Although the role of exercise in diabetes treatment has been misunderstood in the past, specialists have learned more about the circulatory, metabolic, and hormonal effects of exercise in the treatment of diabetes during the last 15 to 20 years.

How does exercise affect glucose levels?
Insulin is produced when the concentration of glucose (a simple form of sugar) in the body increases, such as after eating. Insulin stimulates muscle and fat cells to absorb the glucose they need as fuel for their activities. If you are fasting, your muscles burn fat for energy; after eating, your muscles burn glucose for energy.

During exercise, excess glucose stored by the muscles in the form of a starch called glycogen is first used for energy. Your body burns glycogen for energy most often during short bursts of exercise, such as a quick sprint or a run to catch the bus. As the exercise continues, your muscles take up glucose at almost 20 times the normal rate. With prolonged exercise, free fatty acids become the major material for muscle energy production.

Where do the fuels originate and what is the role of insulin?
Glucose originates mainly from the liver, while free fatty acids originate from the breakdown of fats (triglycerides) in fatty tissue. Insulin levels are usually lower during exercise, allowing more sugar to be released by the liver. However, during exercise, insulin is more effective at stimulating your muscles to take up glucose. Therefore, the overall balance is that blood sugar levels don't change much during exercise.

Why do glucose levels rise in some patients but fall in others, despite similar exercise patterns?
These changes can be attributed to acute effect and sustained effect:

  • Acute effect-In the person whose diabetes is well-controlled, exercise may lower glucose levels since he or she is producing enough insulin and liver glucose production is suppressed. In the person whose diabetes is poorly controlled, he or she is not producing enough insulin. Therefore, liver glucose production is unbalanced and muscle glucose uptake is diminished. This results in an increase in glucose levels and may even lead to ketosis (the production of abnormal amounts of ketones, which are byproducts of fat metabolism). Prolonged strenuous exercise (exceeding 80% of your maximum capacity) may also lead to elevation of blood sugar.
  • Sustained Effect-The stored supply of glycogen in your muscles is depleted after 40 to 60 minutes of moderate intensity exercise. After exercise, glucose flux across muscle increases significantly. This may lead to delayed hypoglycemia.

What are the benefits of regular exercise?
Exercise has many effects-psychosocial, cardiovascular, and metabolic-that may benefit people with diabetes. Regular physical activity helps decrease blood sugar and is an important part of controlling diabetes. Exercise:

  • Burns off extra body fat, helping you to achieve ideal body weight or lose body weight. (Interestingly, improvements in insulin sensitivity may be evident, independent of the weight loss.)
  • Improves muscle strength and blood flow.
  • Increases energy level and enhances work capacity.
  • Improves circulation, oxygen utilization, and endurance (reducing chances of stroke or heart attack.)
  • Increases bone density and strength.
  • May favorably alter the lipid profile. Regular exercise programs have been shown to reduce serum triglyceride levels and raise High Density Lipoprotein (HDL) cholesterol levels. To cause significant changes in HDL cholesterol levels, moderate to heavy exercise (equivalent to running 4 to 8 miles per day) is required.
  • Reduces stress; promotes relaxation and releases tension, anxiety and depression.
  • Improves physical appearance, improving self-esteem, feelings of well-being and self-confidence.

There is a myth that exercise alone will normalize metabolic control. In Type 1 diabetes, exercise has not been shown to significantly improve blood sugar control. However, it is a helpful treatment when combined with nutritional therapy and medication.

Exercise also plays an important role in the management of Type 2 Diabetes (Non-Insulin Dependent Diabetes Mellitus or NIDDM).

What are the potential risks of exercise for people with diabetes?
There are some potential risks of exercise for people with diabetes. The action of hypoglycemic medications, including sulfonylureas and insulin, may be enhanced and result in hypoglycemia (too low a level of glucose in the blood).

Because the patient with diabetes is more likely to have heart disease (with or without symptoms), the risk of arrhythmia (abnormal heartbeat) and ischemia (decreased supply of oxygenated blood) also are increased.

In elderly individuals, anti-gravity exercises may aggravate degenerative joint disease or (more likely) lead to soft-tissue injuries.

In patients with active retinopathy, strenuous exercise may precipitate hemorrhage within the eye or retinal detachment.

Can I stop exercising regularly if I lose enough weight?
No. Recent studies suggest that exercise must be performed regularly since the metabolic changes begin to revert to their original levels within 72 hours of stopping exercise (Burstein et al, Diabetes 34:756). Regular exercise results in lower serum insulin levels but enhanced glucose uptake by skeletal muscle.

Exercise precautions
Before you begin an exercise program, you must tell your physician if you have any of the following conditions:

  • Poor metabolic control
  • Significant micro- or macro-vascular disease
  • Severe peripheral neuropathy
  • Hypoglycemic unawareness
  • Cardiac autonomic neuropathy

Your physician will work with you to correct these problems as much as possible before he or she helps you to develop an individualized and safe exercise program.

How much should I exercise and what type of exercise is best?
This remains the choice of the patient. Improvement in insulin sensitivity and reduction in cardiovascular risk is evident even after relatively mild training. Although aerobic exercise is preferred, resistance exercise in selected groups is safe and also improves glucose control.

Exercise sessions should last for about 20 to 40 minutes. Systolic blood pressure during exercise should be kept below 180 to 200 mmHg. You should exercise at least three times a week.

How should I adjust my meals and medications for my exercise program?
Planning and foresight is essential. The ability and willingness to monitor your blood glucose levels is also crucial. In general, it is better to exercise after meals. Check your blood glucose level before and after exercise and have a source of carbohydrate readily available. Be sure to drink enough fluids during and after exercise to prevent dehydration.

Depending on the time of exercise, reducing the dose of either long-acting insulin or short-acting insulin will be necessary. It is preferable to use the abdomen for insulin injections since absorption of insulin in this area is least affected by exercise.

Prescription for exercise in a patient with diabetes
Here's an example of how your physician will help prescribe the exercise program that is right for you. Your physician will:

  • Determine blood sugar control.
  • Perform a complete cardiovascular examination.
  • Obtain an electrocardiogram (EKG) and stress test, if the patient:
    - is older than age 35
    - has been diabetic for less than 15 years
    - is suspected to have ischemic heart disease or cardiac autonomic neuropathy.
  • Perform neurologic and ophthalmologic examination.
  • Individualize the exercise activity depending on the patient's medical complications. (For example, physicians may restrict jogging in patients with peripheral neuropathy or may restrict isometric and upper-body exercises in patients with hypertension. However, resistance exercises may be performed by younger patients without diabetic complications).
  • Tell the patient to always include a warm-up and cool-down period and to gradually increase the intensity of exercise.

Conclusion
In summary, exercise for patients with diabetes should be thought of like any prescribed agent. An individualized exercise program should be prescribed by a knowledgeable caregiver for a knowledgeable patient. This will help to provide maximum benefit with minimum risk to the patient with diabetes.

For further reading:

  1. Kanj H., Schneider SH and Ruderman NB. 1988. Exercise and Diabetes Mellitus in "Exercise and Sport Sciences Reviews." Volume 16, eds. Pandolf, Kent. Macmillan Publishing Co. New York.
  2. The Physician's Guide to Type 2 Diabetes - Diagnosis and Treatment. Published by the American Diabetes Association.
  3. Diabetes Mellitus and Exercise. eds. J.T. Devlin, E.S. Horton and M. Vranic. 1992. Smith Gordon Publishers, London.

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