Exercise and Diabetes. Sheri R. Colberg

(or cardiorespiratory) exercise is defined as continuous, dynamic exercise that uses large muscle groups and requires aerobic metabolic pathways to sustain the activity (Haskell 2007, Physical Activity Guidelines Advisory Committee 2008). Examples include walking, jogging, running, cycling, swimming, water aerobics, rollerblading, and cross-country skiing. Aerobic exercise has been the mode of physical activity traditionally prescribed for diabetes management and prevention. Most of its benefits in terms of management of blood glucose levels are related to acute and chronic improvements in insulin action. The acute effects of a recent bout of exercise account for most of these improvements, but admittedly are short-lived, whereas regular exercise training generally results in a more lasting effect through different mechanisms (Hawley 2008, Winnick 2008).

       Aerobic Exercise Recommendations for Adults

      The American College of Sports Medicine (ACSM) recommends that most adults engage in moderate-intensity aerobic training for at least 30 min/day on ≥5 days/week (a total of at least 150 min weekly), vigorous-intensity training for at least 20 min/day on ≥3 days/week (at least 60–75 min/week), or a combination of moderate- and vigorous-intensity exercise to achieve a similar amount of training (Haskell 2007, Nelson 2007, Garber 2011). The latest federal guidelines (2008) are similar, but allow for 150 min of moderate or 75 min of vigorous activity weekly (Physical Activity Guidelines Advisory Committee 2008). For additional and more extensive health benefits, adults should increase their aerobic physical activity to 300 min (5 h) a week of moderate-intensity, or 150 min a week of vigorous-intensity aerobic physical activity, or an equivalent combination of moderate- and vigorous-intensity activity (Physical Activity Guidelines Advisory Committee 2008). Additional health benefits are gained by engaging in physical activity beyond this amount. The recommended physical activity guidelines and prescription for adults and older adults are available online through the ACSM (www.acsm.org) or U.S. federal guidelines (www.health.gov/paguidelines).

       Aerobic Exercise Recommendations for Adults with Diabetes

      For individuals with type 1 diabetes (T1D), exercise recommendations are closely aligned with those for apparently healthy people (Haskell 2007, D’Hooge 2011), whereas recommendations for T2D are more reflective of guidelines for obesity, hypertension, and sedentary lifestyles (Sigal 2007; Colberg 2010, 2011) given that ~80% of adults with diabetes are overweight or obese. Most individuals with T2D also have a low aerobic capacity (Boulé 2003), leading to the recommendation that they engage in 150 min of moderate (or higher-intensity) aerobic activity weekly (Colberg 2010, American Diabetes Association 2013). Such individuals should focus on appropriate exercise durations to achieve adequate levels of calorie expenditure (Levine 2005, 2008). Recommendations for women with uncomplicated gestational diabetes (GDM) include engaging in 30 min of moderate intensity activity like brisk walking on most days of the week (150 min weekly) (Committee on Obstetric Practice 2002).

      INDIVIDUAL AEROBIC EXERCISE SESSIONS

      Each aerobic exercise session generally has three distinct parts or phases:

      • Warm-up

      • Conditioning phase

      • Cooldown

       Warm-Up

      Doing a warm-up includes 5–10 min of a physical activity done at a slower speed or lower intensity, such as cycling slowly before picking up the pace. Warming up before moderate- or vigorous-intensity aerobic activity allows a gradual increase in heart rate (HR) and breathing at the start of activity (Physical Activity Guidelines Advisory Committee 2008). It may help reduce muscle injury and facilitates a safe transition from rest to exercise by stretching postural muscles, increasing blood flow, elevating body temperature, and increasing oxygen availability and metabolic rate. Alternately, warming up for a muscle-strengthening activity involves doing exercises with lighter weights or resistance to start.

       Conditioning Phase

      This phase is the most important for achieving fitness and other goals as it includes the actual activities to enhance cardiorespiratory fitness, muscle strength and endurance, or flexibility (depending on the training protocol). Time spent doing the warm-up and cooldown counts toward meeting aerobic activity guidelines only if the activity is of at least moderate intensity (e.g., walking briskly as a warm-up before jogging) (Physical Activity Guidelines Advisory Committee 2008).

       Cooldown

      A cooldown includes at least 3–5 min of doing a lower-intensity activity to help the body gradually recover from the conditioning phase and safely transition back to a resting state. Perhaps most important, cooling down helps prevent blood from pooling in the arms and legs (and thereby lessens the chance of fainting at the end of exercise), as well as aids in the removal of metabolic by-products like lactic acid immediately after exercise. HR monitoring can be effective in monitoring recovery from an exercise bout. If an individual takes medications for hypertension that act as vasodilators, he or she may require a longer period of active cooldown to prevent hypotensive episodes postexercise (Balady 2007).

      EXERCISE PRESCRIPTION FOR AEROBIC TRAINING

      The exercise prescription is an actual plan for an individual to follow to reach his or her physical fitness or diabetes management goals. The five components of typical aerobic exercise prescription should at a minimum include the following:

      • Mode

      • Intensity

      • Frequency

      • Duration

      • Progression

       Mode

      The types, or modes, of aerobic activity that are recommended for individuals with diabetes are highly dependent on their preferences and skill level. Although health-related benefits of improved physical fitness do not depend on the type of aerobic exercise done, the actual fitness gains are somewhat activity specific. Walking is the most common type of physical activity done by individuals with diabetes and often is the most convenient. Other low-impact or non–weight-bearing types of activity, such as cycling, swimming, and aquatic or chair exercises, may be more appropriate for those with complications or coexisting conditions like peripheral or autonomic neuropathy (Browning 2005, Duncan 2005, Colberg 2009). For individuals without lower body joint limitations, jogging and running are acceptable higher-intensity activities.

      Although the mode of physical activity (e.g., walking, running, swimming, chair exercises, rowing, etc.) is important to consider, any type of increased movement appears to initially improve an individual’s fitness level. Sedentary adults or youth who begin to participate in any physical activity experience measurable results in their fitness levels. Individuals should choose activities that can safely and effectively improve cardiovascular endurance and maximize caloric expenditure (Haskell 2007, Nelson 2007, Colberg 2010). A wide range of physical activities can be included in the exercise prescription, and all possible activities that the individual is interested in doing should be considered, if they are safe to perform. As he or she becomes more successful and confident, options can be expanded and other types of physical activities can be added or substituted.

       Intensity

      Determining appropriate aerobic exercise intensity is important for anyone with diabetes. The prescribed intensity must match the individual’s current fitness capabilities. For example, if an activity is too easy, it may not raise fitness levels effectively, but if it is too hard initially, the individual may not be able to complete the workout, may become injured, or may be discouraged from exercising altogether. Physical activity guidelines for adults focus on both moderate-intensity activity and vigorous-intensity activity (Haskell 2007, Physical Activity Guidelines Advisory Committee 2008, Garber 2011). Examples of activities in each of these categories are given in Table 4.1 (Physical Activity Guidelines Advisory Committee 2008). As a rule of thumb, a person doing moderate-intensity aerobic activity can talk, but not sing, during the activity. A person doing vigorous-intensity activity cannot say more than a few words without pausing for a breath.

      Table 4.1 Intensity of Various Aerobic