Blood sugar during training: why exercise sometimes lowers – and sometimes raises it

Three kinds of exertion – three reactions

Exertion isn’t just exertion: my three patterns (n=1)
My three single-case patterns. Easy, long activity such as walking or gentle cycling usually lowers blood sugar. Intense, short training raises it first before it falls later. Activity under stress, such as a call-out, raises it even without food. Working muscles take up glucose partly independent of insulin, while high intensity and stress hormones provide glucose.
Exertion isn’t just exertion (my patterns)
Single-case observation (n=1) – direction, not guarantee

🚶
Easy & long
Walking, easy cycling
↓ usually lowers

🏋
Intense & short
Strength, intervals, sprints
↑ up first

🚒
Under stress
Call-out, time pressure, alarm
↑ raises (without food)

Why this is (simplified)
Working muscles take up glucose – partly insulin-independently. At high intensity or under stress, the

body also provides energy at the same time (stress hormones) – then the balance can briefly tip upward.
After intense exertion the value can fall markedly later. On insulin/sulfonylureas: mind the hypo risk.

Fig. 1: My three patterns – easy cardio lowers, intense sessions raise first, stress activity raises even without food.

1) Easy and long: the reliable lowerer

Walking or gentle cycling is the tool that almost always works for me. Example from my notes: 12 July, 12:50, 96 mg/dl – note “walk 3 km”, 40 minutes. Or the evening when my value jumped over 200 after the beer garden: 3 km in 30–35 minutes, and afterwards the value was normal again (the whole story is here).

2) Intense and short: up first, then down

Evidence level: plausible (matches the known mechanism)

On 14 July I noted at 16:26: “strength training chest and back”, 1 hour 20 minutes – at 145 mg/dl with a rising arrow. That’s typical: with intense effort the body releases stress hormones and provides energy – the liver releases glucose. So the value can rise short-term even though I’m exercising. The lowering effect often comes only afterwards, sometimes markedly.

From the cruise report I also know the other direction: start at 115, after eating 160 and rising, then bike/running → can quickly fall to ~70 mg/dl, then settles around 120–130. Both are the same coin: it depends on intensity, timing and starting point.

3) Under stress: up, entirely without food

The most interesting case in my data: 12 July, 13:52, 138 mg/dl – note “call-out h2.01 urgent door opening”, 13 minutes. That was physical activity, but under tension and time pressure. For me such call-outs tend to drive the value up. The body does exactly what it’s built for: it provides energy because something might happen. Only that the glucose then sits in the blood.

Why this is the case

Working muscles take up glucose – a partly insulin-independent pathway via the muscle contraction plays a role. That’s why movement works even with impaired insulin action. At the same time, high intensity and stress activate the counterparts (adrenaline, cortisol) that provide glucose. What comes out on balance is the sum of both – and it tips one way or the other depending on the situation. More on this in Movement & blood sugar and Stress, cortisol & glucose.

Key takeaways

Easy cardio (walking, gentle cycling) lowers most reliably for me.
Intense training can raise the value first – the drop often comes later.
Activity under stress (e.g. a call-out) raises the value, entirely without food.
Muscle work promotes glucose uptake partly independent of insulin; stress hormones work against it.
The result is the balance of both – so “exercise lowers” is too simple.
On insulin/sulfonylureas and with complications: coordinate with your doctor.