Diabetic Athlete's Handbook: Your Guide to Peak Performance by Sheri R. Colberg, PhD (Continued)
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.
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