclusion is that exercise is unpredictable. The real driver is circulating insulin exposure at the start of activity. The displayed IOB does not fully represent the true physiological insulin exposure. Across all three scenarios, that is the same underlying issue.

The core trade-off, stated plainly

Shorter AIT/DIA improves correction freedom but increases the risk of multiple correction-induced hypos and hides exercise hypoglycaemia risk. Longer AIT/DIA better reflects insulin exposure but may block corrections, leading to sustained hyperglycaemia when carbohydrates are underestimated. Understanding this trade-off is essential. The graphic below illustrates it.

And that leads directly into the full guide.

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, and the four common situations. This page.

Read the hub

Part 1 — The Insulin On Board–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.

Read Part 1

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.

Read Part 2

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.

Read Part 3

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.

Read Part 4

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.

Open Part 5

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.

Open Part 6

Part 7 — Recommended Reading and Resources

The research, references, and further reading that underpin this guide.

Read Part 7

Acknowledgements

A special thank you to Simon Helleputte, MSc, PhD (Faculty of Medicine and Health Sciences, Ghent University) and Joseph Henske, MD, FACE (Director of the Diabetes Program, Professor of Medicine, Division of Endocrinology and Metabolism) for their generous feedback and thoughtful input 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.

Their insights have significantly strengthened the thinking presented here.