Why Exercise Can Send You Low

Exercise is good for you and it complicates Type 1, and both of those are true at once.

The main effect is downwards. Working muscles pull glucose out of your blood, and they keep doing it for hours afterwards while they restock. Increased physical activity is a recognised cause of low blood sugar in people on insulin.

But it is not only downwards. Short, very intense effort — a sprint, a competitive match, heavy lifting — can push glucose up instead, because the body releases stress hormones that liberate stored glucose. This catches people out badly, because a high reading after exercise feels like a contradiction.

As a rough guide: steady moderate activity such as walking, cycling or swimming tends to lower glucose. Brief maximal effort tends to raise it. Mixed activity, like most team sports, can do both within the same hour.

The other thing to understand is timing. The largest risk is often not during the activity but several hours later, including overnight, when your muscles are still refilling.

None of this is a reason to avoid exercise. It is a reason to check before and after, and to learn your own response rather than a general rule.