Last week we published an excerpt from Chapter 4 of Sheri Colberg’s revised, updated, and expanded version of her 2001 book, Diabetic Athlete’s Handbook: Your Guide to Peak Performance. Dr. Colberg has a PhD in exercise physiology, is a Diabetes Health board member, and is herself an athlete with diabetes. Her book draws upon the experiences of hundreds of athletes with diabetes to provide the best advice for exercisers with diabetes, either type 1 or type 2.
Diabetic Athlete’s Handbook has detailed advice on blood sugar regulation, medications, nutrition and supplements, injury prevention and treatment, and mental strategies for maximizing performance and optimizing health, along with detailed regimen advice for more than 100 sports and activities.
Specific insulin advice is included for pump users and those on a basal/bolus insulin regimen (including Lantus, Levemir, Humalog, NovoLog/NovoRapid, and Apidra). The book also has examples from hundreds of athletes with type 1 and type 2 diabetes, along with 10 profiles of elite and amateur athletes, from professional surfers to Iditarod dog mushers.
The book is a “must have” for the athlete in your life.
Sheri Colberg, PhD, also known as Sheri Colberg-Ochs, is an author, exercise physiologist, and associate professor of exercise science at Old Dominion University in Norfolk, Virginia. She was also appointed as adjunct associate professor of internal medicine at Eastern Virginia Medical School, also in Norfolk.
The following is an excerpt from Chapter 2 of Dr. Colberg’s book: “Balancing Exercise Blood Sugars”
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. Table 2.1 is a short list of some of the more important of these variables. After you learn to control some of them 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 (see table 2.2). 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.
Timing of Exercise and Insulin Levels
The timing of exercise may also play a big role in your body’s responses. For instance, you’re less likely to experience low blood sugars if you exercise before breakfast, especially before taking any insulin. At that time of day, you have only your basal insulin (the insulin that covers your body’s need for insulin at rest separate from food intake) on board, so your circulating levels will generally be low, but you usually have higher levels of cortisol, a hormone that increases your insulin resistance, to compensate.
If you exercise after breakfast and a quick-acting insulin injection, your insulin dose may affect whether you get low because the dose will affect your levels of circulating insulin. In one study, exercisers with type 1 diabetes did 60 minutes of moderate cycling starting 90 minutes after taking their regular dose with an insulin pump and eating breakfast. To prevent lows, they reduced their rapid-acting insulin boluses by 50 percent and took no basal insulin. Their morning insulin reductions, however, turned out to be less than afternoon ones made for a similar workout. Thus, if you often develop hypoglycemia during exercise, you might be better off exercising before taking any insulin to cover breakfast instead of afterward or later in the day.
Anyone with type 2 diabetes who still makes insulin is also more likely to have glucose levels drop if exercising after breakfast or another meal (as opposed to before) because of the insulin that is released in response to eating that elevates the levels. Keep in mind, though, that if you exercise long enough without eating, whether you have diabetes or not, you can develop hypoglycemia because of running low on fuels and liver glycogen after not eating overnight, so running a marathon without eating anything beforehand isn’t a good idea.
Regulating Insulin Levels During Exercise
Physical activity is one of the main causes of hypoglycemia in people with tightly controlled diabetes. Exercising with low levels of insulin is indeed a much more normal physiological response. To lower yours, you may need to lower (if possible) your premeal insulin doses. Table 2.4 gives some general recommendations for insulin changes but refers primarily to rapid- or short-acting insulins, not basal ones. (Basal insulins can also be reduced, but for guidelines on doing so, refer to the recommendations for individual sports in part II.)
How much insulin you have in your system between your exercise sessions can also affect how well you do during the next workout. You may end up restoring less muscle glycogen after exercise (or any time) if you don’t have enough insulin or your insulin action is diminished. Although your muscles can take up glucose and restore glycogen mostly without insulin for the first hour following an intense or long bout of exercise, after that time, you need to have enough insulin available to continue stimulating glucose uptake and glycogen storage. If you end up not storing as much glycogen, the next time you exercise your body may depend on greater use of fat, which will lower your ability to exercise and likely cause you to fatigue much more quickly, especially if having low glycogen levels causes you to take up more blood glucose. Keeping your blood sugars closer to normal after exercise also helps you restore glycogen more effectively than if your sugars run high during that time. So, you’ll likely need some insulin after exercise for any carbohydrate that you eat, albeit it a reduced amount.
Read Part 1 which includes a different chapter excerpt from Colberg’s book