Exercise and Diabetes. Sheri R. Colberg

are measurable over time, such as changes in body composition with exercise training or body fatness during weight loss or regain. Success of interventions often is judged on the basis of such changes.

      A frequently measured parameter associated with exercise training is cardiovascular fitness (also known as cardiorespiratory fitness or aerobic fitness), which is the ability of the circulatory and respiratory systems to supply oxygen during sustained physical activity (McGavock 2004, Church 2005, Sui 2008, Lee 2009, Williams 2011). Musculoskeletal fitness includes both muscular endurance and muscular strength, which reflect the ability to continue to perform without fatigue and maximal ability to exert force, respectively (American College of Sports Medicine 2009). Flexibility measures the range of motion available at a joint (Herriott 2004). Finally, body composition in this health-related category refers to the relative amounts of fat and fat-free mass that an individual has (which can affect overall health); the fat-free component includes muscle, bone, body water, organs, and other body parts that are not fat (Lee 2005, Iqbal 2007, Wang 2008, Chomentowski 2009, Bray 2012, de Souza 2012). Excess body fat can affect both physiological and health-related fitness, depending on its relative amount and regional distribution.

      Case in Point: Continued

      A thorough interview with MJ to discuss her personal beliefs, past experiences, preferences, and concerns about increasing her activity levels is required to assist the health-care or fitness professional working with her to gain a better understanding of MJ’s view of physical activity. Any preprogram conversations also should include a discussion of the possibility of developing hypoglycemia during or after physical activity (given her use of a sulfonylurea), the best types of carbohydrates to keep handy during all activities (like glucose tablets or gels or hard candy), and the importance of monitoring blood glucose before and after exercise to establish typical response patterns. Once her exercise routine is established, she may need to talk with her health-care provider about possibly reducing her medication dose to prevent hypoglycemia.

      (Continued)

      HEALTH EFFECTS OF PHYSICAL ACTIVITY AND FITNESS FOR DIABETES

      Although many American adults are not physically active, just 39% of adults with diabetes in the U.S. are physically active—defined as engaging in moderate or vigorous activity for at least 30 min thrice weekly—compared with 58% of other adults (Kirk 2004, Morrato 2007). In spite of the fact that myriad studies have shown that physical activity has a positive impact on diabetes management and prevention of complications, many health-care providers still are hesitant to prescribe it, and fitness professionals may be ignorant of precautions needed to handle the complexities of the disease with exercise as an added variable. Moreover, giving diabetic individuals a full exercise prescription that takes comorbid conditions into account may be too time-consuming for or beyond the expertise of many health-care and fitness professionals.

      Unfortunately, simply instructing diabetic individuals to “exercise more” is frequently not motivating or sufficiently informative to get them regularly and safely active enough to benefit their fitness levels, diabetes control, and overall health (Morrato 2006). Moreover, professional books with exercise information and prescriptions have not been up to date or interactive enough to provide busy professionals with access to the latest tools and recommendations for each individual’s unique circumstances and requirements, and exercise remains woefully underprescribed for individuals with diabetes, almost all of whom can benefit from it immensely.

       Prevention of T2D with Physical Activity

      The “exercise is medicine” dogma launched by the American College of Sports Medicine campaign in 2007 cites diabetes as one of its hallmark diseases. At this point, it has been well established that participation in regular physical activity as part of lifestyle improvements can prevent or delay the onset of T2D (Tuomilehto 2001, Knowler 2002, Duncan 2003, Laaksonen 2005, Hamman 2006, Li 2008). Early on, studies assessing physical activity via self-report showed that higher levels are associated with reduced risk for T2D, regardless of the method of physical activity assessment or types of activities (Helmrich 1991, Manson 1991, Hu 1999). Greater volumes of physical activity may provide more protection, although both moderate walking and vigorous activity are associated with a decreased risk, as is better cardiovascular fitness (Wei 1999, Sui 2008).

