Aerobic versus anaerobic
Why steady effort usually lowers glucose and hard effort can raise it first, how to tell which you are doing, and why it depends on your own fitness.
Aerobic and anaerobic are just names for how hard you are working relative to your own fitness, and they matter here because the two tend to move glucose in opposite directions. Knowing which one an activity is for you is the closest thing there is to predicting what your glucose will do.
- Aerobic means you could hold a conversation: jogging, steady cycling, swimming lengths, a brisk walk. Usually pulls glucose down.
- Anaerobic means you could not: sprints, heavy lifting, an all-out effort. Often pushes glucose up first, then down later.
- It is relative to you. A pace that is easy aerobic for a regular runner is flat-out anaerobic for someone starting out.
- Most real sport is a mix, which is why a match or a class can go up early and down by the end.
- A hypo (a low, below 4 mmol/L, where you feel shaky, sweaty or confused and need fast sugar) can happen during or straight after either kind, and again hours later or overnight.
Two ways of making energy, and how to tell them apart
Aerobic means “with oxygen”. At an effort you can sustain, your muscles get enough oxygen to burn fuel steadily, and you could keep going for a long time. Anaerobic means “without enough oxygen”: past a certain intensity your muscles need energy faster than oxygen can be delivered, so they switch to a quicker route that cannot be sustained for long.
You do not need a lab to tell which you are in. The talk test does it: if you can hold a conversation in full sentences, you are aerobic. If you can manage a few words between breaths, you are on the edge. If you cannot speak at all, you are anaerobic. That is a genuinely reliable guide and it costs nothing.
The important part is that this is relative to your own fitness, not to the activity. Cycling to work is comfortably aerobic for a regular cyclist and properly anaerobic for someone who has not ridden in years, and their glucose will behave differently on the same route at the same speed.
Why one tends to lower glucose and the other can raise it
Aerobic work is steady demand met steadily. Muscles take glucose out of the blood to keep going, and because the effort is sustainable that uptake carries on for as long as you do. Nothing much is telling your liver to panic, so the net result is usually a fall, and a longer session usually means a bigger one.
Anaerobic work is different because it is stressful in the physiological sense. Hard efforts release adrenaline and other stress hormones, and part of what they do is tell your liver to release stored glucose. Someone without type 1 answers that with their own insulin and barely moves; you cannot, so the released glucose has nowhere to go and your reading climbs even though you are working hard.
That rise is usually temporary. Once the effort stops, the hormones fade while the muscle uptake and the restocking carry on, so an anaerobic session commonly goes up during and down afterwards, sometimes well afterwards.
Real sport is a mixture, and that is the useful bit
Almost nothing you actually do is purely one or the other. Football is mostly aerobic running with anaerobic sprints in it. A gym session might be anaerobic lifting followed by an aerobic bike. A hill on a steady ride is a few anaerobic minutes inside an aerobic hour. So the practical question is not “which is this” but “which dominates for me, and when in the session”.
That is worth knowing because it explains shapes that otherwise look random. Going up in the first ten minutes of a match and finishing low is not a broken sensor or a mistake, it is a sprint-heavy start followed by an hour of aerobic running. A weights session that leaves you higher than you started, then drops you at bedtime, is the same story stretched out.
Some people deliberately use this: a short hard effort at the end of an aerobic session is a recognised way of blunting the fall. It is worth discussing with your team rather than trying blind, because how much it does varies a lot between people, but it is a real effect and not folklore.
What decides which one you are in
The same activity moves between the two categories depending on who is doing it and how hard they are pushing. A run at conversational pace is aerobic; the same run with hill repeats in it is both. Nerves add an adrenaline effect on top of whatever the physical effort is doing, which is why a competitive version of a familiar session behaves differently from training. Heat and altitude both make a given effort harder, which nudges you toward the anaerobic end without changing the pace.
Telling them apart in practice
Examples, not instructions or doses.
Forty minutes at a pace where you could chat the whole way. Firmly aerobic, and your glucose drifts down steadily throughout, which is the textbook shape.
Heavy sets with long rests, nothing you could talk through. You finish higher than you started, because the effort released stress hormones and your liver did what it was told.
Twenty minutes of steady work, then intervals at the end. Down through the first half, back up during the intervals, then down again later that evening as everything settles and your muscles restock.