Exercise in Diabetes Management
Maximizing Benefits, Controlling Risks
Russell D. White, MD, with Carl Sherman
Series Editor: Nicholas A. DiNubile, MD
THE PHYSICIAN AND SPORTSMEDICINE - VOL 4 - NO. 27 - APRIL 1999
In Brief: Exercise is a recommended component of diabetes management. In both type 1 and type 2 diabetes mellitus, exercise can increase insulin sensitivity, lower blood glucose, and have positive psychological effects. More markedly in type 2 than in type 1, regular physical activity improves glycemic control, reduces hypertension, and normalizes lipids. Adjustments in insulin dosage, careful blood glucose monitoring, and attention to diet around the time of exercise will help prevent hypoglycemia and hyperglycemia, which are common hazards of exercise in type 1 diabetes. Special precautions are necessary for those who have diabetic complications such as retinopathy or peripheral neuropathy.
Though exercise is highly beneficial to patients who have diabetes mellitus, its potential complications can make it a complex treatment modality. With regular exercise, overall well-being improves, cardiovascular risk factors are reduced, and hyperglycemia is better controlled: These sequelae argue for the inclusion of exercise in diabetes management. The American Diabetes Association (ADA) concludes its position statement on exercise with the injunction that "all patients with diabetes should have the opportunity to benefit from the many valuable effects of exercise" (1).
At the same time, exercise presents risks for some patients, imposing the need for specific activity precautions, careful planning of the timing of physical activity, and realistic limits on the duration and type of activity.
The clinical picture differs between types 1 and 2 diabetes. The former patients often want to exercise but sometimes should not, while the latter almost always should exercise but often don't want to. The exercise prescription must be tailored to the disease, the individual, and his or her fluctuating condition.
The benefits of exercise for people who have diabetes are many (see "Exercise in Practice," below).
Decreased cardiac risk. One widely recognized benefit of exercise is at least as important for diabetic individuals as for the population as a whole: the reduction of risk factors related to cardiovascular disease. Patients who have type 2 diabetes have a twofold to fourfold increase in such risk. Contributing to this risk is dyslipidemia: These patients typically have low high-density lipoprotein cholesterol, elevated triglycerides, and, in some cases, elevated low-density lipoprotein cholesterol (2). Regular physical activity changes these lipid fractions in a favorable direction. Blood pressure decreases with exercise (3). High insulin levels cause hypertrophy of the tunica media—the middle, muscular layer of the vascular wall; this hypertrophy is associated with the sustained hypertension so common in type 2 diabetes. Exercise increases insulin sensitivity, causing insulin levels to decrease. The hypertrophic effects are then lessened, potentially decreasing blood pressure values. Exercise also improves certain coagulation parameters. Improved fitness and exercise have been associated with a decreased incidence of coronary disease in the general population, but direct evidence of such a decrease among patients who have diabetes has not appeared (4).
Weight loss. Desirable for many patients who have type 2 diabetes, weight loss improves metabolic parameters and reduces cardiovascular risk. Exercise has been shown to be a useful adjunct to diet for this population (5). In addition, the psychological benefits associated with exercise—improvements in mood, self-esteem, and quality of life (6)—are particularly important for people who have a chronic disease such as diabetes. Having exercise as part of their treatment regimen allows patients to take an active, positive role in management of their disease.
Improved glucose utilization. That exercise may have a direct therapeutic effect in diabetes has been recognized since ancient times (7). Its ability to lower blood sugar was noted in the preinsulin era, and it was used to reduce the need for insulin in the mid-1920s (8). That exercise can increase insulin sensitivity, in diabetic as well as nondiabetic individuals, was established a quarter century ago (9).
Exercise helps patients use their endogenous insulin more effectively. Improvements in glucose tolerance tests have been shown in the type 2 diabetic with as little as 1 week of aerobic training (10). Improved glycemic control, reflected in glycosylated hemoglobin or fasting glucose measures, has also been documented after 6 to 12 weeks of an aerobic exercise program (11).
In large part, the enhanced glycemic control appears to result not only from the acute effects of exercise—increased skeletal muscle glucose disposal and repeated cycles of glycogen depletion and resynthesis—but also from a true training effect. A single exercise session has been shown to increase insulin sensitivity for 16 hours (12) or longer (13,14), and a particularly intense bout (85% of VO2 max, continued to exhaustion) to increase glucose utilization, 12 hours later, to a level comparable to what is achievable after 12 weeks of moderate training (11). However, insulin sensitivity appears to correlate with capillarization (15), and increased capillary density (as well as a reduction in diffusion barriers) may also enhance positive metabolic changes in obese diabetics who exercise (16). With repeated exercise, muscle hypertrophy (with increased glucose metabolism) and increases in VO2 max occur.
