Why Exercise Feels Complicated with T1D
Exercise is one of the best things you can do for your overall health. However, if you live with Type 1 Diabetes, it can feel like solving a puzzle every time you lace up your shoes. Will you go low? Will you spike? Will your glucose behave completely differently than yesterday even though you did the exact same workout?
The truth is, exercise affects glucose in complex ways, and no two sessions are exactly alike. But with the right strategies and a bit of practice, you can exercise confidently and safely. Thousands of people with T1D run marathons, play competitive sports, and stay active every day.
Aerobic vs. Anaerobic: Two Very Different Effects
Understanding the type of exercise you are doing is the first key to predicting how your glucose will respond.
Aerobic exercise (running, cycling, swimming, brisk walking) tends to lower glucose. Your muscles use glucose for fuel, and insulin sensitivity increases during and after activity. The risk here is going low.
Anaerobic exercise (weightlifting, sprinting, HIIT) can raise glucose temporarily. High-intensity effort triggers stress hormones like adrenaline and cortisol, which tell your liver to release stored glucose. You may see a spike during or right after the session, followed by a delayed drop hours later.
Mixed exercise (team sports like soccer, basketball, or tennis) combines both effects and can be the hardest to predict. The key is to monitor more frequently and learn your personal patterns.
Before You Start: Pre-Exercise Checklist
Before any workout, run through this quick checklist:
Check your glucose and trend. Ideally, start exercise with glucose between 7 and 10 mmol/L (126 to 180 mg/dL) and stable or rising.
If below 5.5 mmol/L (100 mg/dL): Have 15 to 20g of fast-acting carbs and wait until you are above 5.5 before starting.
If above 14 mmol/L (250 mg/dL): Check for ketones. If ketones are present, do not exercise. Treat the ketones first.
Reduce insulin on board. If you have recently bolused, consider delaying exercise or reducing the dose. Active insulin plus exercise is the most common cause of exercise lows.
Have supplies ready. Always carry fast-acting glucose (juice boxes, glucose tabs, or candy) when exercising. Keep them in your pocket, not in your bag across the gym.
During Exercise: What to Watch For
Check glucose every 30 minutes during exercise, or rely on your CGM alerts. If you feel any symptoms of a low such as shakiness, dizziness, or sudden fatigue, stop and treat immediately.
For sessions longer than 45 minutes: You will likely need to consume carbohydrates during the activity. A general starting point is 15 to 30g of carbs per hour of moderate aerobic exercise, but your needs may vary.
Hydration matters. Dehydration can cause glucose to rise because the sugar in your blood becomes more concentrated. Drink water regularly during exercise.
Pay attention to how you feel. CGM readings lag behind your actual glucose by about 10 to 15 minutes. If you feel low but your CGM says you are fine, trust your body and treat.
After Exercise: The Delayed Low
One of the trickiest aspects of exercise with T1D is the delayed low, which can happen 4 to 8 hours after activity, and sometimes even overnight.
After exercise, your body works to replenish the glucose it burned from your muscles (called glycogen replenishment). This process can keep pulling glucose from your bloodstream for hours.
Strategies to prevent delayed lows: - Reduce your basal insulin by 10 to 20% for 4 to 6 hours after intense exercise (ask your care team for guidance) - Have a protein and carb snack after your workout - Set a lower glucose alert on your CGM for the night after exercise - If using a pump, consider a temporary basal reduction before bed
Track your patterns. Keep a simple log of what you did, your glucose before/during/after, and what you ate. After 2 to 3 weeks, patterns will emerge that make planning much easier.
Building Confidence Over Time
The most important thing is to start. Do not let fear of lows keep you from being active. Begin with shorter sessions, monitor closely, and build up gradually.
Every person with T1D responds to exercise differently. What works for someone else may not work for you. The good news is that your body is remarkably consistent. Once you learn your own patterns, exercise becomes much more predictable.
Talk to your endocrinologist or diabetes educator about creating an exercise plan tailored to your insulin regimen. Many diabetes centres offer exercise-specific consultations. And remember that every workout teaches you something new about your body. There is no such thing as a "failed" session, only data for next time.