Tailoring Exercise to Optimize Fitness and Glycemic Control

Krishna V. Bhaskarabhatla, MD, MSc; Richard Birrer, MD, MPH

THE PHYSICIAN AND SPORTSMEDICINE - VOL 32 - NO. 1 - JANUARY 2004

In Brief: Physical activity decreases insulin resistance and can aid in both preventing type 2 diabetes mellitus and managing the disease. Before patients with type 2 diabetes begin an exercise program, they should undergo a preparticipatory clinical assessment and screening for micro- and macrovascular disease. Patients older than 35 and those with additional cardiovascular risk factors, peripheral vascular disease, long-standing disease, or peripheral neuropathy should have an exercise stress test. Adequate glycemic control and selection of appropriate physical activity, such as brisk walking or swimming, are essential to avoid associated complications. Patients on diabetes medications should monitor blood glucose levels and adjust their diet to minimize fluctuations in blood glucose during exercise.

Type 2 diabetes mellitus is a costly and serious disease that afflicts 8% of adults in the United States.¹ Insulin resistance, a defect in insulin action, is a strong risk factor for the development of the disease and associated complications. Insulin resistance is frequently associated with hyperinsulinemia, increased blood pressure, and elevated cholesterol levels and both precedes and predicts type 2 diabetes.^2-5 In insulin-resistant states, endothelial nitric oxide synthesis is reduced and insulin-mediated vasodilation is impaired, resulting in abnormal vascular reactivity.^4,5

The diagnosis of type 2 diabetes is often delayed until complications occur. Cardiovascular disease, manifesting as coronary artery disease, is the leading cause of morbidity and mortality among patients with type 2 diabetes, and many of the primary risk factors for coronary artery disease frequently coexist among these patients.¹

Investigators have found an inverse relationship between physical activity and insulin resistance and the subsequent development of type 2 diabetes.^5,6 Recent clinical evidence strongly supports the role of physical activity in the prevention of type 2 diabetes.^7-9 Physical activity increases insulin-mediated glucose disposal rates^10-12 and improves insulin sensitivity, independent of its effect on weight loss and fat distribution.¹³ Increased exercise also improves endothelial nitric oxide release and insulin-mediated vasodilation and substantially lowers elevated blood pressure, an important cardiovascular risk factor in patients with type 2 diabetes and those who have insulin resistance.^14-16 Exercise-associated improvement in dyslipidemia may be related to an overall increase in hepatic lipase, skeletal muscle lipoprotein lipase, and lecithin-cholesterol-acyl-transferase activity.¹⁷ In addition to its central role in preventing type 2 diabetes, physical activity has established itself as an important component of the management strategy for adults with clinical disease.^7,8

Exercise for 30 minutes or more at a moderate level of intensity on most, preferably all, days of the week can improve insulin sensitivity and glycemic control and decrease the need for oral medications or insulin.^18-25 To ensure that any exercise program is optimally safe, effective, and enjoyable, it is important that individuals at risk of type 2 diabetes, or already affected, undergo health screening before beginning any exercise program.

The Preexercise Assessment

The clinical evaluation should pay particular attention to micro- and macrovascular disease and any long-term complications of diabetes that may affect exercise safety or tolerance or exacerbate underlying disease.^18-25 The evaluation must include a neurologic examination to detect peripheral or autonomic neuropathy and a dilated retinal examination to identify proliferative retinopathy. Laboratory tests should include those for fasting blood glucose levels, glycosylated hemoglobin (HbA₁C) and renal function tests, including screening for microalbuminuria. Adults with type 2 diabetes should have a thorough cardiac evaluation before beginning an exercise program. If abnormalities are present, exercises of the appropriate type and intensity should be selected to avoid the significant risk of worsening complications.

An exercise stress test is recommended for patients older than 35 years who intend to start a program of moderate or vigorous exercise. Other indications for an exercise stress test among type 2 diabetics include presence of one or more of the following: disease duration of 10 or more years, additional cardiovascular risk factor(s), peripheral vascular disease, autonomic neuropathy, or micro- or macrovascular complications. An exercise stress test will help identify abnormal heart rate and blood pressure responses, exercise tolerance, and previously undiagnosed ischemic heart disease.^21,23

Avoiding Complications From Physical Activity

Besides findings in the preexercise assessment, several other considerations are important for those with diabetes who participate in physical activity.

