How Exercise Impacts Blood Glucose

Reviewed by: HU Medical Review Board | Last reviewed: March 2021.

Exercise helps improve blood glucose control and lower A1C values in people with type 2 diabetes. In fact, studies have found that people who exercise 150 minutes or more a week combining resistance and aerobic training had the greatest health improvements.1-5

When you have diabetes and you exercise, you need to be aware of how your blood glucose will be affected. In simple terms, your body uses blood glucose to power your workout. This means your blood glucose level gets lower. Knowing how your body reacts to exercise will help you enjoy yourself and get fit safely. Here are some common questions about exercising with diabetes.1-5

How does exercise lower blood glucose?

In the short-term, physical activity helps increase insulin sensitivity. This means your cells are better able to process glucose during and after exercise. It varies from person to person, but your blood glucose levels can be lower for up to 24 hours after your workout. If you stay active long-term, it may also help lower your A1C.2

In general, you should exercise 1 to 3 hours after eating a meal. This is when your blood glucose levels are most likely to be high enough to feed your muscles the energy they need.

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Should I check my blood glucose during exercise?

You should check and record your blood glucose 15 to 30 minutes before, during, and after exercise. This is especially true if you take insulin or other drugs that can cause hypoglycemia. Recording these numbers will help you and your doctor understand how to adjust your medicine, if that is needed.5

Tracking your numbers several times will help you better understand how your blood glucose changes during different forms of exercise. It also will help you learn how to prevent your blood glucose from getting too high or too low during exercise.

Does exercise cause hypoglycemia?

Hypoglycemia is also called low blood glucose or low blood sugar. If you manage your diabetes with diet and lifestyle changes alone, you are less likely to develop hypoglycemia. If you take insulin or a secretagogue like sulfonylurea to manage your diabetes, you may need to adjust your insulin dose or carbohydrate intake to prevent hypoglycemia. Keep in mind that hypoglycemia is more likely to happen if you:1

  • Skip meals
  • Exercise for a long time
  • Exercise strenuously
  • Do not eat enough carbs before, during, and after exercise

To help manage your blood glucose, you should always keep some carbohydrates with you when you exercise.

Some people develop low blood glucose 4 to 8 hours after their exercise has ended. To avoid this, eat some carbohydrates that absorb into the bloodstream slowly. Examples include dried fruit, fruit jerky, granola bars, and trail mix.1

If you consistently experience low blood glucose during exercise, talk with your doctor. You may need to change your eating times, your carbohydrate intake, or adjust your medicines.

What is the 15-15 rule?

The American Diabetes Association recommends people who feel their blood glucose has gotten too low follow the 15-15 rule:2

  1. Check your blood glucose.
  2. If your reading is 100 mg/dL or lower, eat 15-20 grams of carbohydrates to raise your blood glucose. Options include 4 glucose tablets (4 grams each), 1 gel tube (15 grams), juice (½ cup), regular soda (½ cup), sugar (1 tablespoon), or honey (1 tablespoon).
  3. Check your blood glucose after 15 minutes. Eat another 15 grams of carbs if your number is still below 100 mg/dL.
  4. Repeat these steps every 15 minutes until your blood glucose is at least 100 mg/dL.
  5. Make sure your blood glucose is high enough before continuing your workout.

Should I avoid physical activity if my blood glucose is very high?

In general, if your blood glucose is very high (250 mg/dL or higher), you may need to avoid vigorous physical activity. However, if your blood glucose is slightly high and you feel well, it is generally safe to keep exercising.5

Remember, make sure to drink water before, during, and after exercise to stay well hydrated. Dehydration can raise your blood glucose levels.4

Does exercise affect where I inject insulin?

Inject insulin into a site other than the muscles used during exercise. For example, if you bike, then inject into your arm. If you play tennis or racquetball, inject into the stomach area since both your arms and legs are used in those sports. In general, you should inject your insulin 1 hour to 1½ hours before your workout begins.

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