Exercise plays an important role in the prevention and treatment of type 2 diabetes.
Diabetes was the sixth leading cause of death in 2000, and currently affects more than 17 million people in the United States. The disease has a strong association with obesity, hypertension, stroke, coronary heart disease, intermittent claudication and microvascular diseases (e.g., retinopathy, nephropathy, neuropathy and infection), much of which is preventable. When a person has diabetes, the body does not produce insulin (type 1) or properly use insulin (type 2). Insulin is a hormone that functions to regulate blood glucose levels by transporting blood glucose into cells to be stored and used for energy production. With these conditions, the glucose in the blood can't be used properly and builds up in the bloodstream, despite the muscles and other tissues feeling "starved" for energy. Individuals with diabetes require insulin injections and/or medication that either mimics insulin's actions, or makes tissues more sensitive and responsive to insulin. While at least three specific forms of diabetes have been identified (e.g., type 1, type 2 and gestational), type 2 diabetes is certainly the most common, accounting for approximately 90 to 95 percent of all cases. Based on data from numerous investigations performed over the past several years, it appears that while genetic factors likely play a role, environmental factors (e.g., physical inactivity and poor dietary habits) are largely responsible for the striking rise in this disease. In fact, type 2 diabetes is nearing epidemic proportions, due to a greater prevalence of obesity, poor dietary habits and sedentary lifestyle. Note: Much of the information in this article applies to the role of exercise in the prevention and treatment of type 2 diabetes, although the majority of information also has relevance for those with the type 1 form of the disease. Readers are encouraged to see the section on additional readings for more specific information.
ExerciseThe available evidence clearly shows that regular physical activity is of crucial importance for all individuals with diabetes (more precisely, for all individuals). Although people with type 1 diabetes will certainly benefit from regular exercise training, it should be understood that exercise will not prevent or cure the disease, as type 1 diabetes is auto-immune related. However, data indicate that those who are more physically active are far less likely to be diagnosed with type 2 diabetes, suggesting that exercise may indeed act as a preventive strategy for this form of the disease. This is likely because exercise acts in a similar manner as insulin, in allowing glucose to be taken up from the blood into cells. In other words, an acute bout of exercise can help to transport glucose into cells, clearing it from the bloodstream. This degree of clearance seems dependent on the amount of muscle mass used, and the type, intensity and duration of exercise. Because exercise reliably (more so in obese than lean individuals) improves insulin sensitivity, in addition to favorably affecting other cardiovascular risk factors associated with this disease, it is viewed as one of the principle therapies in the treatment of diabetes. This effect is also observed in non-diabetic individuals, or those with "pre-diabetes," and may help to control "insulin resistance syndrome/Syndrome X." While the effect of exercise is certainly welcome, it should be understood that the changes are short-lived, and are generally lost within hours to a few days following an exercise bout. This observation indicates that regular exercise (perhaps daily or multiple daily sessions) is warranted for optimal glucose control and disease management. In addition to this acute effect of exercise, physical activity promotes other beneficial outcomes in those with diabetes, which may directly affect secondary complications associated with the disease. These include improved mood and lower levels of stress (which independently regulate blood glucose), lower resting and submaximal heart rate and blood pressure, lower triglycerides and total and LDL cholesterol levels, and successful weight management.
Getting startedAs with anyone starting an exercise program, clients with diabetes should undergo a thorough screening procedure prior to beginning. This should include a clinically supervised graded exercise test for individuals older than 35 and for those with longstanding diabetes (more than 10 years), especially in patients with autonomic neuropathy, as they are more prone to altered heart rate and blood pressure regulation and have a greater risk of silent myocardial ischemia. Additionally, all individuals should seek the help of a qualified clinical exercise physiologist who can both write an individualized exercise prescription and monitor progress. Perhaps the biggest concern in those who are diabetic and engage in exercise training is the risk of exercise-induced hypoglycemia, which may occur during or following an acute exercise bout. As general guidelines, people should consume 15 to 20 grams of carbohydrate for each hour of moderate-intensity activity; however, this amount should be adjusted based on need. Therefore, having food and drink (mainly carbohydrate) available and ready to consume is important, as well as a glucometer in order to adjust both the nutrient ingestion and medication dosage. The management protocol should aim to balance activity level, nutrient intake and medication dosage so that blood glucose levels stay within a safe range (this appears to be within 100 to 200mg/dl). If fasting glucose is greater than or equal to 250 mg/dL with the presence of ketones (monitored through urine test strips), or greater than or equal to 300 mg/dL without the presence of ketones, exercise should be avoided. If glucose is less than 100 mg/dL prior to exercise, carbohydrate (20 to 30 grams) should be consumed. The best precautions are for clients to monitor and record blood glucose levels before, during and following activity, especially when first beginning a program. If exercising outdoors, it may be a good idea for clients to do so on both warm, humid days, as well as on cool, dry days, as temperature and humidity will alter hydration status and perceived level of exertion. Note that dehydration can affect blood glucose levels and heart function adversely. Therefore, exercisers should stay well-hydrated. This may be best achieved by consuming plenty of cold fluids before, during and after exercise sessions. If exercising for prolonged periods (more than one hour), a diluted (5 to 6 percent) carbohydrate beverage should be considered. However, it is important to adjust the amount of carbohydrate based on blood glucose levels. Some exercisers may need more and some may need less carbohydrate. While these are general guidelines, it is important for each individual to determine which protocol of activity, nutrient intake and medication works best for them. With practice, this can be achieved easily, and most individuals can participate safely in all of the activities that they enjoy.
