The key concept in this section is that device‑displayed insulin on board is a mathematical estimate, while physiological insulin exposure reflects the actual biological insulin still influencing tissues.
Rapid-acting insulin analogues (aspart, lispro, glulisine, faster aspart, ultra-rapid lispro) do not have a single fixed duration.
In broad terms, they:
- begin working within ~10–20 minutes
- often peak around ~60–120 minutes
- continue exerting biological effect for ~4–8 hours
But here’s the point most IOB explanations miss:
A small bolus behaves differently from a large bolus. The “tail” stretches as the dose increases. This matters clinically because the boluses that create the most problems (post-meal highs, exercise hypos, correction stacking) are often the larger ones.
One of the clearest ways to think about insulin dose size is units per kilogram (U/kg), which normalises dose relative to body weight and better reflects the physiological exposure created by a bolus.
You might assume this does not apply to the newer ultra-rapid insulins such as Lyumjev. However, the prescribing information itself shows otherwise (thanks to Joseph Henske, Professor of Medicine, University of Arkansas, for highlighting this). Pharmacodynamic studies of ultra‑rapid lispro demonstrate faster early insulin exposure, but still show a prolonged tail of biological activity, particularly at larger doses. Faster onset therefore does not eliminate the dose‑dependent extension of insulin exposure.
The table below shows how the same rapid-acting insulin dose expressed as units per kilogram (U/kg) translates into actual units across different body weights — alongside a conceptual estimate of how long meaningful biological activity may persist.
These durations are rough estimates, not fixed pharmacokinetic limits. Real insulin action varies between individuals and is influenced by injection site, insulin sensitivity, activity, and stacking. The purpose here is to help visualise and personalise a key principle:
As insulin dose per kilogram increases, both peak timing and the duration of the insulin tail tend to extend.
| Body Weight | 0.05 U/kg | Peak / Tail | 0.1 U/kg | Peak / Tail | 0.2 U/kg | Peak / Tail | 0.3 U/kg | Peak / Tail |
|---|---|---|---|---|---|---|---|---|
| 25 kg | 1.25 U | 1 hr / 4 hr | 2.5 U | 1.5 hr / 6 hr | 5 U | 2 hr / 7 hr | 7.5 U | 2.5 hr / 8 hr |
| 50 kg | 2.5 U | 1 hr / 4 hr | 5 U | 1.5 hr / 6 hr | 10 U | 2 hr / 7 hr | 15 U | 2.5 hr / 8 hr |
| 75 kg | 3.75 U | 1 hr / 4 hr | 7.5 U | 1.5 hr / 6 hr | 15 U | 2 hr / 7 hr | 22.5 U | 2.5 hr / 8 hr |
| 100 kg | 5 U | 1 hr / 4 hr | 10 U | 1.5 hr / 6 hr | 20 U | 2 hr / 7 hr | 30 U | 2.5 hr / 8 hr |
Peak action is not disappearance
A common mental model is: “insulin does its main work after 2 hours.”
Peak action does not represent the end of insulin activity. Reduced effect does not mean no effect.
This matters because a lot of T1D decision-making happens precisely in the window where insulin is fading but still meaningful:
- the 2–4 hour window after a meal
- the “I’m still high, so I’ll correct again” window
- the “I’ve got time for a quick workout” window
If you treat this window as “insulin-free” because a display says 0.0, you will repeatedly be surprised by outcomes that are actually predictable once you understand exposure.
Circulating insulin is what drives tissue effects
Physiologically, what matters is insulin exposure to tissues — muscle, fat, liver — which is driven by absorption into circulation and clearance over time.
That’s why two people can take the same dose and experience different outcomes:
- absorption variability (site, temperature, lipohypertrophy)
- clearance differences (especially the kidneys)
- changes in sensitivity (sleep, stress, hormones, recent activity)
Device IOB is a mathematical estimate. Your physiology is a dynamic biological system.
Portal vs peripheral: the gradient is reversed in T1D
In people without diabetes, insulin is secreted into the portal vein and first passes through the liver. The liver sees high insulin concentrations; peripheral tissues (muscle and fat) are exposed to lower concentrations.
In T1D, insulin is delivered into subcutaneous tissue and enters the peripheral circulation first. That reverses the natural gradient.
This point is crucial for T1D management and deserves its own graphic and table (thanks again to Joseph H).
