Recently, I saw two very different clients with a very similar problem: difficulty controlling blood glucose levels during physical activity.
One was a 62-year-old man named Anthony who has Type 2 diabetes. Anthony’s wife likes him to take a 45-minute walks after dinner. “It’s good for him to help him lose weight,” she said. “But he keeps getting low on the way home. I give him extra food, but it just defeats the whole purpose. Maybe we should give it up.”
The other was a teenage girl, Liz, whose father expressed concern over how her diabetes was affecting her swimming performance. “We need her blood sugars tight when she competes,” he said. “Trouble is, she goes low during practice, so we give her extra carbs. Then she goes way up high for the meets, so she gets tired and doesn’t swim well. And if we cover the high, she winds up low during the night or even the next day. Up and down. Up and down.” Thus goes the cycle.
Managing blood glucose levels during physical activity is important not only for preventing low blood glucose (hypoglycemia), but also for improving physical performance. Research has shown that strength, speed, flexibility, and stamina are all impaired by high blood glucose levels. It is not usually dangerous to exercise with a high blood glucose level, as long as you cannot detect ketones in your urine. (The presence of ketones in urine indicates a significant insulin deficiency, which must be corrected before you exercise.) Even without ketones, however, your physical performance will be influenced by even moderately elevated blood glucose levels.
Blood glucose regulation involves a complex interaction between factors that raise blood glucose and factors that lower blood glucose. Factors that raise blood glucose include carbohydrate in the diet and stress hormones, which cause insulin resistance and extra glucose production by the liver. Factors that lower blood glucose include insulin, other diabetes medicines, and physical activity, which enhances insulin sensitivity. A change in one of these factors requires a change in one or more other factors to keep the balance.
Most of the time, physical activity lowers blood glucose levels. With increased activity, muscle cells become much more sensitive to insulin (whether it is insulin your body produces or insulin that you inject). This enhanced insulin sensitivity may continue for many hours after exercising, depending on the extent of the activity. The more intense and the more prolonged the activity, the longer and greater the enhancement in insulin sensitivity.
With enhanced insulin sensitivity, insulin exerts a greater force than usual. If a unit of insulin usually covers 10 grams of carbohydrate for you, it might cover 15 or 20 grams during or after exercise. Similarly, if a unit of insulin normally lowers your blood glucose by 50 mg/dl, it might lower it by 75 mg/dl during or after exercise.
Occasionally, physical activity raises blood glucose levels. Some forms of physical activity, most notably high-intensity, short-duration exercises and competitive sports, can produce a short-term rise in blood glucose levels followed by a delayed drop. This is due primarily to the stress hormone production, or “adrenaline rush,” that accompanies these kinds of activities.