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Diabetes: Controlling Blood Sugar during Exercise

You may become frustrated with the ups and downs of blood sugar levels when you exercise, especially if you are taking insulin to control your diabetes. But it is important to remember that your reward for your efforts is a healthier heart and healthier blood vessels. You can avoid problems by keeping good exercise records and learning how to adjust your medicine, what you eat, and your exercise schedule.

How does exercise affect my blood sugar level?

Exercise usually helps lower your blood sugar. It helps your body burn more sugar. This is because insulin works better during exercise. Exercising can be a good way to lower a high blood sugar (as long as you do not have ketones in your blood or urine).

Sometimes blood sugars go up with exercise. This may happen because you are excited and are releasing a hormone called adrenaline. This is a normal response. The adrenaline causes sugar to be released from stores in the muscle and liver. The higher blood sugar usually happens in the first hour of exercise.

How can I prevent low blood sugar (hypoglycemia) during exercise?

Check your blood sugar before, during, and after exercise.

The best way to know how exercise affects your blood sugar is to do a blood sugar test before, during, and after exercise. This is especially important if you have just been diagnosed with diabetes or if you are starting an exercise program. You should keep good records of your exercise and the results of your blood tests. If you do the same exercise at the same time of day and with your usual insulin dose and a similar starting blood sugar level, you will learn the effect of exercise on your blood sugar. You will know how you need to adjust your insulin and snacks to avoid low blood sugar.

In your records include:

  • the date and the time you started exercising
  • your blood sugar just before exercise
  • what your last dose of insulin or medicine was before exercise
  • the time you took the insulin or pills
  • the time you stopped exercising
  • your blood sugar just after exercise.

You can review your records with your healthcare provider if you are having problems. These records can be very helpful to your provider as she or he works with you to control your blood sugar. If you are starting an exercise program or if you have had a change in your medicines, it’s important to track your blood sugar response to exercise. Check your blood sugar every few hours until bedtime. It’s also important to do this if you have changed the type of exercise or how long you exercise. Your blood sugar may keep being lower for several hours.

Eat before heavy exercise.

If you are going to exercise around mealtime, eat your meal first. When it is possible to choose your exercise time, try to start the exercise 30 to 60 minutes after a meal or snack.

  • Your body absorbs liquids more quickly than solid food. Liquids generally prevent low blood sugar reactions for just 30 to 60 minutes after you drink them.
  • Your body digests solid foods more slowly. Solid foods usually keep the blood sugar level up for at least 2 to 3 hours.

If your blood sugar is less than 100 milligrams per deciliter (mg/dL), or 5.5 millimoles per liter (mmol/L), eat a carbohydrate snack (that is, at least 15 grams of carb) before exercise. Test your blood sugar 15 to 30 minutes later. Do not exercise until your blood sugar is higher than 100 mg/dL (5.5 mmol/L).

Have extra snacks available during exercise.

You should always have a source of sugar handy.

  • Keep a sugar packet, sugar cube, or a glucose tablet in your pocket or fanny pack for an emergency. Joggers' wallets for athletic shoes are another way you can carry these safety snacks.
  • Also keep a sandwich or similar snack (peanuts or granola bar) nearby. The effects of a sugar packet on your blood sugar may last only a few minutes.

It is often hard to guess the amount of a snack you will need for a particular activity. If you exercise within an hour after a meal, you may not need an extra snack. If you are not physically fit, your blood sugar may drop more quickly than if you were more fit. It is very useful to check your blood sugar to figure out what snack works best for you. If your blood sugar is low (for example, below 100 mg/dL, or 5.5 mmol/L), you need a larger snack than when your blood sugar is high. The type of snack may depend on the length of the activity.

  • Snacks such as milk or juice are used for short-term (30 to 60 minutes) activities because they contain carbohydrates that are quickly absorbed. Milk is better than juice because it has protein. Add more food, such as crackers or bread if the activity is to last longer.
  • Snacks that include protein and fat along with carbohydrate are good for long-term activities. A good snack might be a cheese or meat sandwich with a glass of juice.

Extra water is also important, particularly during hot weather. A general rule is to drink 8 ounces of fluid for every 30 minutes of vigorous activity. Liquids such as milk and fruit juices help replace water, salts, and carbohydrates.

You may need to change your insulin dosage.

Before you try a new activity, talk with your healthcare provider about any changes you might need to make in your insulin dosage. Avoid exercising when insulin is working at peak level, which means it is keeping your blood sugar at its lowest level. Your provider can tell you when your insulin is at its peak. Talk to your provider about adjusting your insulin dose to fit your exercise needs and schedule.

You may need to change the injection site.

Your activity and where you inject the insulin can affect how quickly you absorb the insulin. Exercise increases blood flow in the part of the body that is moving. The increased blood flow causes a faster absorption of insulin.

  • If you inject insulin into an arm or leg that you will be using a lot during exercise, your body may absorb the insulin too fast. For example, if you are going to run, don't inject insulin into your leg. If you are going to play tennis, avoid injecting into your tennis arm.
  • The abdomen is usually a good injection site before strenuous exercise.

Make sure others know.

Wear a medical alert bracelet or necklace. If you are on a team, it is important for your teammates and coach to know about your diabetes. Make sure one of your teammates knows where your sugar snacks are kept. Remember that when you have a low blood sugar level during a sporting event, you need to rest at least 10 minutes after eating some sugar to let your blood sugar go up.

What is delayed hypoglycemia?

Delayed hypoglycemia means you have low blood sugar several hours after the exercise is over. It may happen from 3 to 12 hours after exercise. It may cause an insulin reaction in the middle of the night. It can happen because extra sugar in the blood goes back into storage in the muscle. Hormone changes with sleep (for example, lower adrenaline levels) may also cause the delayed reaction. The best ways to keep from having this delayed reaction to exercise are:

  • Eat a snack after exercise or extra carbohydrate at the next meal.
  • Eat a longer-lasting snack (a solid food including carbohydrate, protein, and fat) at bedtime.
  • Reduce your insulin dose.

If you have delayed hypoglycemia, discuss it with your healthcare provider. You may need to change your medicine dosage or schedule.

Abstracted from the book, "Understanding Diabetes," 10th Edition, by H. Peter Chase, MD (available by calling 1-800- 695-2873).
Published by RelayHealth.
Last modified: 2011-05-16
Last reviewed: 2011-03-03
This content is reviewed periodically and is subject to change as new health information becomes available. The information is intended to inform and educate and is not a replacement for medical evaluation, advice, diagnosis or treatment by a healthcare professional.
© 2011 RelayHealth and/or its affiliates. All rights reserved.
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