By Corey Ellis, MD
Member AMSSM
Wright State Orthopedics and Sports Medicine - Dayton, OH

According to the American Diabetes Association (ADA), 23.6 million people have Diabetes Mellitus (DM), of whom 186,300 are under the age of 20. An estimated 57 million more people are thought to have pre-diabetes. The large numbers of people affected combined with the increasing popularity of triathlons means that many participants are bound to be diabetic or pre-diabetic.

What effects does exercise have on diabetes? Immediate benefits of exercise include a decrease in blood glucose levels following exercise, thus decreasing insulin requirements. An exception can occur if a high blood sugar level is present at the onset of exercise. Long term benefits are substantial, including weight loss, improved cholesterol levels, decreasing blood pressure, a perceived improvement in quality of life, and better glycemic control.

Multiple studies have indicated that glycemic control is improved with physical activity. For example, subjects who participated in exercise regimens 30-60 minutes long, three to four times per week, and done at 50-80% intensity measured by VO2 max had HbgA1c levels decreased 10-20% from baseline, with greater improvements generally seen in DM type II.

Cardiovascular disease is known to be multifactorial, but recent studies have shown that physical activity alone can be a predictor of cardiovascular mortality in men with DM type II. Other specific risk factors of cardiovascular disease are also affected by exercise. Cholesterol improvements have been seen, but not consistently. Diabetic patients with hypertension would also benefit by lowering blood pressure. Further studies have gone on to show physical activity alone, independent of BMI, affects overall mortality in men with DM type II. Most studies involving weight loss include dieting, but still support physical activity as a role in weight loss. Physical activity minimizes the effect of other complications, including retinopathy and neuropathy. These studies suggest that physical activity, even without weight loss, is beneficial.

Training guidelines for diabetics can be extensive, but allow safe participation when possible. Type I and Type II diabetics taking oral hypoglycemic medications and/or insulin to lower blood glucose levels are at increased risk for hypoglycemia. Type I diabetics are also more prone to ketoacidosis due to high blood glucose levels. For a safe start in triathlons, good blood glucose control should be established prior to start of any exercise program. Daily doses of medications, including insulin, should be stable. Blood glucose should be measured before, after, and sometimes during exercise. This will allow the athlete to make appropriate changes to doses as requirements change.

I like to recommend checking blood glucose during exercise if it will last longer than two hours for moderate intensity, or longer than 1 hour for high intensity. Once exercise starts, blood glucose levels generally decrease as glucose is used as an energy source. But if blood glucose level is greater than 250-300 mg/dl at the start, blood glucose levels may actually rise during exercise due to counter-regulatory hormones. Depending on the level of intensity and length of activity, actions may need to be taken to prevent hypoglycemia or hyperglycemia. The following graph may be used as an initial guide, but should be adjusted based on the experience of the individual.

For athletes taking insulin, experience will prove to be helpful in dosing prior to and during exercise. As a general guide, decrease insulin dose that will peak during planned exercise. This may be long or short acting insulin. Hypoglycemia is more common in Type I diabetics, especially if dosing insulin multiple times per day. Symptoms of hypoglycemia include sweating, nervousness, hunger, headache, and may progress to confusion or loss of consciousness. Avoid exercising muscles injected with short-acting insulin for 1 hour. Consider exercise in the morning or during the day. Avoid evening exercise, as evening exercise will decrease insulin need over night or even next day, possibly inducing hypoglycemia. Extra monitoring may be needed through the night.

Despite the best efforts of athletes, complications can occur. Precautions should be taken to prevent serious harm. Athletes should carry identification as someone with diabetes. Easily digested carbohydrates should be available in case of emergency. Gels and tablets are good ways to rapidly raise blood glucose. Continuing to monitor is important to make changes to medication dosage. Reduction in overall insulin dose is common, especially in first few months.

Patients with resting heart rate greater than 100 bpm may have autonomic neuropathy, which is more common in long standing diabetics. Other indications include abnormal thermoregulation and blood pressure responses during postural changes. Caution should be taken with autonomic neuropathy because of increased risk of silent heart attack and sudden death. Peripheral neuropathy may cause decreased sensation of the feet. For these athletes, it is important to wear comfortable shoes and do regular inspections for trauma or wounds.

References and Additional Sources
 
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 2. Church TS, Cheng YJ, Earnest CP, Barlow CE, Gibbons LW, Priest EL, Blair SN. Exercise capacity and body composition as predictors of mortality among men with diabetes. Diabetes Care. 2004 Jan;27(1):83-8.
 3. Diabetes Prevention Program Research Group: Reduction in incidence of type 2 diabetes with lifestyle intervention or metformin. N Engl J Med 346:393-403, 2002
 4. Kanaya AM, Narayan KM. Prevention of type 2 diabetes: data from recent trials. Prim Care. 2003 Sep;30(3):511-26.
 5. Mellion M, Walsh WM, Madden C, Putukian M, Shelton G. Team Physician's Handbook 3rd Ed. Hanley & Belfus, Inc: Philadelphia, 2002
 6. Ruderman N, Devlin JT, Schneider SH, Krisra A, Eds. Alexandria, VA, American Diabetes Association, 2002.
 7. Tuomilehto J, Lindstrom J, Eriksson JG, Valle TT, Hamalainen H, Illanne-Parikka P, Keinanen-Kiukaaniemi S, Laakso M, Louheranta A, Rastas M, Salminen V, Uusitupa M: Prevention of type 2 diabetes mellitus by changes in lifestyle among subjects with impaired glucose intolerance. N Engl J Med 344:1343-1350, 2001.
 8. WWW.diabetes.org
 9. Zinman B, Ruderman N, Campaigne BN, Devlin JT, Schneider SH; American Diabetes Association. Physical activity/exercise and diabetes. Diabetes Care. 2004 Jan;27 Suppl 1:S58-62.