Exercise as an Added Variable in Balancing Blood Sugars
By Sheri Colberg, Ph.D., FACSM
As all people with diabetes know, a constant balancing act is required to keep blood sugars in a normal range. Exercise presents its own special set of problems for control. The challenge of adding exercise into the mix as one more variable to figure out can feel overwhelming at times. The more you understand about what makes your blood sugars go down (or sometimes up) during exercise, the easier it becomes to control and the more confident you can be about doing activities and staying in control of your diabetes.
Exercise as an Added Variable
Any muscular activity increases your body’s use of blood glucose, which can cause you to develop hypoglycemia more readily during or following exercise. Much of your blood sugar response has to do with how much insulin is in your bloodstream, along with how well that insulin is working. If your insulin levels are high during an activity, your muscles will take up more blood glucose and you’re more likely to end up with low blood sugars. You can even end up with late-onset hypoglycemia, which can occur for up to 48 hours after you exercise (more on this topic later in this chapter).
On the other hand, doing any exercise when your blood sugars are too high—especially when you have ketones, produced as a by-product whenever your body tries to use stored fat instead as an alternative fuel, which indicate a lack of insulin in your body—can cause them to go even higher. Exercising under those conditions can put you into diabetic ketoacidosis (DKA), a condition that results from the combination of elevated blood sugars and insulin deficiency. DKA causes your liver to produce ketones that make your blood too acidic, which can be life threatening and land you in the hospital. Certain types of exercise, such as intense resistance workouts, can also raise your blood glucose levels (as explained in the following section), regardless of whether you have diabetes.
Because so many variables can potentially affect your blood sugar responses to exercise, especially if you use insulin or certain other diabetic medications, on some days you may feel like giving up! Don’t, though, because regardless of any frustration that you may feel from time to time, the health benefits of being active far outweigh the drawbacks. After you learn to control some of the variables and anticipate their effects, a somewhat predictable pattern will emerge over time to help you better predict your blood sugar responses to similar exercise. The best way to deal with the multitude of variables that can affect you during exercise is to learn your unique responses to all of them by checking your blood glucose levels before, (occasionally) during, and after exercise.
Hormonal Responses to Exercise
Studies of people with type 1 and type 2 diabetes have shown that extremely intense exercise like resistance training, weight lifting, or near-maximal anaerobic workouts can actually cause an immediate rise in your blood sugar levels, primarily resulting from your body’s hormonal response. Intense exercise causes the release of several hormones that increase the production of glucose by your liver and reduce your muscular uptake of it. These hormones include epinephrine (more commonly known as adrenaline) and norepinephrine, which are released by the sympathetic nervous system (the one that allows your body to respond to physical or mental stressors with an increased heart rate), as well as glucagon, growth hormone, and cortisol. The effects of these glucose-raising hormones can easily exceed your body’s immediate need for glucose, especially because exercise done at high intensity can’t be sustained for long. The result is an immediate rise in your blood sugars during and following short bouts of intense exercise.
You may experience some insulin resistance immediately after intense exercise, which can last for a few hours. For instance, after doing near-maximal cycling to exhaustion, one group of people with type 1 diabetes using insulin pumps experienced elevated blood glucose levels for two hours following the activity. Your body will likely need some supplemental insulin to bring your blood sugar levels back to normal. Similarly, in type 2 diabetic exercisers, blood glucose levels also rose for one hour in response to maximal cycling, as did their levels of circulating insulin because they were still making their own. Even if you don’t have diabetes, your body will increase its release of insulin following such workouts. After these hormonal effects wane, your blood sugars can easily drop later on while your body is working hard to restore the muscle glycogen that you used during the activity.
This column is excerpted from Diabetic Athlete’s Handbook (available November 6, 2008 from Human Kinetics), which contains essential exercise-related information and examples for type 1 and type 2 diabetic exercisers. Look for it in stores or find links to places to buy it online on www.shericolberg.com, along with additional information.
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