The number of cases of diabetes mellitus (DM), a group of metabolic diseases characterized by elevated blood glucose levels (hyperglycemia), is growing to epidemic proportions, both in the United States and around the globe (1). According to recent predictions, the worldwide incidence will likely rise from 171 million people to 366 million by the year 2030, but many fear that this projection is an underestimate because the actual number in 2005 turned out to be far higher than anticipated. Because this chronic disease's potential to cause disabling health complications (such as kidney disease, lower limb amputations, heart disease, nerve damage, and blindness) and early death, it is vital that blood glucose levels be effectively controlled to manage diabetes effectively and prevent or lower the risk of these problems.
Diabetes is related to both insulin and glucose levels in the bloodstream. Normally, when an individual ingests carbohydrates or protein at rest, the pancreas releases insulin, an endocrine hormone made in and released by the pancreatic beta cells that stimulates the uptake of circulating glucose into muscle and fat tissues. Without adequate levels of insulin and insulin action, blood glucose levels can rise to abnormally high levels, contributing over time to the development of health complications. Thus, the primary goal of all diabetes management is effective control of blood glucose within normal or nearly normal levels.
Most cases of DM are either type 1 (T1 DM) or type 2 (T2 DM) (2). Representing 5% to 10% of cases, T1 DM is primarily an autoimmune disease in which the ability to make and release insulin is destroyed by the body's own immune responses, requiring exogenous insulin administration. Type 2 DM, however, accounts for 90% to 95% of all cases and is the result of a combination of reduced insulin action in target tissues (insulin resistance), combined with an inadequate compensatory release of insulin over time. When the pancreas in an individual with T2 DM can no longer produce enough insulin to overcome the resistance to insulin's action, particularly in skeletal muscle, elevated blood glucose levels result (Table 1). People who are insulin resistant without having hyperglycemia are considered to have prediabetes, putting them at high risk for developing T2 DM (or gestational diabetes during pregnancy).
Although additional insulin can be injected with a needle, infused with a pump, or inhaled and absorbed through the lungs, certain lifestyle improvements themselves can prevent T2 DM and better control either T1 DM or T2 DM without the need for extra insulin (although individuals with T1 DM will always have to replace the insulin they cannot make on their own). Physical activity generally enhances the action of insulin, be it naturally released by the pancreas or supplemental.
WHY THE ACTION OF INSULIN MATTERS
Effective control of blood glucose levels relies on the relative balance between insulin and glucose-raising hormones such as glucagon and adrenaline (3), which are released by conditions such as exercise, physical or mental stress, and hypoglycemia (low blood glucose levels). Insulin alone lowers blood glucose levels by stimulating the uptake of glucose into insulin-sensitive cells, where glucose is used for energy or stored for later use. Whereas the brain and the nervous system need a constant supply of glucose, which they can take up without insulin, the insulin-sensitive tissues (primarily muscle, adipose, and liver) require an adequate supply of insulin to enable them to store excess glucose after food intake. An individual who is sensitive to insulin usually needs only relatively small amounts of this hormone to keep glucose levels in the reference range (70 to <100 mg/dL), but someone who is insulin resistant will require substantially more to cause the same glucose-lowering effect.
Insulin resistance, or ineffective insulin action, is associated with numerous health risks. It usually results in hyperinsulinemia, or high circulating insulin levels, which are associated with greater damage to blood vessels and contribute to arterial plaque formation and heart disease. Hyperinsulinemia also has been associated with high blood pressure, obesity (particularly intra-abdominal), osteoporosis (thinning bones), and certain types of cancer like colon, breast, and prostate. However, the culprit in all of these conditions is likely the ineffectiveness of the insulin rather than elevations in circulating levels (4,5). In contrast, most athletic individuals with and without diabetes have lower circulating insulin levels and require less insulin release for carbohydrate and other food intake (6), meaning that their bodies are very sensitive to insulin and their risk for these health problems is substantially reduced.
Although a hallmark of T2 DM, an insulin-resistant state can occur in T1 DM as well. In fact, a growing number of individuals with T1 DM have "double diabetes," meaning that, in addition to not being able to make insulin (characteristic of T1 DM), they also are insulin resistant like most people with T2 DM (7). In their case, improvements in insulin action can result in reduced insulin dosages and better control of blood glucose levels. Fortunately, individuals with diabetes or prediabetes can lower insulin resistance and improve insulin's action by modifying their lifestyles, particularly by improving their exercise habits.
