Healthcare professionals recommend exercise to help improve blood sugar levels and insulin sensitivity in people with type 1 or type 2 diabetes. Exercise also reduces the risk of heart disease and death.

However, exercise can cause hypoglycemia, or low blood sugar, especially in people with type 1 diabetes and, less commonly, in people with type 2 diabetes who use insulin.

Low blood sugar and the fear of going low can be barriers to exercise participation. But there are strategies that people can use to reduce the occurrence of low blood sugar, such as eating additional food and reducing insulin doses before and after exercise.

Exercise increases uptake of glucose by your muscles and liver so that your body can use it for energy. Your body then takes glucose from your blood to rebuild these stores, thereby lowering blood sugar. Exercise also makes your body more sensitive to insulin, so you will need less insulin during and after activity.

However, the type, intensity, and duration of activity influences the impact on blood sugar and insulin sensitivity:

  • Aerobic exercises generally decrease blood sugar levels during and after workouts.
  • Endurance exercises or longer duration of exercise can result in greater blood glucose use and higher risk of hypoglycemia.
  • Resistance or high intensity exercises can cause your blood sugar to rise during and after exercise.
  • Mixed activities such as short bursts of high intensity in a session with moderate aerobic exercise can lead to less risk of hypoglycemia and better glucose stability.

Low blood sugar can occur during and after exercise. To help prevent this, if you use multiple daily injections, you can reduce your basal, or long-acting, insulin dose in the morning before exercising. If you use an insulin pump, you can suspend your pump at the start of exercise. It’s important that you don’t suspend it for more than 90 minutes.

Another option is to reduce your basal rate 30 to 60 minutes before exercising and continue until after you complete your exercise.

You may also need to reduce your bolus, or mealtime, insulin. If you plan to do mild to moderate aerobic exercise within 2 to 3 hours after your mealtime insulin, you may need to decrease that insulin by 25 to 75 percent based on how long you plan to exercise.

If you plan to do prolonged high intensity or anaerobic exercise, healthcare professionals don’t recommend an adjustment.

You can talk with your doctor about how to adjust your insulin dose before exercise.

It can help if your pre-workout blood sugar is between 90 to 250 milligrams/deciliter (mg/dl). If your blood sugar is less than 90 mg/dl, ingest 15 to 30 grams (g) of a carbohydrate about 15 to 30 minutes before exercise. Choose a type of carbohydrate that your body can absorb quickly, such as:

  • glucose tablets
  • hard candies
  • fruit
  • fruit juice
  • crackers

You can repeat it every 30 minutes during exercise based on repeat blood sugar testing. Exercising for less than 30 minutes or doing very high intensity exercises may not require an additional carb intake.

If your blood sugar is high, which can be above 250 mg/dl, check your urine for ketones. Do not perform any exercise if ketones are present. Correct the high blood sugar and wait to exercise until there are no longer ketones in your urine.

If there are no ketones, you can do mild to moderate intensity exercises. Avoid high intensity, however, as this can worsen hyperglycemia, or high blood sugar.

Sustained high intensity workouts can raise your blood sugar levels. This is due to the release of stress hormones such as adrenaline and cortisol, as well other counter regulatory hormones such as glucagon, which raise blood sugar by stimulating your liver to release glucose. Your blood sugar may be high during and even after your workout.

It’s important not to give a correction dose during your workout. After you finish, to lower your blood sugar, you can hydrate with water or do a light aerobic cooldown. If this does not work, you can then give correction, but half of what you would usually give for the same blood sugar level.

If your workout is longer or more intense than you planned, you could be at risk for hypoglycemia while exercising. Try to check your blood sugar every 30 minutes during longer periods of exercise. If your blood sugar is less than 90 mg/dl, have a snack containing 15 to 30 g of carbs and continue with your workout.

If your blood sugar is less than 70 mg/dl, you may start to have symptoms of hypoglycemia. In this case, stop exercising and treat the low blood sugar. Do not restart your exercise until the low blood sugar corrects.

Hypoglycemia occurs when your blood sugar is less than 70 mg/dl. Symptoms can vary from person to person, so the only way to know for sure is to check your blood sugar. Some early symptoms include:

  • feeling shaky
  • sweating
  • clamminess
  • hunger
  • increasing heart rate

As blood sugar continues to drop, you may experience:

  • irritability or anxiety
  • dizziness
  • sleepiness
  • slurred speech
  • weakness
  • blurred vision

With severely low sugar, usually less than 40 mg/dl, there can be:

  • confusion
  • seizures
  • loss of consciousness
  • death

If you have symptoms of low blood sugar and cannot test, go ahead and treat. We use the “15-15 rule” to treat mild to moderate hypoglycemia. This is taking 15 g of carbohydrates and waiting 15 minutes before testing blood sugar. If blood sugar is still less than 70 mg/dl, repeat the process.

Severe low blood sugars are medical emergencies. Your family members, friends, or workout partners can call 911 if you’re unconscious or give you emergency glucagon to raise your blood sugar.

Blood sugar can continue to drop 4 to 8 hours after you complete exercise. This is because muscles use up their glycogen stores during intense, prolonged activity and need replenishing. You can help prevent low blood sugar by eating carbohydrates after exercise that absorb slowly, such as a granola bar or trail mix.

You may also need to decrease your insulin dose after exercise.

A small 2013 study found that decreasing the bolus dose of insulin by 50 percent at the meal following exercise helped to prevent early-onset hypoglycemia up to 8 hours after exercise.

If you use multiple daily injections, reducing basal insulin by 20 percent that day can help prevent low blood sugar. If you use an insulin pump, decreasing your basal rate by 20 percent for 5 to 6 hours after exercise can reduce your risk of going low during the night.

Many factors can affect your insulin dose adjustments. You can talk with your doctor about how to adjust your insulin after exercise to help prevent low blood sugar.

If you exercise at night, especially after you eat dinner with the usual insulin dose at the meal, you can often be at an increased risk of low blood sugars overnight.

However, if this is best time for you given your lifestyle, you can reduce your risk by decreasing your evening insulin doses and having a post-exercise snack.

Dr. Kelly Wood is an ABMS board certified endocrinologist and internal medicine physician who treats adults with diabetes, thyroid disease, osteoporosis, and other hormonal conditions. She achieved her fellowship in endocrinology from University of Wisconsin Hospital and Clinic