If you’ve had type 1 diabetes for any length of time you’ve probably noticed that your blood sugar tends to go down after you exercise. This is normal. Exercise is an excellent way to help control your blood sugar, but you must be mindful of how exercise works to reduce blood glucose and what you can do to prevent low blood sugar levels during, and especially following, exercise.
Why glucose goes down after exercise
When you exercise, your body uses the sugar stored in your liver, muscles and in your blood. The sugar stored in your liver and muscles is called glycogen. During the first 15 minutes of activity most of the sugar used for fuel comes from the blood or the muscles. After 15 minutes the sugar stored in the liver is tapped for fuel. After 30 minutes, the glycogen reserves in your muscles and liver are starting to run out and you switch over to using stored fat for fuel.
This means that you are using up your stored glucose. As you do, your glucose levels go down. What most people with diabetes don’t realize is that it can take four to six hours - for some, up to 24 hours - to replace the used sugar in the form of glycogen in the muscles and liver. During this period your blood sugar may continue to lower from that same exercise session.
What you can do to prevent low blood sugar following exercise
- Check your blood sugar before you exercise. You want your blood glucose reading to be above 100 mg/gL and below 250 mg/dL. This helps to ensure that you are starting your exercise with a blood glucose level that is less likely to send you into a hypoglycemic episode. If your blood sugar is below 100 mg/dL you will want to eat 15 carbohydrates, wait 15 minutes and check your blood again to ensure that it is above 100 mg/dL before exercising.
- Try not to exercise at the peak of your insulin action. Exercising when your insulin peaks increases your risk of a rapid drop in your blood sugar. Try to anticipate when you will exercise and plan it around the peak action points.
- Avoid late evening exercise. You should attempt to stop your exercise at least two hours before you intend to sleep at night so that you can assess how your exercise is affecting your blood sugar. If you exercise right before bedtime you increase the risk of a nighttime hypoglycemic reaction that could be serious. If your blood sugar is less than 100 mg/dL before bed, you might consider doubling your snack or, if possible, reducing your insulin dosage to lessen the risk of a low blood sugar reaction while you sleep.
- Take a pass on a post-workout sauna, steam room or hot-tub session. Each of these is relaxing, but they all continue to keep your heart rate up and may contribute to lower your blood glucose as a result.
- Check your blood glucose immediately after you exercise and for several hours afterward. It makes sense to most people with type 1 diabetes that they should check their blood sugar shortly after exercise to ensure that it is at a safe level. But far fewer would think to check their blood sugar again two to four hours after exercise to check for a delayed drop of their blood sugar. If you notice that your blood sugar is lower at this two- to four-hour post-exercise check, you should check it again in another two to four hours or until you are certain your glycogen from exercise has been replaced and you no longer see a lowering of your glucose.
- For longer workouts, eat an extra snack before you exercise. If you anticipate an exercise session of more than 30 minutes, you might consider taking an extra 15 carbohydrates to help cover for the additional glucose you will be using. Regardless, you will want to pause your activity after 30 minutes and check your blood to ensure that your blood sugar is in an acceptable range.
Joslin Diabetes Center. "Why Is My Blood Glucose Sometimes Low after Physical Activity?”