Exercise prescriptionBecause many of the cellular adaptations that occur through exercise training are muscle specific, it follows that a program aimed at improving glucose control should involve all major muscle groups, and be performed using differing modalities (e.g., walking, cycling, swimming, weight training, etc.). In addition, a low-intensity warm-up and cool-down of five to 10 minutes (including stretching exercises) should always precede and follow each exercise bout. Both aerobic exercise and weight training could be performed on the same day, or they can be alternated. Of main importance is the consistency of the physical activity. Aside from benefiting metabolically, regulating medication and food intake in order to maintain stable glucose levels will be much easier if the activity pattern remains relatively consistent from day to day. Aerobic. A general aerobic (walking, cycling, swimming, etc.) conditioning program should consist of exercising three to five days per week, for 20 to 60 minutes per session, at a light to moderate intensity (e.g., 50 to 80 percent heart rate reserve: [220 - age - resting HR] x 0.50-0.80 + resting HR). Both the duration and intensity should be progressed cautiously based on individual tolerance. The amount of exercise may be spread into multiple daily sessions, if preferred (e.g., 15 minutes, three times per day). However, keep in mind the importance of consistency. Diabetics should strive for a total energy expenditure equal to 1,000 to 2,000 kcal per week, with the higher end being reserved principally for those needing to lose weight. Anaerobic. Aside from commonly prescribed aerobic exercise, it is also beneficial to engage in regular resistance exercise (weight training) aimed at increasing the degree of lean body mass (muscle) and decreasing body fat. This form of exercise will aid in kcal expenditure, and will have a similar effect on blood glucose regulation as aerobic exercise. In addition, the diabetic exerciser can also expect to experience an increase in muscular strength and endurance, and an enhanced sense of vigor. A general resistance-exercise program should consist of one or two sets of 10 to 15 repetitions to near fatigue for all of the major muscle groups. This should be performed two or three times per week, with two days of rest between each session. It is important to note that there are numerous ways to design an effective program aimed at increasing strength and lean body mass. The exercises chosen, number of sets and repetitions, length of rest intervals, frequency and duration of sessions, etc., should be specific to the individual, and should be changed at least once every four to six weeks for optimal progress. (Refer to the section on additional readings for more specific information.)
Potential concernsWhile it is well-established that exercise has both positive acute and chronic effects for those with diabetes, it also has to be used cautiously under some circumstances. First, as with anyone beginning an exercise program, a physician should perform a complete examination. For the diabetic, this should include a screening for both macro and microvascular complications that may be worsened by exercise, in addition to a graded exercise test when indicated. Further, all individuals should wear an ID tag indicating their condition and emergency contact information. Exercise-induced hypoglycemia is a concern, but can be controlled by taking the appropriate precautionary measures (e.g., having carbohydrate available and using when needed, checking blood glucose regularly, remaining well-hydrated, performing insulin injections in non-exercised muscles, adjusting the medication dosage on exercise days). Additionally, those with extensive retinopathy should avoid strenuous or pounding exercise, such as heavy resistance training and running. Further, those with peripheral neuropathy should pay close attention to their foot care (i.e., appropriate shoes and socks to keep feet dry), and should consider using non-weight-bearing exercise modes such as cycling and swimming to avoid skin and joint complications that may go undetected if they have impaired protective sensation. Infections can be lessened with appropriate care and routine foot checks. Lastly, those with autonomic neuropathy should have clinical exercise testing performed prior to beginning an exercise program, and should avoid exercise in extreme environments due to problems with thermoregulation. Beyond these more common concerns, individuals may also have their own specific limitations and needs. It is up to each person to be responsible enough to become educated on his or her condition and to take the appropriate precautions.
A quick note on dietThere exists a vast literature on the role of small, frequent meals (often referred to as a "nibbling" dietary pattern) in the management of blood glucose and insulin levels, information that all diabetic patients and their physicians should be aware of. Briefly, consuming the typical "two to three meals per day" plan is often a recipe for poor blood glucose control and disease management. The literature supports the role of smaller, macronutrient- balanced (protein, carbohydrate, fat), unprocessed, fiber-rich meals spread evenly throughout the day for optimal disease management. Such knowledge should be integrated into a well-rounded fitness program.
SummaryExercise training in the diabetic individual may prevent, delay or somewhat correct the adverse cellular changes that impair glucose regulation. Beyond this, the benefits of regular physical activity to improve cardiovascular function, muscular strength and endurance, psychological status and body composition have been well-documented. As such, regular exercise should be accepted and endorsed as an absolute must for all diabetics. It will improve their quality of life and reduce the risk of diabetic-related complications. It is crucially important for all to understand the fundamental role that exercise plays in both disease management and in controlling disease progression.
Additional Reading Please note that this article should simply serve as an overview. For additional information, readers are referred to the following: ACSM. Position Stand: Exercise and Type 2 Diabetes. Medicine and Science in Sports and Exercise 32(7): 1345-1360, 2000. ADA/ACSM. Joint Position Statement: Diabetes Mellitus and Exercise. Medicine and Science in Sports and Exercise 29(12), 1997. Colberg, S., and E. Horton. The Diabetic Athlete. Human Kinetics: Champaign, Ill., 2000. Gordon, N.F. Diabetes: Your Complete Exercise Guide (The Cooper Clinic and Research Institute Fitness Series). Human Kinetics: Champaign, Ill., 1993. Hornsby, W.G., and A.L. Albright. Diabetes. In ACSM's Exercise Management for Persons with Chronic Diseases and Disabilities, pp. 133-141, Durstine and Moore (eds). Human Kinetics: Champaign, Ill., 2003 (2nd ed.). Ivy, J.L., T.W. Zderic and D.L. Fogt. Prevention and treatment of non-insulin dependent diabetes mellitus. Exercise and Sport Science Reviews 27: 1-35, 1999.