For those who prefer words!
| Physiological | Without Type 1 Diabetes | With Type 1 Diabetes | Practical Consequences | Implications for Exercise, Fat Storage, and Glucose Control |
|---|---|---|---|---|
| 1. Where insulin is delivered | Insulin is secreted directly from the pancreas into the portal vein, reaching the liver first. | Insulin is delivered into the subcutaneous tissue via injection or pump and then slowly absorbed into the systemic bloodstream. | The liver receives insulin immediately in people without diabetes, but much later in T1D. | Exercise risk depends more on circulating systemic insulin in T1D because insulin is already in the bloodstream rather than concentrated in the liver first. |
| 2. Initial handling of meal glucose | The liver captures and stores a large proportion of incoming glucose from the gut. | Because portal insulin is low, the liver captures less of the incoming glucose load. | More glucose escapes into the systemic circulation after meals. | Higher circulating glucose combined with circulating insulin increases the likelihood of glucose swings during activity. |
| 3. Glucose entering the bloodstream | Only a relatively small amount of glucose leaves the liver and enters the bloodstream. | A larger portion of glucose reaches the bloodstream quickly, producing the familiar post-meal spike. | Even with a perfect carbohydrate ratio, glucose may rise temporarily after eating. | Higher glucose exposure before exercise can create unpredictable behaviour depending on insulin exposure (IOB). |
| 4. Systemic insulin levels | Most insulin acts locally in the liver; systemic circulating insulin levels remain relatively low. | Insulin circulates widely in the bloodstream because it must travel from the subcutaneous tissue to all organs. | Peripheral tissues (muscle and fat) see higher insulin exposure. | Higher systemic insulin increases exercise-related hypoglycaemia risk when activity occurs within several hours of a bolus. |
| 5. Glucose storage in muscle and fat | Glucose storage in muscle and adipose tissue is modest because hepatic buffering controls glucose entry into the bloodstream. | Larger amounts of glucose are stored in muscle and fat due to higher circulating insulin and glucose. | Greater metabolic exposure to insulin in peripheral tissues. | Over time, this can increase fat storage risk and also amplify exercise glucose drops because muscle uptake is already insulin-stimulated. |
Simply,
- The liver is under-insulinised, so glucose output is not suppressed, especially post-meal and during stress.
- Muscle and fat is over-insulinised, and when combined with activity, insulin exposure can feel “way too strong”, even 3-4 hours after the insulin was administered.
This reversal of insulin exposure between the portal vein (liver) and peripheral tissues (muscle and fat) sits at the heart of the IOB problem. In T1D, insulin is delivered in the opposite direction to how human physiology was designed to use it.
This physiological exposure is precisely why exercise responses in Type 1 diabetes depend strongly on the insulin exposure present at the start of activity
Exercise “supercharges” insulin exposure
Exercise is often taught as if it simply “uses glucose”. In T1D, it’s more accurate to think of exercise as a multiplier on insulin effects.
During physical activity, several things happen at once:
- Muscle blood flow increases (faster delivery of insulin and glucose to working tissue).
- GLUT4 transporters increase at the muscle surface (insulin-dependent and insulin-independent pathways).
- Insulin sensitivity rises (sometimes sharply).
- Insulin clearance by the kidneys is reduced, making more insulin available to the muscle cells (insulin becomes stronger!)
This is where the difference between displayed IOB and physiological insulin exposure becomes important. A device may display little or no IOB while circulating insulin remains sufficient to amplify exercise‑driven glucose uptake.
This is why two exercise sessions that look identical on paper can behave completely differently:
- Morning fasted activity with no bolus insulin often leads to the glucose drifting up
- The same activity in the afternoon 2–4 hours after a bolus can drop the glucose fast
The difference is not the exercise itself — and it is not only morning cortisol. The key variable is the insulin exposure entering the session, shaped by the cumulative bolus and correction doses earlier in the day.
This is why substantial bolus reductions (25-75%) are typically required when exercising within two hours of a meal bolus — the insulin exposure is still near peak.
A familiar scenario: 0.0 IOB… and still dropping
Consider this sequence:
- 12:00 — lunch bolus
- 14:00 — device displays 0.0 IOB
- 14:15 — exercise begins
- 14:35 — glucose falls faster than expected
Many people interpret this as: “exercise is unpredictable”.
Often it’s the opposite. The physiology is predictable. The display is not necessarily aligned to the physiology — especially when device settings are optimised for corrections rather than insulin exposure.
Biology says:
Rapid‑acting insulin commonly exerts meaningful biological effects for approximately 4–6 hours, with duration influenced by dose size and individual physiology.
Most device models offer an AIT/DIA of ~2–8 hours, often defaulting to ~3–4, and most diabetes teams and individuals shorten 2 hours.
Why the mismatch?
Because devices are not modelling pharmacology alone. They are balancing safety and correction behaviour, using a simplified model that often lumps meal and correction insulin together.
Key Principles
- Insulin duration depends on dose per kilogram
- Insulin still has meaningful biological effect well after peak
- Device IOB displays may not represent this exposure
“If insulin behaves this way biologically, the next question becomes: how do diabetes devices attempt to model it?”
Part 2 will explain the different mathematical models for calculating IOB, why it often under-represents physiological exposure, and why that under-representation is frequently done on purpose.
The IOB Guide for T1D
- Hub: The Insulin On Board Guide for T1D
- Part 1 – The Insulin On Board–Physiology Mismatch
- Part 2 – Different Models For Calculating Insulin On Board
- Part 3 – Choosing a Device-Specific Insulin On Board Settings: What Are You Optimising For?
- Part 4 – The Future of calculating Insulin On Board: combining correction behaviour and exercise hypoglycaemia risk
- Part 5 – GNL Exercise Insulin on Board Calculator for T1D
- Part 6 – Reccomended Reading and Resources
