When I exercise, sometimes my blood sugar goes up, and sometimes it goes down. Why is that?
Welcome to the world of diabetes management, where sometimes things aren’t as simple as they may seem. Many people living with diabetes are instructed early on about the risk of hypoglycemia during exercise, since physical activity lowers blood sugar and makes us more sensitive to insulin. And while these are often true, there is actually much more to consider. (Be sure to check with your diabetes care team before starting or modifying an exercise program.)
Working muscles do indeed burn glucose for energy. The more (and bigger) muscles used, the more intense the exercise, and the longer the activity lasts, the more glucose is burned up. That’s why a half-hour of swimming (which uses just about every muscle in the body) at a fast pace will lower the blood glucose much more than a half-hour of lifting weights (which uses isolated muscle groups) or 20 minutes of casual walking (which is shorter, less intense and uses the legs only). (Read more about how many calories you may burn by doing strength training, swimming, walking and other activities.)
But that is not the only thing influencing our blood sugar while we’re exercising. There may also be counter-regulatory (stress) hormones produced, as well as food digesting from a previous meal or snack. These, of course, are raising the blood glucose level at the same time that physical activity is lowering it. It is the balance of these factors that matters. If the physical activity lowers the glucose more than hormones and food are raising it, then the blood sugar level will drop. If the hormones and food raise the glucose more than the activity lowers it, then the blood sugar will go up. It’s as simple as that.
Counter-regulatory/stress hormones include adrenaline (epinephrine/noreipephrine), cortisol, growth hormone and glucagon. All stimulate the liver to release glucose stored in the liver into the bloodstream, and cause a degree of insulin resistance. When produced in large amounts, they can contribute to a significant rise in blood sugar. Some, such as growth hormone and cortisol, are naturally produced in larger amounts in the early morning hours. That’s why exercise performed in the morning may result in less of a blood sugar drop (or even a rise) compared to the same exact exercise performed later in the day.
The other hormones are produced in response to stress (real or perceived). During exercise, “stress” can take the form of intense competition, nervousness, maximal efforts, pain or injury. That’s why a soccer game against your arch rivals may cause the blood sugar to rise, whereas a practice makes it drop. The same can be said for maximal weight lifting (high weight, low reps) compared to lighter weight lifting (low weight, high reps). The amount of glucose burned in both activities is equal, but the hormone response varies considerably.
The effect of previously consumed food is a bit clearer. Rapidly digesting carbohydrates such as bread, crackers, juice or cereal, consumed shortly before a workout, will likely raise the blood sugar during the exercise session. Slower-digesting carbohydrates, such as large/mixed meals and foods containing ample amount of fat or fiber, consumed a few hours prior, may still be digesting and raising the blood sugar during the workout.
Another factor that may impact the blood sugar response during exercise is the presence (or absence) of insulin in the bloodstream. When large amounts of insulin are present, such as after a mealtime dose is administered, the more likely the blood sugar will drop. However, when insulin levels are very low, as is the case when an insulin pump has been disconnected or suspended (or when the basal insulin has been reduced), glucose levels may have a tendency to rise.
Blood glucose management is a continual balancing act. It is important to consider the various factors that influence blood sugar at any given time. On one side of the scale are physical activity and insulin. On the other side are food and the multitude of hormones that raise blood sugar. Work with your healthcare team to devise strategies for keeping the scale from tipping too far towards one side or the other.
Gary Scheiner, MS, CDE*, is a Certified Diabetes Educator and Masters-Level Exercise Physiologist who has lived with type 1 diabetes for more 28 years. He was named 2014 Diabetes Educator of the Year by the American Association of Diabetes Educators, and has written six books, including Think Like A Pancreas. Scheiner and his clinical staff provide diabetes management consultations worldwide via phone and the Internet through his practice, Integrated Diabetes Services. Scheiner is a paid contributor for The DX. All opinions contained in this article reflect those of the contributor, and not of Sanofi US, its employees, agencies, or affiliates.
*“Certified Diabetes Educator” and “CDE” are certification marks owned and registered by the National Certification Board for Diabetes Educators (NCBDE). NCBDE is not affiliated in any way with Sanofi US. NCBDE does not sponsor or endorse any diabetes-related products or services.
© 2015 The DX: The Diabetes Experience