Exercise preparation. Patients, especially those who take sulphonylureas or insulin, should monitor blood glucose and urine ketone levels. If their blood glucose levels are less than 100 mg/dL, they should have a snack. Patients should delay exercise until they correct metabolic abnormalities such as greatly elevated blood glucose levels (those exceeding 250 mg/dL) and any associated ketosis.²⁶ During physical activity, fluid should be taken early and frequently in quantities sufficient to compensate for losses from sweating (as reflected in exercise-mediated body weight loss), or the maximal amount of fluid tolerated. Dehydration can have adverse effects on cardiac function and blood glucose levels.²⁰

Glucose metabolism, therapy, and exercise. During physical activity, energy demands escalate, and counterregulatory hormones such as glucagon and catecholamines maintain normal blood glucose levels essential for proper central nervous system function.²¹ Although blood glucose regulation during exercise in patients with type 2 diabetes differs from that in patients without diabetes in several ways, elevated blood glucose concentrations usually decline toward normal levels with moderately intense exercise, and exercise-induced hypoglycemia is rare.^21-24

However, in patients with type 2 diabetes, insulin administered for glycemic control can attenuate or even block the increased mobilization of glucose and other energy substrates induced by physical activity, and hypoglycemia may ensue.²¹

Patients who take sulfonylureas, but not biguanides (metformin), thiazolidinediones (piglitazone, rosiglitazone) or meglitinides (repaglinide), may encounter similar problems with hypoglycemia. Hence, the dosage of sulfonylurea or insulin medications may need to be decreased prior to exercise to prevent hypoglycemia.^21-24,27

In patients with advanced disease, as insulin deficiency ensues, an excessive release of counterregulatory hormones during physical activity may increase already high levels of glucose and ketone bodies and can even precipitate diabetic ketoacidosis.²¹ Patients whose diabetes is treated by diet alone do not need to take supplemental food before, during, or after exercise, except when exercise is exceptionally vigorous or of long duration (eg, marathon, biathlon, triathlon).^21-24 Of important note, patients who are at high risk of hypoglycemia should wear a diabetes identification bracelet that is clearly visible at all times while exercising.

Complications and exercise. Exercise can lead to several other complications besides hypoglycemia. Risks of exercise may include exacerbation of previously undiagnosed underlying cardiovascular disease, with the development of angina pectoris, myocardial infarction, or cardiac arrhythmias.^21-24 Patients with proliferative retinopathy are at risk for retinal or vitreous hemorrhage or retinal detachment when they engage in extremely strenuous activity or exercise that involves Valsalvalike maneuvers.

Minimizing complications. Patients who have peripheral neuropathy should avoid running, jogging, or similar exercises, because these activities place them at greater risk of soft-tissue and joint injuries.^21-24 In patients with a loss of proprioception, the following exercises are considered safe: swimming, bicycling, rowing, chair exercises, arm exercises, and other non–weight-bearing exercises. Prolonged walking, jogging, and step and treadmill exercises are considered unsafe.

Patients who have autonomic neuropathy may have impaired heart-rate (chronotropic) and blood-pressure (inotropic) responses and are at risk of postural hypotension.^21-24 Degenerative joint disease may be exacerbated by weight-bearing exercise.

Those with diabetes must be taught to monitor closely for blisters and potential damage to their feet, both before and after exercise. Proper footwear is essential, especially for those with peripheral neuropathy. Other important measures to minimize trauma to the feet are to use silica gel or air midsoles as well as polyester or blend (cotton-polyester) socks to prevent blisters and keep feet dry.^21-24

Selecting an Appropriate Exercise Program

Exercise programs should contain activities tailored to the individual patient.

General guidelines. An exercise program should consist of moderately intense aerobic exercises that can be sustained for 30 minutes or longer and do not result in a sustained heart rate in excess of 60% to 70% of the person's predetermined maximal rate.^21-24 If the patient does not have hypertension or proliferative retinopathy, some resistance training or high-intensity exercises may also be well tolerated. Each exercise session should incorporate a 5- to 10-minute preexercise warm-up and a 5- to 10-minute postexercise cooldown of low-intensity aerobic exercise (walking, cycling) or slow, rhythmic, and stretching exercises (calisthenics) to prevent musculoskeletal injuries. Depending on the patient's level of physical conditioning, physical activity longer than 30 minutes as tolerated is encouraged.

Choosing the regimen. The preferred mode of aerobic exercise is brisk walking for a minimum of 30 minutes on most days of the week. As physical fitness improves and the patient becomes familiar with the sensations associated with aerobic training, other large-muscle activities, such as swimming, cycling, rowing, cross-country skiing, and aerobic dance, can be introduced to provide variety. These and other activities—for example, stair climbing, hiking, calisthenics, bicycling, rowing, swimming, and sports such as tennis, racquetball, soccer, basketball, and touch football—are especially beneficial when performed regularly.²⁸

Measuring effort. A useful marker to assess the workload performed for an activity is the metabolic equivalent (MET). One MET is the amount of oxygen consumed while sitting at rest (basal metabolic rate) and is 3.5 mL O₂/kg/min.^28,29 The workload of selected activities in METs has been compiled (table 1).²⁹

