Anaerobic Exercise
See why sprints and heavy lifts push blood sugar up, then down again hours later.
Anaerobic exercise above roughly 85% of your maximum heart rate triggers a stress response that temporarily raises blood sugar — even while you are burning fuel at a very high rate.1,2 This makes it one of the most counterintuitive situations in diabetes management.
This chart is designed to help you visualize how blood sugar, insulin on board, and insulin sensitivity interact during and after a short, intense workout.
These curves model rapid-acting insulin — aspart, lispro, or glulisine.3 If you use Regular or another mealtime insulin, the concept holds but the curve shifts later and runs longer; the Bolus Insulin Types simulator compares them side by side.
Pay attention to:
- Why blood sugar rises during and just after an anaerobic effort
- When the glucose spike peaks and begins to fall
- Why the hours after the workout carry a hidden low risk despite the earlier high
What’s happening here?
During anaerobic exercise, your body releases stress hormones — primarily adrenaline and cortisol — that signal your liver to release stored glucose rapidly.1 This release outpaces how fast your muscles can use it, so blood sugar rises even though you are working hard.
Unlike aerobic exercise, which typically lowers blood sugar during the workout, anaerobic effort can push blood sugar up significantly. The rise often peaks 5–15 minutes after the hardest effort ends, not during it.
Stress hormones cause the liver to release glucose faster than muscles can absorb it. Insulin on board is temporarily less effective, and insulin under the skin may absorb a little slower while blood flow is diverted to working muscle. Blood sugar rises.
Stress hormones clear. Blood sugar peaks and begins falling. Insulin on board starts working more effectively — sometimes more than expected — so a correction given for the high acts faster and stronger than it would at rest.
Depleted muscles pull glucose from the blood to replenish stores, with or without insulin. Insulin sensitivity is significantly elevated, so the same dose lowers blood sugar much more than usual. The risk of a delayed low is highest here.
The day after
- Insulin sensitivity can remain elevated for up to 24–48 hours after intense exercise
- Meals and corrections during this window may need smaller doses than usual
Applying this to your own life
Anaerobic workouts create two separate management challenges that happen back to back: a high during and just after the effort, followed by an elevated low risk in the hours that follow.
When doing intense workouts, watch how your blood sugar responds during the workout itself and for several hours afterward. A CGM is especially useful here because the delayed low can happen well after the workout feels finished.
- Expect the high, then the low. Note how far blood sugar rises, how long the peak lasts after you finish, and how fast it falls in the first hour.
- Go easy on the correction. Sensitivity climbs as the stress hormones clear, so a full correction for the post-workout high can land as a low.
- Watch the evening. Lows 2–6 hours later can arrive during sleep if you trained late.
- Keep notes. Peak, drop, any delayed low, and how your next meal’s insulin compared with a rest day.
Sources
- Riddell MC, Gallen IW, Smart CE, et al. Exercise management in type 1 diabetes: a consensus statement. Lancet Diabetes Endocrinol. 2017;5(5):377–390. doi.org
- Garber CE, Blissmer B, Deschenes MR, et al. Quantity and quality of exercise for developing and maintaining cardiorespiratory, musculoskeletal, and neuromotor fitness in apparently healthy adults. Med Sci Sports Exerc. 2011;43(7):1334–1359. doi.org
- Hirsch IB. Insulin analogues. N Engl J Med. 2005;352(2):174–183. doi.org