Exercise Guide — Part 1

Aerobic Exercise and Type 1 Diabetes

Aerobic exercise tends to lower glucose — and the size of that drop is dominated by insulin on board, not the exercise itself.

Overview

This page explores how to build reasonable starting plans for aerobic (endurance) exercise — activities at a consistent pace such as running, cycling, fast walking, swimming, rowing, and stepping.

Aerobic exercise usually lowers glucose in type 1 diabetes. The mechanism here is that the size and speed of that drop is dominated by starting conditions — especially insulin on board.

This page follows the GNL approach: major in the majors. Start with insulin on board, then starting glucose, then trend arrows. Aerobic physiology comes after — because it explains the direction, not the risk.

A quick recap from the Exercise Guide introduction: aerobic exercise usually drops glucose, and that drop is usually an insulin-on-board issue.

Major 1: Insulin on board — the main driver

Most large drops during aerobic exercise are not caused by the exercise itself. They are caused by aerobic exercise supercharging insulin action when bolus insulin is still active.

The same session can behave very differently depending on how much bolus insulin remains active. This is why timing relative to the last meal tends to matter more than the activity itself.

The 90-minute window

When exercise happens 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.

Chart illustrating why bolus reductions tend to be needed when aerobic exercise falls within 90 minutes of a meal

The Three-Hour Rule

One heuristic simplifies aerobic 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. It is not a guarantee — it is a reliable starting point that many people find reduces hypos, corrections, and mental load.

Major 2: Starting glucose value

Starting glucose matters because aerobic exercise usually pulls glucose down. Starting higher may provide more buffer; starting lower reduces that margin. This is why a pre-exercise check tends to be informative before beginning any session.

Major 3: Trend arrows — direction and speed

Carbohydrate decisions during aerobic exercise work best when they are based on both glucose value and the trend arrow. Numbers without direction are incomplete information.

As a starting heuristic, many people check the CGM before exercise and re-check at regular intervals during activity — commonly every 20–30 minutes. Combining glucose value with trend arrow helps inform whether carbohydrate is likely to be needed.

Why does glucose tend to drop during aerobic exercise?

Aerobic exercise opens an alternative pathway into muscle cells, allowing glucose uptake with less reliance on insulin. This is often called the “side door” mechanism.

Diagram of the insulin-independent glucose uptake side-door mechanism during aerobic exercise

This mechanism is powerful — and it becomes the dominant risk factor when bolus insulin is still active. That is why the first part of this page focuses on insulin on board and timing.

After exercise: the delayed risk

Hypoglycaemia risk can remain elevated for hours after aerobic exercise, particularly after sessions lasting 45 minutes or longer. Muscles replenish glycogen stores over several hours, and insulin sensitivity may remain elevated in that window. This is worth keeping in mind when planning the next meal or bolus.

CGM trace showing delayed hypoglycaemia risk following longer aerobic exercise sessions

Building a starting plan

The algorithm below gives a starting plan and two adjustment rows. The approach is to begin with the “Starting plan”, then use the outcome as data. If glucose ran high the first time, use the “Went high” row next time. If glucose ran low, use the “Went low” row. This is the CGM-as-feedback loop in practice.

Aerobic exercise starting plan algorithm with adjustment rows for high and low glucose outcomes

Generic worked example

Meal at 08:00, exercise at 09:00 for 60 minutes, then lunch afterwards. Building a plan using the algorithm and reviewing the outcome:

Before exercise: reduce meal bolus — start with a sensible reduction based on timing and past CGM data
During exercise: check CGM before and every 20–30 minutes; use glucose value plus trend arrow to inform carbohydrate intake using the chart for body weight
After exercise: consider whether the next bolus warrants reducing; be alert for delayed hypoglycaemia after longer sessions
Review: repeat the starting plan if it worked; if glucose ran high or low, use the corresponding adjustment row next time

Carbohydrate charts and calculators

Personalised PDF calculators — choose the device you use:

Carbohydrate charts covering body weights from 10–60+ kg — available in both mmol/L and mg/dL:

mmol/L device — carbohydrate charts for 20 minutes of aerobic exercise

Carbohydrate chart for aerobic exercise using a mmol/L CGM device, body weight 10–30 kg

Carbohydrate chart for aerobic exercise using a mmol/L CGM device, body weight 40–60+ kg

mg/dL device — carbohydrate charts for 20 minutes of aerobic exercise

Carbohydrate chart for aerobic exercise using a mg/dL CGM device, lower body weight range

Carbohydrate chart for aerobic exercise using a mg/dL CGM device, higher body weight range

Optional deep-dive video

This video shows how to build aerobic exercise plans and apply them during a 40-minute rowing session. It goes into more depth on the physiology — skip it if you just want the practical steps above.

Watch: building and applying an aerobic exercise plan during a rowing session (YouTube)

Key principles for aerobic exercise

Aerobic exercise tends to lower glucose — the size of the drop is usually determined by insulin on board
The 90-minute window: when bolus insulin is near its peak, glucose drops tend to be larger and faster
The Three-Hour Rule provides a reliable starting point for more predictable sessions
Always check glucose value plus trend arrow together — neither alone gives the full picture
Delayed hypoglycaemia risk can persist for hours after longer sessions
Use CGM outcomes as feedback to refine the next plan — the glucose never lies

This content is for educational exploration only. It describes average responses and general principles. It is not medical advice and cannot replace individual clinical guidance from your diabetes care team.

Continue the guide

← Exercise Guide overview Part 2: Mixed and Team Sports →

Back to the Exercise Guide overview