Before you start: glucose, ketones and insulin
Where your glucose should be before exercise, when not to train today, and the published starting points for reducing insulin.
This page covers the decisions you make before you move: whether your glucose is in a good place to start, whether today is a day to skip, and how insulin is usually adjusted for planned exercise. The numbers are published starting points — the ones your diabetes team will work from — not personal doses.
Where your glucose should be when you start
| Reading before you start | Steady aerobic | Hard or interval work |
|---|---|---|
| Below 5.0 mmol/L | Take 10–20 g of glucose, wait until above 5.0 | Same — treat first, then start |
| 5.0 to 6.9 | Take 10 g of glucose, then go | Fine to start as you are |
| 7.0 to 10.0 | The sweet spot. Go. | Go, but expect it to climb |
| 10.1 to 15.0 | Go | Go, but expect it to climb further |
| Above 15.0 | If unexplained, check ketones first | Check ketones first (see below) |
Source: Riddell et al. 2017, Panel 1.
The direction your glucose is already moving matters as much as the number. Falling fast at 7.0 is not the same as steady at 7.0. If you use a CGM, look at the trend arrow, not just the reading: an arrow pointing down at 7 mmol/L means treat before starting.
When not to train today
A myth worth dropping: "don't exercise above 14"
That rule is wrong as usually stated. Every guideline ties the decision to ketones, not to the glucose number by itself.
Mildly high glucose does not measurably hurt exercise capacity. What makes exercise dangerous is a shortage of insulin with ketones building up — and that is a different thing from a high number on its own. That is why the table above says "check ketones first" rather than "don't go".
Taking less insulin for planned exercise
These are the published starting points for exercise within about 90 minutes of a meal dose. Bring them to your team as the basis for a plan; don't apply them on your own.
| Reduce the meal dose before exercise lasting | 30 min | 60 min |
|---|---|---|
| Easy aerobic | −25% | −50% |
| Moderate aerobic | −50% | −75% |
| Hard aerobic | −75% | — |
| Flat out or anaerobic | No reduction | — |
Source: Riddell et al. 2017, Tables 2 and 3.
Cutting the meal dose by as much as 75% does not appear to raise ketones during exercise.
Background (basal) insulin
On a pump: reducing the basal rate 60 to 90 minutes beforehand works better than switching the pump off. If you do suspend it, keep it short — Diabetes Canada says only for activity of 45 minutes or less, and never leave it off beyond about two hours.
On injections: cutting long-acting insulin is not routine advice, because it affects the whole day. It is an option for a tournament, a camp, or a week of much heavier activity than usual — something to plan with your team.
Where you inject matters
Don't inject into a muscle you are about to work: insulin leaves a working limb faster. In practice: not the thighs before a run, and the abdomen rather than the legs before leg day. It is a small, practical change that people rarely hear about, and it can make a difference straight away.