TABLE 1. Energy Requirements for Selected Physical Activities Activity Workload in METs Calisthenics, walking (3-3.5 mph), noncompetitive softball, badminton (social doubles), archery, fly fishing, horseback riding (trot), small-boat sailing, household activities: mopping, window cleaning, stocking shelves, packing or unpacking light or medium objects, plumbing, raking leaves 3-4 Walking (3.5-4 mph), social dancing, golf (carrying bag), table tennis, badminton (singles), tennis (doubles), noncompetitive baseball, calisthenics (moderate), swimming (light), rowing or canoeing (3 mph), gardening, light carpentry, painting, using power saw on hardwood, pushing a power mower or cart 4-5 Walking (4-5 mph), tennis (singles), scuba diving (warm water), competitive badminton, heavy calisthenics, water skiing, cross-country hiking, moderate swimming, dancing (rhumba, square), snow shoveling, hand lawn mowing, splitting hard wood, carrying or lifting weights (20-29 kg [45-64 lb]), exterior carpentry, backpacking (2.2 kg [5 lb]) 5-7 Heavy labor, handball, squash, running (6-7 mph) >9 MET = metabolic equivalent (3.5 mL O2/kg/min) Adapted from Birrer R: Writing a prescription for exercise, in Birrer R (ed): Sports Medicine for the Primary Care Physician, ed 2. Boca Raton, FL, CRC Press, 1999, p 149.

Strength training. Resistance exercise is necessary to counter the muscle atrophy induced by aging.²⁸ Patients should perform a variety of 8 to 10 resistance activities to work most of the major muscle groups. One to three sets of 8 to 15 repetitions should be undertaken to exercise each major muscle group. Thus, attention should be directed to movement at each of the major joints, including hip flexion and extension, knee flexion and extension, ankle dorsiflexion and plantar flexion, shoulder flexion and extension, and elbow flexion and extension.

Patients do not have to rely on resistance equipment. They can exercise the same joints using inner tubes and elastic bands, cuff and hand weights, free weights, dumbbells, and wall pulleys. Individual contractions should not be held for more than 5 to 6 seconds to avoid a large increase in cardiac afterloading. Such training should be performed at least twice per week.

The Wealth of Benefits

Given the current evidence of efficacy for physical activity in patients with type 2 diabetes, mechanisms for adopting a physically active lifestyle should be a major therapeutic goal and public health priority, along with assuming a prudent diet. Increasing physical activity levels can be achieved by several means, including public health programs and community-based interventions. Much of the required activity can be incorporated into daily routine—for example, a walk to and from a commuter rail station. Individual or group activities are appropriate, and many patients find variety to be important for sustaining interest.

For those new to exercise or with significant complications, supervised exercise programs may be beneficial. Most patients, however, do not require formal supervision once the initial assessment is completed and an appropriate exercise program is established. Healthcare professionals play an important role in educating patients about how to incorporate physical activity into their lives and how to participate safely in an exercise regimen.