HOW PHYSICAL ACTIVITY IS BENEFICIAL
Almost all physical activity has a positive effect on insulin action. In fact, in comparing the benefits of leading a physically active lifestyle with other lifestyle improvements (e.g., dietary interventions and medications), being active by itself has the greatest benefit on glycemic control and is more effective than attempting a combination of lifestyle improvements that includes exercise (8). However, depending on the type of activity-how hard it is, how long it lasts, and what type of exercise it involves, the improvement in insulin action can vary (9). Almost all activities cause muscles to use some stored glycogen as fuel, but more intense bouts, and longer duration moderate bouts, may affect insulin action for up to 1 to 2 days, the period during which blood glucose is being used to restore muscle glycogen. Less intense activities (e.g., gardening) cause a greater reliance on mobilized fat stores as fuel, resulting in minimal use of glycogen stores.
Therefore, moderate-intensity or higher intensity physical activities are recommended to most effectively enhance insulin action and glycemic control, particularly when an individual participates in a fixed duration of activity (9). A study of sedentary insulin-resistant middle-aged adults who engaged in 30 minutes of moderate-intensity walking 3 to 7 days per week for 6 months found that participants succeeded in reversing their insulin resistance without changing their diets or losing any body weight (10). Likewise, in older adults, low- to moderate-intensity "walking" on a minitrampoline for 20 to 40 minutes 4 days per week over a 4-month period also enhanced glucose uptake without any additional insulin release or loss of abdominal fat. Even in young and obese individuals, insulin action apparently improves after 1 week of daily moderate aerobic training without weight loss or a true training adaptation in muscle (11).
Resistance exercise seems to be at least equally effective as aerobic exercise in enhancing insulin action. In a recent study on people with T2 DM, 4 to 6 weeks of moderate (40% to 50% of the one-repetition maximum) resistance training improved their insulin sensitivity by 48%, even without causing any measurable changes in their body fat or muscle mass (12). Similarly, older men newly diagnosed with T2 DM undertaking 16 weeks of progressive resistance training just twice a week gained muscle mass (0.5 ± 1.1 kg, compared with a 0.4 ± 1.0-kg loss in the weight loss only group), lost body fat (particularly intra-abdominal), and greatly enhanced their insulin sensitivity-all while consuming 15% more daily calories (13).
Interestingly, in sedentary individuals, 6 months of thrice-weekly participation in either endurance (aerobic) or resistance exercise training has been found to improve glucose use, but their mechanisms of action may vary somewhat (14). Undeniably, resistance training results in changes in muscle tissue, including potential increases in muscle mass, allowing for higher overall glucose uptake and storage in these insulin-sensitive tissues. Endurance training was alternately found to more directly enhance muscular glucose uptake by increasing the total amount of enzymes that process and store glucose as muscle glycogen, along with a greater activity immediately after exercise.
In these and other studies, a significant portion, although not all, of the effect on insulin action seems to result from the last bout of physical activity, making regular participation crucial for continued enhancements in insulin action (15). Even in healthy older women, 9 months of participation in higher intensity aerobic exercise enhanced insulin action more than moderate- or low-intensity exercise when energy expenditure (300 Kcal per session) was held constant, likely because of greater transient effects from the higher intensity workouts after 72 hours when they were retested (9). Both training groups also experienced some longer lasting effects from becoming more physically trained.
WHY IT MATTERS WHERE BODY FAT IS STORED AND HOW EXERCISE CAN HELP
Recent research has shown that T2 DM may be preventable and prediabetes reversible with weight loss. A follow-up analysis of the Diabetes Prevention Program study showed that weight loss was the dominant predictor of lower diabetes incidence (16). However, whereas reduced caloric intake and increased physical activity were the main factors that predicted weight loss, increased physical activity was most important for weight maintenance after loss. For every kilogram of weight loss, participants experienced a 16% reduction in diabetes risk even after adjusting for changes in diet and activity. Among the 495 participants not meeting weight loss goals (a 5% to 7% loss of body weight) after 1 year, those who achieved the physical activity goal had a 44% lower diabetes incidence, showing the independent effect of physical activity regardless of weight changes. Exercise largely reduces the rate of loss of muscle mass while dieting, making measures of body composition or circumference a more meaningful way to assess improvements in body composition than weight loss changes alone.