The disease-modifying benefits for patients with type 1 diabetes are not as marked. The same increase in insulin sensitivity occurs, and insulin requirements are reduced with physical training (17). But there is often no corresponding improvement in glycemic control. This may reflect dietary changes that come with physical activity. One investigator tracked food consumption in patients with diabetes who participated in a cycling program and found increased calorie intake on exercise days (18).
Enhanced socialization. Exercise and organized sports allow diabetic patients to participate in social activities. Children (for whom exercise-management guidelines are the same as for adults) want to participate in activities like youth soccer, Little League baseball, or biking trips. Mature diabetic patients enjoy tennis, swimming, racquetball, and other sports with friends or colleagues. This active participation promotes socialization, peer acceptance, and personal esteem.
Hypoglycemia. The principal risk of exercise among those who have diabetes is hypoglycemia; it is of greater concern for patients who have type 1 diabetes than for those who have type 2. In nondiabetic individuals, increased glucose utilization during exercise triggers an array of homeostatic responses. Insulin declines, and concentrations of counterregulatory hormones (glucagon, growth hormone, catecholamines and cortisol) rise, increasing hepatic gluconeogenesis. But in the person who has type 1 diabetes, there is no endogenous source of insulin to modulate, and the counterregulatory mechanisms are likely to be impaired, particularly after 5 years or more of diabetes (12,19,20).
Exercise also appears to enhance the absorption of exogenous insulin, raising the risk of hypoglycemia still further (21). This may depend in part on the injection site: Absorption increases most from exercising extremities. The abdomen is therefore advocated for insulin administration, as absorption is more predictable from this site than from the extremities.
Other strategies for avoiding exercise-induced hypoglycemia include adjusting insulin dose and/or food intake before exercise, ingesting carbohydrate snacks during sustained activity, and monitoring glucose levels scrupulously (even, in some cases, during the activity and with added frequency after the activity is stopped) (3). This approach has been shown to be effective even for athletes who engage in strenuous endurance exercise such as competitive cross-country skiing (22).
Although normal metabolic responses to exercise may be impaired in type 2 diabetes (plasma insulin may not decline, and liver glucose production may not increase), the resulting drop in blood glucose rarely approaches the point of hypoglycemia. It can do so, however, particularly in those taking sulfonylureas or requiring exogenous insulin (23). Adjustments in food intake and insulin and medication dose may be required.
Delayed hypoglycemia. Delayed hypoglycemia is a more insidious danger. It often occurs at night, 6 to 15 hours after exercise, but may develop as long as 28 hours after exercise. One prospective study (24) found that 48 of approximately 300 young patients with type 1 diabetes had at least one such episode in a 2-year period. It was not limited to those in excellent metabolic control, and it most often occurred in individuals who increased their training regimen.
This phenomenon reflects both increased glucose uptake and glycogen synthesis in skeletal muscles (the residual effect of exercise-heightened insulin sensitivity) and subsequent hepatic glycogen synthesis in stores depleted by exercise (25). Increased food intake, reduced insulin dose, and careful blood glucose monitoring in the period after exercise may forestall delayed hypoglycemia. Because glycogen is replenished more slowly in liver than in muscle, carbohydrate requirements may be increased for up to 24 hours after prolonged exercise (3). This phenomenon is usually caused by inadequate replenishment of glycogen stores immediately postexercise and in the ensuing hours.
Hyperglycemia. The opposite crisis, hyperglycemia, is a risk for patients who are in poor metabolic control. Without adequate insulin, muscle cells cannot utilize glucose during exercise, glucagon-induced production of glucose from the liver is unopposed, and fatty acids are mobilized to supply fuel. The result: increasing hyperglycemia, ketosis, and acidosis (16). This is one reason blood glucose monitoring before exercise is essential.
Patient selection. Virtually all individuals who have diabetes can benefit from exercise (see the Patient Adviser, "Exercising With Diabetes: Tips, Strategies, and Precautions," page 77). The potential gains are such that physicians should actively support the efforts of patients who have diabetes to increase physical activity to the extent of their capacity. In some cases—for example, for an obese individual with type 2 disease—regular exercise should be aggressively promoted as one of the foundations of management. Some physicians recommend that patients begin their exercise program in a monitored situation such as a wellness center, or with a partner.