References

1. Erkelens DW: Insulin resistance syndrome and type 2 diabetes mellitus. Am J Cardiol 2001;88(7B):38J-42J
2. Lillioja S, Mott DM, Spraul M, et al: Insulin resistance and insulin secretory dysfunction as precursors of non-insulin-dependent diabetes mellitus: prospective studies of Pima Indians. N Engl J Med 1993;329(27):1988-1992
3. Haffner SM, Stern MP, Hazuda HP, et al: Cardiovascular risk factors in confirmed prediabetic individuals: does the clock for coronary heart disease start ticking before the onset of clinical diabetes? JAMA 1990;263(21):2893-2898
4. Caballero AE, Arora S, Saouaf R, et al: Microvascular and macrovascular reactivity is reduced in subjects at risk for type 2 diabetes. Diabetes 1999;48(9):1856-1862
5. Helmrich SP, Ragland DR, Leung RW, et al: Physical activity and reduced occurrence of non-insulin-dependent diabetes mellitus. N Engl J Med 1991;325(3):147-152
6. Eriksson KF, Lindgärde F: Poor physical fitness, and impaired early insulin response but late hyperinsulinaemia, as predictors of NIDDM in middle-aged Swedish men. Diabetologia 1996;39(5):573-579
7. Knowler WC, Barrett-Connor E, Fowler SE, et al: Reduction in the incidence of type 2 diabetes with lifestyle intervention or metformin. N Engl J Med 2002;346(6):393-403
8. Tuomilehto J, Lindstrom J, Eriksson JG, et al: Prevention of type 2 diabetes mellitus by changes in lifestyle among subjects with impaired glucose tolerance. N Engl J Med 2001;344(18):1343-1350
9. Pan XR, Li GW, Hu YH, et al: Effects of diet and exercise in preventing NIDDM in people with impaired glucose tolerance: the Da Qing IGT and Diabetes Study. Diabetes Care 1997;20(4):537-544
10. Rosenthal M, Haskell WL, Solomon R, et al: Demonstration of a relationship between level of physical training and insulin-stimulated glucose utilization in normal humans. Diabetes 1983;32(5):408-411
11. Schneider SH, Amorosa LF, Khachadurian AK, et al: Studies on the mechanism of improved glucose control during regular exercise in type 2 (non-insulin-dependent) diabetes. Diabetologia 1984;26(5):355-360
12. Kriska AM, Pereira MA, Hanson RL, et al: Association of physical activity and serum insulin concentrations in two populations at high risk for type 2 diabetes but differing by BMI. Diabetes Care 2001;24(7):1175-1180
13. Kriska A: Can a physically active lifestyle prevent type 2 diabetes? Exerc Sport Sci Rev 2003;31(3):132-137
14. Petrie JR, Ueda S, Webb DJ, et al: Endothelial nitric oxide production and insulin sensitivity: a physiological link with implications for pathogenesis of cardiovascular disease. Circulation 1996;93(7):1331-1333
15. Stewart KJ: Exercise training and the cardiovascular consequences of type 2 diabetes and hypertension: plausible mechanisms for improving cardiovascular health. JAMA 2002;288(13):1622-1631
16. Roberts CK, Vaziri ND, Barnard RJ: Effect of diet and exercise intervention on blood pressure, insulin, oxidative stress, and nitric oxide availability. Circulation 2002;106(20):2530-2532
17. Lehmann R, Engler H, Honegger R, et al: Alterations of lipolytic enzymes and high-density lipoprotein subfractions induced by physical activity in type 2 diabetes mellitus. Eur J Clin Invest 2001;31(1):37-44
18. Nelson KM, Reiber G, Boyko EJ, et al: Diet and exercise among adults with type 2 diabetes: findings from the third National Health and Nutrition Examination Survey (NHANES III). Diabetes Care 2002;25(10):1722-1728
19. Pate RR, Pratt M, Blair SN, et al: Physical activity and public health: a recommendation from the Centers for Disease Control and Prevention and the American College of Sports Medicine. JAMA 1995;273(5):402-407
20. Zinman B, Ruderman N, Campaigne BN, et al: Physical activity/exercise and diabetes mellitus. Diabetes Care 2003;26(suppl 1):S73-S77
21. Expert Committee on the Diagnosis and Classification of Diabetes Mellitus: American Diabetes Association: clinical practice recommendations 2002. Diabetes Care 2002;25(suppl 1):S1-S147
22. Devlin JT: Diabetes and exercise: the risk-benefit profile, in Ruderman N, Devlin JT (eds): The Health Professional's Guide to Diabetes and Exercise. Alexandria, VA, American Diabetes Association, 1995, p 34
23. Chipkin SR, Klugh SA, Chasan-Taber L: Exercise and diabetes. Cardiol Clin 2001;19(3):489-505
24. Peirce NS: Diabetes and exercise. Br J Sports Med 1999;33(3):161-172
25. Boule NG, Haddad E, Kenny GP, et al: Effects of exercise on glycemic control and body mass in type 2 diabetes mellitus: a meta-analysis of controlled clinical trials. JAMA 2001;286(10):1218-1227
26. Kitabchi AE, Umpierrez GE, Murphy MB, et al: Management of hyperglycemic crises in patients with diabetes. Diabetes Care 2001;24(1):131-153
27. Inzucchi SE: Oral antihyperglycemic therapy for type 2 diabetes: scientific review. JAMA 2002;287(3):360-372
28. Shephard RJ, Balady GJ: Exercise as cardiovascular therapy. Circulation 1999 23;99(7):963-972
29. Birrer RB: Writing a prescription for exercise, in Birrer RB (ed): Sports Medicine for the Primary Care Physician, ed 2. Boca Raton, FL, CRC Press, 1994, pp 143-156

Dr Bhaskarabhatla is an attending physician in the department of family medicine, and Dr Birrer is president and chief executive officer, both at Saint Joseph's Regional Medical Center in Paterson, New Jersey. Address correspondence to Richard Birrer, MD, Saint Joseph's Regional Medical Center, 703 Main St, Paterson, NJ 07503; e-mail to birrer@sjhmc.org.

Disclosure information: Drs Bhaskarabhatla and Birrer disclose no significant relationship with any manufacturer of any commercial product mentioned in this article. No drug is mentioned in this article for an unlabeled use.