Glycogen stores in the liver and skeletal muscles need to be replenished following each bout of physical activity, which is accomplished via an increased rate of blood glucose uptake during recovery from exercise until the depleted glycogen stores are fully replaced. This activity may take 24 to 48 h to complete, during which time both insulin sensitivity and fat oxidation in skeletal muscles usually are enhanced (Cartee 1989, Goodpaster 2003, Boon 2007). Combined hyperinsulinemia and hyperglycemia during moderate exercise in individuals with T1D, however, does not suppress the release of hepatic glycogen to maintain blood glucose levels (Chokkalingam 2007b), although total body glucose uptake is increased by having higher insulin levels (Chokkalingam 2007a).
Glucose-raising hormones. Endogenous insulin secretion normally decreases during exercise in people without diabetes and in most people with T2D who still secrete insulin. Its suppression is an essential step in allowing hepatic glucose production to ramp up to maintain the balance of glucose in the blood (Galbo 2007, Szewieczek 2009). Exercise causes the release of glucose-raising hormones like epinephrine and norepinephrine in an intensity-dependent manner, with exponentially more being released in response to intense compared with moderate- or low-intensity activity (Manetta 2002, Kreisman 2003). Other hormones like glucagon, cortisol, and growth hormone significantly influence how the primary fuel substrates (i.e., carbohydrate, protein, and fat) are mobilized and used for energy production (Kreisman 2003). Exercise-induced changes to the secretion of key hormones, as shown in Table 1.4, allow alternate fuels to be made available as energy sources while maintaining glucose homeostasis. In individuals dependent on exogenous insulin, these counterregulatory hormone responses can be altered. For example, growth hormone secretion during exercise in T1D has been found to be normal as long as normal blood glucose levels are maintained, but it is suppressed during hyperglycemic conditions (Jenni 2011).
Table 1.4 Metabolic and Exercise Responses of Hormones
Health Benefits of Chronic Exercise Training
The benefits of regular physical activity participation are numerous. Current evidence suggests physical activity improves insulin action, lowers blood glucose levels, improves BMI (commonly placing individuals into categories of normal, overweight, or obese based on weight and stature), and reduces multiple risk factors for cardiovascular disease (Kang 1996, Cuff 2003, Goodpaster 2003, Kriska 2006, Zoppini 2006, Hordern 2008). These important metabolic changes demonstrate the significant role that physical activity and exercise have in the prevention and management of T2D, in particular. Blood glucose management with physical activity as an added variable in T1D can be more challenging, but most of the same metabolic and overall health benefits are possible for these individuals as well (as shown in Table 1.5).
Table 1.5 Health Benefits of Regular Physical Activity Participation
Many research studies have found lower levels of cardiovascular fitness in individuals with prediabetes and T2D, in particular (Church 2005, Simmons 2008, Sui 2008, Lee 2009), but even in some children and adults with T1D (Williams 2011). Fitness gains are certainly possible for individuals undergoing training, but professionals should take this finding into account when prescribing appropriate exercise training regimens for anyone with diabetes. On the other hand, not everyone with diabetes has a lower fitness level to start. For example, a recent study examined whether glycemic status influences aerobic function in women with T1D and whether aerobic function is reduced relative to healthy women (Item 2011). There were no differences, however, between the two groups, either in the oxidative enzyme activity or in capillary-to-fiber ratio. Their mitochondrial capacity depended on the A1C level in untrained women with T1D, but it was not reduced relative to untrained healthy women. Exercise training in children with T1D has been shown to effectively reverse endothelial dysfunction and improve physical fitness, demonstrating that engaging in appropriate training is important for all individuals with any type of diabetes (Trigona 2010, Seeger 2011).
Case in Point: Wrap-Up
In MJ’s case, because she has been sedentary and is obese, it is best to start with exercise done at a lower intensity and progress her slowly with structured activities to avoid the development of athletic injuries, exercise nonadherence, or lack of motivation, with a goal of increasing her amount of physical activity gradually over a period of weeks to months.
Exercise Program Goals
Activity: The initial focus will be on lifestyle physical activity that MJ enjoys doing and can easily fit into her lifestyle, such as daily walking.
Intensity, Frequency, and Duration: Because MJ is currently inactive, she should be encouraged to start with short activity bouts of low- to moderate-intensity forms of physical activity that can be incorporated into her daily routine, such as doing 5–10 min of slow walking several times daily, 5–6 days a week. The exercise duration then can be increased gradually to 10 min per session, three times a day, and the walking speed increased slowly. Short intervals interspersed into her normal walking will improve her fitness levels more quickly and assist in management of blood glucose levels and postactivity blood pressure as well.
Progression: Long-term exercise goals should focus on progressively increasing amounts and frequency of activity to reach the minimum recommended levels of 150 min of moderate or vigorous exercise spread throughout the week, and MJ should be advised to add in some resistance training at least 2 days a week as well.
Daily Movement: For weight management purposes, MJ’s goal is to maximize her caloric expenditure each week with planned activities, but also by adding in greater daily movement during nonexercise times (i.e., more daily living activity). The professional working with her can help her to identify physical activities that she might be interested in trying to incorporate into her daily lifestyle, even if it is simply to stand more each day or take more daily steps (which can easily be measured by wearing a pedometer).
Behavioral Changes: MJ’s self-efficacy in being active can be enhanced by helping her to set realistic and specific goals that she can successfully accomplish, such as making a list of five ways to be more active throughout the day and trying one of them each week. She should be encouraged to ask questions and come up with her own ideas and suggestions for overcoming her exercise barriers and becoming (and staying) more physically active.
For most people with any type of diabetes, exercise can be undertaken safely and blood glucose levels can be managed effectively to the overall benefit of their diabetes management and health. Professionals who interact with people with any type of diabetes have an important and challenging role in helping them become and stay more physically active to manage their diabetes and their health.
Professional Practice Pearls
• Although diabetes is categorized into four main types, most individuals have T2D (90–95% of cases), are overweight or obese, and have a sedentary lifestyle.
• T1D can occur at any age; when its onset is during adulthood, it can be classified as LADA and may (at least initially) be misdiagnosed as T2D.
• Although exercise is technically a planned activity that is a subset of the broader term physical activity, they both involve bodily movement and can be used interchangeably.
• The terminology of fitness is intended to be inclusive rather than athletic in nature when considering what affects an individual’s fitness level and health outcomes.
• The potential health benefits of regular physical activity participation are numerous and include lowering the risk for development of T2D.
• Individuals with diabetes, particularly those with T2D, may be starting their exercise programs with cardiovascular