Preparticipation evaluation. The pre-exercise examination includes a careful assessment for vascular and nervous system complications that could be worsened by certain activities. Watch for signs and symptoms of peripheral arterial disease, including intermittent claudication, cold feet, decreased or absent pulses, subcutaneous tissue atrophy, and hair loss (1). Diminished deep tendon reflexes, monofilament sense, vibratory sense, and/or position sense in the feet may signal peripheral neuropathy. Careful evaluation of the feet for ulceration is also important. Eyes must be examined for diabetic retinopathy according to ADA schedules.
Assessment for autonomic neuropathy includes history of heat intolerance, dependent edema, impotence, and hypoglycemic unawareness. Specific testing includes resting tachycardia, fall in systolic blood pressure within 2 minutes of standing (greater than 20 mm Hg is abnormal; greater than 30 mm Hg is diagnostic), R-R interval variation (less than 10 beats per minute is abnormal when the patient lies supine and breathes six times per minute), and lack of pupil dilation in a dark room.
Stress testing is generally recommended for individuals at risk for heart disease who anticipate engaging in moderate to intense exercise. Among patients with diabetes who are considered at increased risk are those who are older than 35 years; have had type 2 diabetes longer than 10 years or type 1 longer than 15 years; have any additional risk factor for coronary artery disease; or have microvascular disease, peripheral vascular disease, or autonomic neuropathy (1).
Recommended activities. Most of the research documenting the benefits of exercise among patients with diabetes involves aerobic activity. A workable goal for many patients is aerobic exercise at 50% to 70% of VO2 max for 20 to 60 minutes per day, 4 to 7 days per week. Exercise sessions should begin with a gentle 5- to 10-minute warm-up and stretching of the muscles to be exercised, and conclude with a 5- to 10-minute cool-down period.
Nearly all people who have diabetes can incorporate upper-body strength training with light weights and high repetitions. More strenuous strength training may be acceptable for young people who have diabetes, but it is not recommended for older patients or those who have long-standing diabetes.
Patients are most likely to continue in programs that involve activities they enjoy. Working with patients to find activities that are pleasant, safe, and effective will go a long way in encouraging compliance.
Both patient and physician must be aware that diabetes and its sequelae may make some types of exercise inadvisable or even dangerous. Meticulous attention to diabetes management factors—blood glucose monitoring, diet, insulin, and medication—is the key to trouble-free exercise.
For all patients. Blood glucose monitoring is an essential part of the exercise prescription. Patients should monitor their blood sugar before and after exercise, and (for those with type 1 diabetes) every 20 to 30 minutes during prolonged exercise.
According to the ADA (1), patients should avoid exercise if glucose levels exceed 300 mg/dL (or 250 mg/dL with ketosis), and take a carbohydrate snack before exercise if levels fall below 100 mg/dL. I, however, recommend postponing exercise in type 1 individuals when glucose exceeds 200 mg/dL. With higher glucose levels, osmotic diuresis occurs and produces deyhydration. This results in a worsening metabolic state and poor performance. Instead, patients should correct dehydration and the insulinopenic state before exercising.
Patients who have autonomic dysfunction may need to be reminded that they will not have the symptoms (eg, tachycardia, sweating) that might ordinarily warn of hypoglycemia.
Meal timing is important. Exercise should be scheduled 1 to 2 hours after a meal, or when insulin is not at its peak activity. Morning exercise is ideal; those who have type 1 diabetes should avoid evening exercise when possible.
While food intake before exercise does not usually need to be increased if insulin is adjusted appropriately, carbohydrate snacks should be taken during prolonged activity or at any sign of hypoglycemia. To prevent delayed hypoglycemia after exercise that is intense or prolonged enough to deplete glycogen reserves, the patient must adequately replenish glycogen stores within 1 to 2 hours of exercising, then maintain the levels in the ensuing 24 hours.
Adjustment of medication dosage is often critical. A multidose insulin regimen makes it easier to adjust dosage when exercise is anticipated. Generally, short-acting insulin should be decreased by 30% before exercise of less than 1 hour, 40% if 1 to 2 hours, and 50% if greater than 3 hours. Insulin lispro (an insulin analogue), with its immediate onset and short half-life, makes it easier for patients to avoid hypoglycemia. Patients using an insulin pump may try reducing the basal rate by 50% 1 to 2 hours prior to and during exercise. If exercise is planned immediately after a meal, they should reduce the premeal bolus by 50%.
