Insulin on Board and T1D: Understanding Its Real Meaning

In T1D, insulin exposure is the dominant variable driving post-meal control, correction effectiveness, exercise response, and glucose volatility.

If insulin exposure is misunderstood, the whole system becomes harder to interpret. People start doing “weird” things — not because they’re reckless, but because the system in front of them doesn’t match what their body is doing.

Major in the majors: this guide is about 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. It’s a practical, mechanism-first framework for understanding what “insulin on board” actually means biologically — and why the number your device shows can be both useful and misleading.

This graphic and the 90‑second TL;DR that follows are a taster of what the full guide offers and explain most of the core concepts covered in the guide.

Exercise IOB Calculator

The GNL Exercise Insulin on Board Calculator for Type 1 Diabetes is designed for people who choose shorter Active Insulin Time (AIT) settings to optimise glycaemic control but still want a more realistic understanding of exercise-related hypoglycaemia risk.

All you need to enter is:

your current glucose
recent bolus insulin doses
the type of activity you are about to perform (aerobic, mixed, or anaerobic)

The calculator then estimates:

your exercise-related insulin exposure risk
a suggested carbohydrate intake for 30 minutes of activity to help prevent hypoglycaemia

It is designed as a practical guide to support safer exercise decisions when insulin exposure may not be fully reflected on the device screen.

Hopefully, approaches like this will become integrated into diabetes devices in the future, allowing people with Type 1 diabetes to benefit from both flexible correction behaviour and realistic visibility of insulin exposure.

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 (IOB) 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 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 active when you start activity.

Insulin action in the body typically lasts 4–6 hours (sometimes longer). But most bolus calculators use a shorter IOB duration (often 2–4 hours) so corrections are not blocked.

Short IOB (2–3 hrs)
- Easier corrections
- More freedom to treat highs
- Less visibility of insulin still active during exercise
Long IOB (4–6 hrs)
- Better visibility of insulin exposure
- Better exercise hypo risk awareness
- Corrections may feel slower or restricted

The Three IOB Models You Will See

1. No IOB deduction from corrections

Full corrections are always delivered
IOB was displayed but ignored by the calculator

2. Correction: IOB only

Only correction insulin blocks future corrections
Meal insulin does not

3. Total IOB (meal + correction)

All insulin blocks further corrections
Most protective against stacking

If you choose a shorter IOB duration, it helps to think about exercise risk using simple time‑based heuristics.

<3 hours since last bolus: High exercise hypo risk
3–4 hours: Moderate insulin exposure.
>4 hours: Low exercise hypo risk

If exercise safety is the priority, set the IOB duration to around 4–6 hours. Be aware that this may restrict correction doses, meaning extra insulin may sometimes need to be given manually, which increases the hypoglycaemia risk.

If you prefer greater precision, you can use the Glucose Never Lies® Exercise IOB Calculator for Type 1 Diabetes.

That is the short version. If you want to go deeper, the full 5-part guide explains:

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

The full guide is written for well-informed people living with T1D and clinicians. It’s deliberately more technical and conceptual than most IOB explanations, because insulin exposure is not a settings problem — it’s a physiology problem: absorption, dose size, clearance, liver vs muscle signalling, and the way exercise amplifies insulin action. It’s made up of four parts and is intended to be worked through in sequence, but feel free to jump around as you need.

If the concept of insulin exposure isn’t clear, the following situations often appear confusing or contradictory.

Four Common IOB Situations in Daily Life

Almost everyone living with Type 1 diabetes (T1D) 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 these situations 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 Active Insulin Time (AIT) / Duration of Insulin Action (DIA) is set to 2 hours. You have been told that shorter settings help achieve tighter glucose control.

• 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, obviously, why would you?

Within 15 minutes, your glucose falls rapidly, and you are treating a significant hypo.

The conclusion many people draw is:

• Exercise is unpredictable
• Exercise is dangerous

In reality, the physiology is not unpredictable. The device display saying “0.0 IOB” is massively under-representing the insulin still circulating — and exercise simply amplified its effect.

Example 2 – High IOB blocks a correction

Your AIT/DIA is set at 5 hours, either because you were advised that this reflects real insulin duration or because the device uses a longer default model.

• 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 lowers glucose by 2.0 mmol/L (36 mg/dL)

You calculate that you need roughly 4 units to correct. But when you attempt a correction:

• The device says you still have 4.4 units IOB
• It offers no correction

You know you are still rising. So you:

• Override the calculator
• Ghost-carb
• Or inject using a pen

Your glucose returns to target. Later, you may be told that you should “trust the technology.”

You may think “Foxtrot Oscar!”

What is often missed is this:

• 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
• But it does provide a more realistic picture of insulin exposure for exercise risk

Example 3 – Short AIT and repeated corrections

Your AIT/DIA is set to 2 hours to allow more aggressive corrections.

Two hours ago:

• You ate 100 g of carbohydrate
• You gave 10 units for the meal
• Your glucose was 13.9 mmol/L (250 mg/dL)
• You added a 4-unit correction

Total insulin delivered: 14 units

Two hours later:

• Glucose is 12.5 mmol/L (220 mg/dL)
• Your pump now shows 0.0 IOB

The pump offers another 3-unit correction. You take it. Ninety minutes later, you are hypoglycaemic. What happened?

• The device’s IOB model had reached zero
• Your physiology had not

The earlier correction of insulin had not fully cleared.

• Roughly 2 units were still active

The system could not warn you about stacking because:

• The model cannot separate meal insulin from correction insulin

Over time, this creates a difficult situation. You may begin to:

• Stop trusting the calculator
• Or stop trusting your own instincts

Example 4 – The same exercise gives three different results

You wake up and take the dog for the same 20-minute walk on three different mornings. The walk is identical each day, and you are using an AID/HCL system. But the outcomes are completely different.

Day 1: 0.0 units IOB

• You wake up and go for a walk.
• Glucose remains stable.

Day 2: −1.0 units IOB (negative IOB)

• You do the same walk.
• Glucose rises during activity.

Day 3: 1.1 units IOB

• You do the same walk again.
• Glucose falls rapidly.

From the person’s perspective:

• The behaviour is identical
• The activity is identical
• Yet the glucose response is completely different despite not eating breakfast or delivering a bolus.

The natural conclusion is that exercise is unpredictable, but the real driver is not the exercise. The key variable is circulating insulin exposure at the start of activity. Across these scenarios, the same underlying issue appears. The displayed IOB does not fully represent the true physiological insulin exposure.

This has two important consequences:

• Shorter AIT/DIA improves correction freedom but increases the risk of multiple correction-induced hypos and hides exercise hypo risk
• Longer AIT/DIA better reflects insulin exposure but may block corrections, leading to sustained hyperglycemia when carbohydrates are underestimated

Understanding this trade‑off is essential, and the graphic below illustrates it.

And that leads directly into the full guide.

But first.

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.

I am also deeply grateful to Professor Michael Riddell (York University, Canada) for his ongoing support, patience with my 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.

How to use this guide

Ideally, work through the guide in sequence, as each section builds on the previous one.

However, if you are already familiar with IOB concepts, feel free to jump between sections and explore the parts most relevant to you.

Insulin on Board (IOB) Guide for T1D

TL;DR

How to use this guide

The IOB Guide for T1D