Blood sugar, also called glucose, is one of the body's main sources of energy. Physical activity can cause blood sugar levels to go higher or lower than the normal range, depending on a number of factors. Understanding how to maintain healthy blood glucose levels during exercise is particularly important for people with diabetes.
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How the Body Uses Glucose
The body gets most of its glucose supply by metabolizing the carbohydrates in food. The body stores some of this glucose in the liver and muscle tissue for later use, in the form of glycogen. The liver also produces its own supply of glucose to keep blood sugars stable between meals and overnight, according to the University of California, San Francisco.
Normally, the body maintains balanced blood sugar levels (what doctors call blood glucose homeostasis) by means of an intricate, tightly controlled system. Insulin and glucagon are two of the primary hormones in this system, according to Kaiser Permanente. Insulin helps move glucose out of the blood and into the cells, which use it for fuel. This process lowers blood sugar. If blood sugar levels drop too low (known as hypoglycemia), the body releases glucagon, which signals the liver to release stored glucose.
People with diabetes have systems that either don't make enough insulin (type 1) or can't use insulin effectively (type 2), per the National Institute of Diabetes and Digestive and Kidney Diseases. As a result, glucose builds up in their bloodstream, causing high blood sugar (known as hyperglycemia). Insufficient insulin action — and the medications used to manage it — can cause significant blood sugar fluctuations during exercise for people with diabetes.
How Exercise Affects Blood Sugar
The body relies on two sources of fuel during physical activity: glucose and fat, according to Harvard's Joslin Diabetes Center. Prolonged or intense forms of exercise deplete blood glucose and glycogen stores as follows:
- During the first 15 minutes of exercise, the body relies mainly on blood glucose or muscle glycogen (which is converted back into glucose) for fuel.
- As exercise continues, the body starts re-converting liver glycogen into glucose.
- After 30 minutes, the body starts to rely more on body fat.
- The body will eventually rebuild its glycogen stores (typically within four to six hours), but exercise can lower blood sugar levels for up to 24 hours.
Exercise also makes the body more sensitive to the effects of insulin, meaning that more glucose is moved out of the blood and into the cells, according to the American Diabetes Association. Additionally, exercise prompts the cells to take in more glucose to meet the increased energy demands. This can also lead to lower blood sugars.
Exercise does not normally cause problematically low blood sugar for people without diabetes. In fact, reducing blood sugar levels through regular exercise is one of the best strategies to help prevent the development of diabetes, according to the U.S. National Library of Medicine.
Exercise, Blood Sugar and Diabetes
Exercise (along with a healthy diet) is a very important part of diabetes management, according to Harvard Health Publishing. As mentioned, being active helps lower blood sugar levels via several different methods.
But if a person with diabetes takes too much blood sugar-lowering insulin or doesn't eat enough carbs, the combination of insulin and exercise can cause blood sugar levels to drop too low. "If a person is taking [blood sugar-lowering] medication, they need to take that into account — and may need to make an adjustment [to their medication] for exercise," Samar Hafida, MD, an endocrinologist at Harvard's Joslin Diabetes Center, tells LIVESTRONG.com.
Some diabetes medicines, including insulin and those in the sulfonylurea class, can cause hypoglycemia during exercise if not managed correctly.
People with diabetes who are taking blood sugar-stabilizing medications such as metformin don't need to worry about low blood sugar during exercise, Dr. Hafida explains. These medications don't actively lower blood sugars; rather, they help with blood glucose maintenance, she says.
If you are taking insulin for your diabetes, be sure to discuss your exercise strategy with your doctor. You should also be aware of the symptoms of hypoglycemia, which include hunger, irritability, dizziness, fatigue and irregular heartbeat, according to the Mayo Clinic.
In some cases, exercise can also cause high blood sugar in people with diabetes, according to the Joslin Diabetes Center. If blood sugar levels are already high before exercise, the extra glucose that the body pumps out during physical activity can push those blood sugar levels even higher. Since people with diabetes have insufficient insulin action, their systems may not be able to manage all that extra glucose, leading to hyperglycemia.
If you have diabetes, always check your blood sugar before exercising. Depending on your blood glucose level, these are the steps you should take, according to the Cleveland Clinic:
- Less than 140 mg/dL: Eat 15 grams of carbs before starting exercise
- Between 150 and 180 mg/dL: This is the best blood sugar range before starting exercise for people with diabetes
- Over 300 mg/dL: Wait before starting exercise, and possibly take a small dose of insulin per your doctor's instructions
Be sure to check your blood sugar after exercise as well, to make sure that your blood sugar isn't too high or too low.
- Joslin Diabetes Center: "Why Is My Blood Glucose Sometimes Low after Physical Activity?"
- University of California, San Francisco: "The Liver & Blood Sugar"
- Kaiser Permanente: "How the Body Controls Blood Sugar"
- National Institute of Diabetes and Digestive and Kidney Diseases: "What Is Diabetes?"
- American Diabetes Association: "Blood Sugar and Exercise"
- National Library of Medicine: "How to Prevent Diabetes"
- Harvard Health Publishing: "The Importance of Exercise When You Have Diabetes"
- Mayo Clinic: "Hypoglycemia"
- Joslin Diabetes Center: "Why Do Blood Glucose Levels Sometimes Go Up after Physical Activity?"
- Cleveland Clinic: "Glucose Control: Why Timing Your Exercise After Meals Matters"