Patients who have type 2 diabetes may need to lower their dose of sulfonylurea medications as their insulin sensitivity improves. Other oral antidiabetic drugs are not as likely to contribute to exercise-induced hypoglycemia, but agents such as acarbose (oral alpha-glucosidase inhibitors) block absorption of disaccharides, so hypoglycemia, should it occur, will require ingestion of glucose- or fructose-containing foods or drinks. Patients who become dehydrated while taking metformin hydrochloride can develop lactic acidosis.
Adequate hydration is important for anyone before and during sustained exercise, but particularly for patients who have diabetes, given the adverse impact of dehydration on blood glucose and cardiovascular function.
Alcohol is a powerful inhibitor of gluconeogenesis, and its effects can persist 20 hours after consumption. Patients should be counseled not to drink alcohol at night if they plan to exercise the following morning.
The preparticipation evaluation is a good time to ensure that patients choose proper footwear for exercise. To prevent blisters and other injuries, shoes should fit well, have cushioned midsoles (silica gel or air), and have a toe box that is wide and that extends in length beyond the longest toe. Socks also should fit well and be made from material that wicks moisture to keep the feet dry.
When exercising, patients should wear their diabetes identification shoe tag or bracelet in such a way that it is clearly visible.
Specific concerns. Patients who have any degree of autonomic dysfunction should be taught to monitor exercise intensity by perceived exertion rather than heart rate.
Those who have autonomic neuropathy are at increased risk of adverse cardiovascular events during exercise. Any dizziness, weakness, or shortness of breath should alert the physician to the possibility of cardiac disease. Such patients should complete an exercise stress test and have monitored exercise sessions. Orthostatic hypotension is common in these patients after upright exercise; thus, bicycling or swimming is more appropriate than running or brisk walking. Autonomic neuropathy may also disrupt thermoregulation; avoiding exercise in heat and cold, along with careful attention to hydration, is important.
Peripheral neuropathy increases the danger of injury to the feet. If the condition is mild, suitable footwear can provide sufficient protection, but if it is more severe, weight-bearing exercise is best avoided. Swimming, bicycling, and rowing are among the physical activities appropriate for those who have loss of protective sensation.
Patients who have retinopathy should be wary of strenuous activities that can precipitate hemorrhage or retinal detachment. This warning applies particularly to activities that cause sudden increases in blood pressure, such as weight lifting or hill sprints. Patients who have more severe disease require further restriction: For example, jogging and racket sports are discouraged for those who have proliferative diabetic retinopathy.
When there is any risk of hypoglycemia, activities that could be acutely dangerous in the context of impairment—scuba diving or rock climbing, for example—should be avoided.
Finally, it must be borne in mind that each individual has his or her own metabolic response to exercise, which is extremely important to recognize in the diabetic athlete. No general guidelines can take the place of intelligent self-observation and regular glucose monitoring in developing an individualized plan to facilitate safe, enjoyable exercise.
Exercise in Practice
The practical benefits (and limitations) of exercise for patients who have diabetes are suggested by a study (1) of 200 patients with type 2 and 55 with type 1 disease who enrolled in a physical training program. The protocol consisted of at least 30 minutes of aerobic exercise, 3 to 4 days per week.
At 3 months, VO2 max significantly increased among both patient groups; blood pressure and heart rate during exercise declined. Modest reductions in body weight were observed in the type 2 group, who also had significant reductions in fasting plasma glucose, glycosylated hemoglobin, and triglycerides. Among the patients who had type 1 diabetes, insulin requirements fell by 9%. (Other studies demonstrate that glycosylated hemoglobin levels do not improve with regular exercise in those who have type 1 diabetes (2).)
While virtually all insulin-treated patients experienced at least one episode of hypoglycemia, only two elderly individuals required hospitalization. Overall, 12% of patients had exercise-related injuries that required at least 1 week away from the program.
Compliance was problematic, particularly among individuals who were referred to the program by their physicians: Only 32% of referred patients participated fully after 3 months, compared with 72% of self-referrals.
Dr White is associate director of the family practice residency and director of the sports medicine fellowship program at Bayfront Medical Center in St Petersburg, Florida. He is a member of the American College of Sports Medicine and the American Medical Society for Sports Medicine and serves as a crew chief for the US Olympic Committee. He has had type 1 diabetes for 40 years without complications. Mr Sherman is a freelance writer in New York City. Dr DiNubile is an orthopedic surgeon in private practice in Havertown, Pennsylvania, specializing in sports medicine and arthroscopy, and is director of Sports Medicine and Wellness at the Crozer-Keystone Healthplex in Springfield, Pennsylvania. Address correspondence to Russell D. White, MD, Bayfront Medical Center, 700 Sixth St S, St Petersburg, FL 33701-4891: e-mail to [email protected].