Long considered a cornerstone of diabetes management, exercise has been underemphasized as a therapeutic treatment. This is not without reason, as the effects of exercise on blood glucose levels in people with Insulin Dependent Diabetes (IDDM) is physiologically complex, and requires individual tailoring rather than a rigid, uniform prescription. As always, the most effective way to integrate exercise is by being adequately informed.
In nondiabetic individuals, the mechanism for maintaining normal blood glucose levels during exercise is regulation of glucose output from the liver. Three hormones effect the delivery of glucose from the liver: adrenaline, which inhibits the release of insulin; plasma glucose levels; and, norepinephrine. These hormones allow for increases in blood glucose and free fatty acids (FFA) needed by muscle tissues which are exerted during exercise. In addition, the intensity and duration of exercise, level of conditioning of the person exercising, and food intake prior to exercise all effect the relative amount of glucose and FFAs used during exercise.
Individuals with IDDM, lack this complex this metabolic balance. In persons with IDDM, exercise may have one of three effects, depending on the person’s blood glucose levels and concentration of insulin available If a person initiates exercise with a blood glucose level between 120 and 200 mg/dl, and at a time when his or her insulin is not peaking, increased glucose uptake by the muscle tissue is matched by glucose production from the liver, and low blood sugar is avoided. If a person initiates exercise with blood glucose levels below 100 mg/dl, and at the peak of his or her insulin activity, glucose from the liver is inhibited, and hypoglycemia results. Finally, if exercise is initiated during insulin deficiency, the exercise will greatly enhance the production of glucose from the liver, potentially leading to the rapid onset of ketosis.
Translating this into practical terms, if a person with IDDM begins to exercise with relatively stable metabolic control, adequate blood glucose levels can be expected during and after exercise. However, because exercise is often a spontaneous event, and may vary in intensity and length, there are a number of helpful strategies for avoiding hypo- or hyperglycemia. These include:
[Note about exercise: these suggestions are based on intensive excercise (400+ calories/hour), sustained for at least one hour.]
- Always take insulin more than an hour before exercise.
- Lower the insulin dose that will have peak action during exercise by 30-50%, depending on duration and intensity of intended exercise.
- Do not inject insulin into the part of the body most active during exercise (for example, for bicycling, use your arm or abdomen, rather than your leg).
- Decrease your post-exercise insulin dose (by up to 30% depending on intensity and duration of exercise).
- Exercise after meals whenever possible.
- Eat supplemental carbohydrate snacks at least every 30 minutes. Snacks should be between 60-90 calories. A medium orange is 65 calories; a medium banana is 105 calories; 8 ounces of Gatorade is 39 calories.
- Also increase carbohydrate intake both 24 hours before and after exercise, again depending on intensity and duration.
- Monitor blood glucose before, during and after exercise, recording results in a diary.
- Monitor blood glucose approximately every 30 minutes during prolonged exercise.
- Delay exercise if blood glucose is over 250 mg/dl, and ketones are present.
- Delay exercise if blood glucose is under 100 mg/dl, or eat some carbohydrate before you begin.
- Be aware that overnight low blood sugar may result from exercise, and adjust by increasing your evening snack and/or reducing evening insulin dose.