Exercise affects glucose differently depending on the type, intensity, and your insulin timing. A CGM gives you real-time data to navigate workouts safely — but knowing how to interpret that data is the key.
Why Exercise Complicates CGM Readings
Physical activity affects CGM accuracy in two ways: sensor placement (arm movement, pressure on adhesive) can cause compression artifacts, and the rapid glucose changes during exercise amplify the CGM’s natural lag time.
Aerobic Exercise (Running, Cycling, Swimming)
Aerobic exercise typically lowers blood glucose — sometimes significantly. Plan for:
- Glucose often drops during sustained cardio lasting 30+ minutes
- A pre-exercise snack of 15-30g carbs if starting below 120 mg/dL
- Setting a higher low alert threshold during exercise (some athletes set it to 90 mg/dL to give more warning time)
Anaerobic / High-Intensity Exercise (HIIT, Weight Lifting)
Short intense bursts often temporarily raise glucose — the opposite of aerobic exercise. This is due to stress hormones (adrenaline, cortisol) releasing liver glucose. You may see a rise of 30-60 mg/dL during a heavy lifting session, followed by a drop hours later as muscles refuel.
Post-Exercise Hypoglycemia
One of the most dangerous patterns: glucose appears stable or even elevated during exercise, then drops sharply 4-8 hours later (overnight). Your CGM alarms are essential for catching this. Use a lower overnight alarm threshold on heavy workout days.
Sensor Placement for Exercise
The back of the upper arm is a reliable site for most activities. For contact sports or activities with arm impact, the abdomen (for Dexcom users) or upper buttocks (approved for younger users) may stay more secure. Apply a Tegaderm or similar adhesive overlay for high-sweat activities.
Practical Checklist Before Each Workout
- Check glucose and trend arrow — do not start aerobic exercise below 100 mg/dL with a downward arrow
- Have fast carbs accessible (gels, glucose tablets)
- Set a temporary exercise alarm in your CGM app
- Check glucose again 30-60 minutes into longer sessions