How important is weight loss to improving insulin action? The relationship between impaired insulin action and visceral (deep, intra-abdominal) adipose content is relevant in individuals with T2 DM, as most have abdominal obesity. Free fatty acids released from visceral fat directly enter the liver and have a detrimental effect on insulin action there, so loss of this type of fat potentially has the greatest positive benefit. Researchers also recently examined the effects of dieting (a low-calorie diet high in healthy fats), aerobic exercise (moderate walking for 50 minutes three times a week), or a combination of diet and exercise in older women with T2 DM (17). After 14 weeks, only women who exercised, irrespective of weight loss, experienced a significant decrease in visceral fat levels. The strongest predictor of improved insulin action was enhanced fasting fat oxidation, and both exercise and weight loss (but not diet alone) increased it. In fact, dieting by itself reduced subcutaneous and total abdominal fat but not visceral adipose tissue unless study participants also exercised (17). Moreover, exercisers in this study maintained all of their muscle mass, in support of prior studies that have shown that regular exercise results in greater retention of muscle mass in all dieters, not just diabetic individuals.
CHOOSING THE BEST TYPES OF PHYSICAL ACTIVITY
At present, the U.S. Surgeon General recommends that all individuals engage in at least 30 minutes of moderate intensity aerobic activities most days of the week. The American College of Sports Medicine and the American Heart Association recently released updated guidelines recommending that healthy adults, aged 18 to 65 years, engage in moderate-intensity aerobic physical activity for a minimum of 30 minutes on 5 days each week or vigorous-intensity aerobic physical activity for a minimum of 20 minutes on 3 days each week (18).
The recommendations were updated for older individuals as well and apply to all adults who are 65 years and older and to anyone 50 to 64 years old with clinically significant chronic conditions or functional limitations that affect movement ability, fitness, or physical activity (19), which includes many older people with diabetic complications. In general, these guidelines are similar to those for healthy adults in terms of the recommended frequency and duration of exercise, but the intensities are relatively lower to reflect the reduced physical capacity of most aging adults. Moreover, the recommended activities include more stretching and flexibility exercises, along with balance practice to prevent falls.
These recommendations stress that regular physical activity, including aerobic and muscle-strengthening exercise, is essential to healthy aging. For diabetic individuals in particular, because the effect of a single bout of aerobic exercise on insulin action generally lasts up to 24 to 48 hours for most, depending on the duration and intensity of the activity, it is further recommended that they not go more than 2 consecutive days without aerobic physical activity (15). For sedentary individuals, simply starting a moderate walking program will enhance insulin action.
Exercise stress testing is not considered necessary for healthy individuals with a low-to-moderate risk of heart disease desiring to participate in moderate-intensity activities like brisk walking. However, individuals with a high risk of cardiovascular diseases (including most diabetic individuals) who plan on participating in moderate, and especially vigorous, aerobic exercise are advised to undergo such testing first (15,20). Guidelines to preexercise stress testing for diabetic individuals are given in Table 2.
It may be particularly important to incorporate vigorous exercise in a training program because of its potential cardioprotective effects (21). In a recent study of older individuals with diabetes, even when they were already walking at least 10,000 steps a day, their fitness and diabetes control benefited from doing "pick up the pace" training that consisted of walking at a 10% faster pace for 30 minutes 3 days a week (22). For a person who usually takes 90 steps per minute, walking 10% faster would mean taking 99 steps per minute instead. Any activity that is done at a higher intensity, though, has a greater potential to result in hypoglycemia either during or after exercise. As a precaution, individuals who take insulin or certain sulfonylureas (i.e., diabetic medications that increase insulin release) should monitor blood glucose levels before, possibly during, immediately after, and several hours later to learn their usual glycemic responses and to prevent hypoglycemic episodes from occurring (15).
Along with aerobic exercise, regular resistance training is recommended for individuals with T2 DM (15,20). In healthy adults, the new American College of Sports Medicine and the American Heart Association guidelines call for aerobic exercise and additionally recommend that 8 to 12 repetitions of 8 to 10 strength-training exercises be done at least twice a week (18). If a diabetic individual is already participating in regular aerobic physical activities, additional benefits can be gained by adding moderate- to high-intensity resistance workouts (23), which have been found to be safe even when done by diabetic men with an increased risk of cardiovascular events (15).
