During activity, injected or pumped insulin cannot be 'shut off' like the body's own insulin, so too much glucose is taken up by both muscle contractions and the high levels of insulin, says Sheri R. Colberg, Ph.D., professor of exercise science at Old Dominion University in Norfolk, Virginia.
To avoid hypoglycemia when doing physical activity, monitor your blood sugar before and after exercise. Here are some other guidelines from Colberg and the American Diabetes Association:
Eat a small carbohydrate-containing snack before exercising if your blood glucose is 100 mg/dl or lower. Wait about 10 to 15 minutes before starting your activity.
Eat a snack if you plan to exercise for more than 60 minutes, plan to do a more intense workout than usual, or if the weather is warmer or cooler than usual.
Always carry a small snack that's high in sugar or carbohydrate. The average 150-pound adult needs 20 grams of carbohydrate for every half-hour of moderate exercise. Some snack choices include sports drinks and gels and easily absorbed carbohydrate sources, such as jelly beans and energy bars.
Watch for symptoms of hypoglycemia during exercise. If you feel weak, lightheaded, cold, or clammy, stop and check your blood glucose. If it's low, treat it with a pure source of glucose, such as glucose tablets or gel.
Become familiar with the ways different activities affect your blood sugar levels. Measure blood sugar before and after exercise. Keep a written record of what the activity was, how long you did the activity, what you ate, and blood glucose levels before and after. Over time, you'll better understand how activity affects your blood sugar levels and insulin doses.
For insulin pump users, lower basal insulin if you're planning more than 90 minutes of activity. Shorter bouts of exercise require a small carbohydrate snack or a reduction in your bolus insulin.
Avoid rigorous exercise if your fasting blood glucose is more than 250 mg/dl and you have significant amounts of ketones in your urine. Your body produces ketones when you burn fat for energy. You also should choose low-intensity movements if blood glucose is greater than 300 mg/dl.