Preventing Low Blood Sugar During and After Exercise

Low blood sugar (hypoglycemia) is one of the most common and concerning challenges for people with diabetes who are active—especially those using insulin or glucose-lowering medications. With the right strategies, you can significantly reduce your risk of lows during activity and in the hours or days afterward.

⚠️ When Are You Most at Risk of Exercise Lows?

Hypoglycemia is more likely when:

You start with a low glucose or a downward CGM trend
There’s excess insulin on board (IOB) from a recent bolus
You’ve had a low in the past 24 hours
The exercise is spontaneous, prolonged, or involves untrained muscles
You’re performing a new activity or increasing intensity
You’re exercising in cold weather or unpredictable conditions
Basal or bolus insulin hasn’t been reduced
You’re recovering from illness or have been inactive recently

How to Prevent Hypoglycemia Before, During, and After Exercise

🔍 1. Pre-Exercise Strategies

Start with a safe glucose level: Ideally 90–180 mg/dL, trending stable or slightly up
Check your CGM or meter: Look at trend arrows and IOB
Use Exercise Mode or a temp basal: Begin 60–120 minutes before activity for AID or pump users
Reduce bolus insulin: For meals within 2 hours of exercise, lower the dose by 30–50% depending on intensity
Eat fast-acting carbs if glucose is <90 mg/dL or if IOB is high
Avoid starting activity if you’ve had a recent low</strong >—your counter-regulatory responses may be blunted

🚶‍♂️ 2. During Exercise Tips

Monitor your glucose every 30–60 minutes (more often during intense or long workouts)
Carry fast carbs with you: glucose tabs, juice, sports drinks, gummies, or candy
Use the ExCarb system: Know how many grams of carb your body will burn based on the activity

Example: A 150 lb person running 6 mph for an hour may burn ~50g of carbs</em >
Eat small amounts of carbs during prolonged activity: Every 45 minutes is a good rule of thumb
Pay attention to symptoms: If you’re sweating, shaky, or confused, treat immediately—even if the CGM looks fine

Rebuild Glycogen Faster to Reduce Delayed Hypoglycemia
After prolonged or strenuous exercise, you want to rebuild muscle glycogen quickly once the exercise stops. There is a 20- to 30-minute window following exercise when muscles are primed to restore depleted glycogen. Consuming carbs and protein for muscle repair right after exercise lets muscle glycogen stores quickly rebuild. Chocolate milk provides a convenient way to provide both carbs and protein just after exercise. Fast glycogen rebuilding means less glucose will be drawn out of the blood in the following hours, with less risk of a low glucose during the night, even on an AID system. Carb intake just after exercise reloads your glycogen and prepares you for exercise the next day. A small carb bolus may be needed to cover these carbs and improve glycogen uptake. Higher carb intake increases muscle glycogen storage for endurance and performance. On a high-carb diet, a trained marathon runner can run for about four hours before exhaustion. Many athletes “fuel up” muscle glycogen stores to improve performance by eating a high-carb meal covered with a carb bolus the evening before major exercise events.

Rebuild Glycogen Faster to Reduce Delayed Hypoglycemia

After prolonged or strenuous exercise, you want to rebuild muscle glycogen quickly once the exercise stops. There is a 20- to 30-minute window following exercise when muscles are primed to restore depleted glycogen. Consuming carbs and protein for muscle repair right after exercise lets muscle glycogen stores quickly rebuild. Chocolate milk provides a convenient way to provide both carbs and protein just after exercise.

Fast glycogen rebuilding means less glucose will be drawn out of the blood in the following hours, with less risk of a low glucose during the night, even on an AID system. Carb intake just after exercise reloads your glycogen and prepares you for exercise the next day. A small carb bolus may be needed to cover these carbs and improve glycogen uptake.

Higher carb intake increases muscle glycogen storage for endurance and performance. On a high-carb diet, a trained marathon runner can run for about four hours before exhaustion. Many athletes “fuel up” muscle glycogen stores to improve performance by eating a high-carb meal covered with a carb bolus the evening before major exercise events.

Excerpt from Pumping Insulin with Automated Insulin Delivery

🌙 3. After Exercise: Preventing Delayed Lows

Exercise can cause glucose to fall 6 to 36 hours later, particularly at night, due to:

Rebuilding glycogen stores in muscle and liver
Increased insulin sensitivity
Ongoing low-level muscle glucose uptake

💡 Strategies to avoid overnight lows:

Eat extra slow-acting carbs at dinner or bedtime
Reduce basal insulin by 10–40% for 6–12 hours (use temp basal if on a pump or AID system)
Consider an extended Exercise Mode overnight on your AID system
Set CGM alerts a little higher (e.g., 90 mg/dL low threshold)
For strenuous or long-duration activities, monitor trends into the next day

🧠 Muscle Memory and Hypoglycemia

When starting a new activity (e.g., gardening, canoeing, or snow shoveling), muscles absorb more glucose to prepare for future efforts. This glycogen-loading phase can pull glucose from the blood for several hours, increasing the risk of lows—especially at night. Once an activity becomes routine, this effect stabilizes.