For many, a combination of both types of training offers the greatest overall health benefits. For older individuals with T2 DM, a combination of aerobic and resistance training may afford even greater improvements in insulin sensitivity and a more significant decrease in abdominal fat than aerobic training alone, with retention of muscle mass as a side benefit (23,24). In fact, combination training (aerobic and resistance work) is recommended for everyone to optimize functionality with aging, glucose disposal, and insulin action and to prevent or reduce sarcopenia (muscle wasting) and osteoporosis (15,18,24,25).
PLANNING DAILY ACTIVITIES
For all of the reasons discussed, individuals should consider varying the type and intensity of exercise from day to day (Table 3). Such a variable approach to training could potentially improve insulin action to a greater extent, and for a more prolonged period after exercise, than would performing one type and intensity of exercise repeatedly (15,22). To improve glycemic control, the American Diabetes Association and many others recommend at least 150 minutes per week of moderate-intensity aerobic physical activity and/or 90 minutes weekly of vigorous aerobic exercise done over at least 3 days per week, while allowing no more than 2 consecutive days without physical activity (15,20).
Likewise, resistance work (with weights or rubber resistance bands) is most beneficial when done at a moderate or hard intensity, such as using a weight or resistance that allows an individual to complete at least 8 but not more than 12 repetitions for each set while completing one to three sets on each exercise (15). This training regimen will allow exercisers to maximize both overall muscle mass gains and the use of muscle glycogen during each workout. The goal is to progress to doing three sets of 8 to 12 repetitions of each exercise (15). In the absence of physical contraindications, all individuals with T2 DM should ideally participate in resistance exercise 3 nonconsecutive days a week.
Given that the effects of more intense or longer duration exercise may be more extended, the best plan is to intersperse these types of training sessions throughout the week or to do a harder activity every other day to maximize its insulin-enhancing effects (15). Doing "hard" and then "easy" or "moderate" days of activity is a good way to keep motivation high, prevent athletic injuries, and continue doing regular exercise training. An example of an alternating schedule of activities for individuals to optimize insulin sensitivity can be found in Table 3.
In planning out daily activities, it also is important to keep exercise adherence in mind. Many individuals may fail to begin, or start only to later drop out of, regular exercise programs. Although their reasons are many, they often include a perceived lack of time, injuries from workouts (particularly when using higher intensity), inconvenience, and loss of motivation. By varying their activities from day to day, individuals can make time more easily; they are less likely to get athletic injuries; they can pick activities that are convenient for them on any given day; motivation to exercise is likely to stay higher; and the exercise will feel easier to do because there will be adequate time to recover fully between harder workouts (26,27).
OTHER MEANS TO ENHANCE INSULIN ACTION
It is well known that losing excess body fat (particularly from the abdominal region) can help to improve insulin sensitivity, along with other factors (shown in Table 4). For instance, simply avoiding elevated blood glucose levels after eating can improve insulin's effectiveness. Individuals can prevent such glycemic excursions by choosing foods with a high fiber content and lower glycemic index and glycemic load (such as vegetables and legumes), balancing food intake with medicines (insulin or certain oral medicines that can cause hypoglycemia like the sulfonylureas DiaBeta and Micronase), and exercising before meals to enhance the body's ability to process carbohydrates in meals (i.e., taking advantage of heightened insulin sensitivity right after exercise). Insulin action also is improved by getting enough sleep, moderating emotional stress levels (which exercise can help with), and reducing the body's level of inflammation (which also can be accomplished through physical activity and better food choices) (28,29).
In conclusion, individuals can exert a large amount of control over how effectively their insulin works, and being insulin sensitive has many health benefits. If an individual exercises, a heightened sensitivity of the tissues will result in a lower requirement for exogenous or administered insulin. For anyone using insulin-sensitizing medicines, these prescriptions will work more effectively, and they also may need to take smaller doses when engaging in regular physical activity (15). Regardless of the medications used, improving insulin action-particularly when accomplished through increased and varied physical activity-is a winning proposition for diabetes control and prevention, as well as overall health.
CONDENSED VERSION AND BOTTOM LINE
Regular physical activity, both aerobic and resistance, has important and beneficial effects on insulin action and overall blood glucose control. Moreover, enhanced insulin action through frequent bouts of physical activity and longer lasting training effects also is important in the prevention and control of diabetes, along with possibly visceral fat loss. Type 2 diabetes is largely preventable and controllable through these means.
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Keywords:© 2008 American College of Sports Medicine
Blood Glucose; Exercise; Insulin Sensitivity; Prevention; Type 1 and 2 Diabetes