      In three larger intervention studies, the role of physical activity in T2D prevention in high-risk individuals has been examined (Diabetes Prevention Program [DPP] Research Group 2002, Knowler 2002, Pan 1997). Amazingly, all their results were similar, showing that modest weight loss (~7% reduction in initial body weight) combined with increased physical activity and dietary improvements (a healthier low-calorie, low-fat diet) significantly reduces the incidence of developing T2D and that these lifestyle changes were more effective in older individuals. The first of these studies, the Chinese Da Qing study, included an exercise-only treatment arm that showed that even modest changes in exercise (i.e., 20 min of mild or moderate, 10 min of strenuous, or 5 min of very strenuous exercise 1–2 times a day) can reduce diabetes risk by 46% compared with 42% for diet plus exercise and 31% for diet alone (Pan 1997). Both the Finnish Diabetes Prevention Study (Eriksson 1999, Tuomilehto 2001) and the DPP (Knowler 2002) included intensive, lifestyle modifications targeting modest weight loss, dietary improvements, and an average of 30 min of daily, moderate physical activity (Eriksson 1999, Tuomilehto 2001). The DPP, which studied 3,234 high-risk men and women, consisted of medication (metformin), lifestyle modification, and control groups (Knowler 2002). In the DPP, intensive lifestyle modification resulted in a 58% risk reduction that was almost twice the impact of metformin use (31% reduction). Although weight loss was the dominant predictor of a lower incidence of diabetes, the DPP showed that regular participation in physical activity can reduce risk by 44% even when modest weight-loss goals are not achieved (Laaksonen 2005, Hamman 2006, Lindstrom 2006). In summary, physical activity does play a role in preventing T2D in high-risk individuals, across ethnic groups, and in both sexes (Kosaka 2005, Ramachandran 2006).

      Moderate exercise like brisk walking reduces risk of T2D (Helmrich 1991, Hu 1999, Hu 2001, Kosaka 2005, Ramachandran 2006). A meta-analysis that assessed the preventive effects of moderate-intensity physical activity reported that walking on a regular basis, typically done for 2.5 h per week or more, significantly reduces diabetes risk (Jeon 2007). The preventive effects of resistance training have not been studied to date. Likewise, although T2D is increasing in prevalence in children and adolescents, no trials have been completed that address whether physical activity or exercise prevents T2D in youth.

      Limited studies suggest that to prevent and manage T2D, goals for youth should include limiting daily screen time (television, computer, or video game) to <60 min per day and doing at least 60 min of daily physical activity (McGavock 2007). The Treatment Options for Type 2 Diabetes in Adolescents and Youth (TODAY) study (Zeitler 2012) assessed the importance of physical activity as part of a behavioral lifestyle intervention to prevent T2D in youth. The results suggested that metformin combined with lifestyle intervention (e.g., greater physical activity) was intermediate in their ability to manage blood glucose levels and not significantly different from the use of medications (either metformin alone or metformin plus rosiglitazone).

       Acute Effects of Physical Activity on Insulin Action and Hormones

      Insulin. Most benefits of physical activity on diabetes management and prevention of T2D are likely due to acute and chronic improvements in insulin action (Boulé 2001, O’Gorman 2006, Galbo 2007). The acute effects of a recent bout of exercise account for the majority of these improvements, but are short-lived, whereas regular exercise training generally results in a more lasting effect. Although responses can vary, most people with any type of diabetes experience a decrease in their blood glucose levels during mild- and moderate-intensity exercise and up to 2–72 h afterward, necessitating frequent participation to maintain enhanced insulin action (Cartee 1989, Boulé 2001, O’Gorman 2006, Galbo 2007).

      Although exercise cannot prevent T1D, it can lessen the potential decrements in insulin action that can occur due to inactivity; even individuals with T1D can become similarly insulin resistant and require larger insulin doses to manage blood glucose levels when not engaging in regular physical activity (Okamoto 2011). Circulating insulin levels during exercise must usually be lowered with