Exercise and Diabetes. Sheri R. Colberg

in Adults

      Many health benefits of regular physical activity are available to individuals with type 1 diabetes (T1D) or latent autoimmune diabetes in adults (LADA), regardless of their age at onset. For example, regular exercise lowers blood pressure, lipid levels, body weight, cardiovascular and mortality risk, and risk of complications while raising cardiorespiratory fitness levels, muscular strength, quality of life, and sense of well-being (Moy 1993, Laaksonen 2000, Costacou 2007, Herbst 2007, Heyman 2007, Bishop 2009, Conway 2009, Trigona 2010, D’Hooge 2011, Maahs 2011). People with T1D can have lifestyle habits (including sedentary behaviors) that lead to greater insulin resistance and “double diabetes,” with symptoms of both T1D and type 2 diabetes (T2D) (Purnell 1998, Orchard 2003, Kilpatrick 2007). Although an acute bout of exercise improves insulin sensitivity in those with T1D as well, little or no improvement in glucose control has been demonstrated after regular exercise training, even though insulin doses generally decrease (Ebeling 1995, Roberts 2002, Ramalho 2006).

      Case in Point: Aerobic Exercise Rx for a Young Adult with T1D

      MF is a 20-year-old woman currently attending college who has had T1D since the age of 16 years. Although she has achieved reasonable control over her blood glucose levels—even though she is out of the “honeymoon period”—she has gained around 15 lb and is taking higher insulin doses than ever before to keep control over her daily glucose fluctuations. Her college classes are stressful for her this semester, and she has slacked off on doing any regular physical activity (although she rides her bike to classes and walks around a lot). Her goals are to lose the extra 15 lb she gained (her best weight is around 130 lb) and get her A1C value back below 6.5% (ideally no higher than 6.0%, though).

      Resting Measurements

      Height: 66 inches

      Weight: 145 lb

      BMI: 23.4 (in a “normal” range, but heavy for her)

      Heart rate: 82 beats per minute (bpm)

      Blood pressure: 125/70 mmHg

      Fasting Labs

      Plasma glucose: Variable (65–285 mg/dl, controlled with insulin)

      A1C: 6.9%

      Cholesterol: Not tested recently but has always been good in the past

      Medications

      Basal-bolus insulin regimen: 20 units of insulin glargine at bedtime, insulin lispro for meals, snacks, and corrections (usually 1–6 units per injection, based on carbohydrate intake)

      Questions to Consider

      1. What type of aerobic exercise should MF start doing to reach her goals?

      2. What would be an appropriate exercise Rx for her with regard to exercise intensity, frequency, and duration?

      3. How should her exercise training progress over time?

      4. What precautions should MF take, and does she have any exercise limitations?

      (Continued)

      BENEFICIAL EFFECTS OF AEROBIC EXERCISE

      Physical activity in the form of aerobic exercise should be included as part of the diabetes self-management program for individuals with T1D. Throughout life such individuals are fully capable of and gain many health benefits from engaging in a physical fitness plan (Colberg 2009).

       Health Benefits of Regular Activity

      Appropriate diabetes regimen changes must be made if improved blood glucose management is an expected outcome of regular physical activity by these individuals. Although regular exercise may not necessarily improve overall glycemic management, it still confers protective effects for individuals with T1D of all ages. Sedentary behavior has been associated with poor glycemic management in both adult and pediatric populations of T1D, even though regular exercise training has not been shown to improve glycemic control in most studies, likely due to the difficulty associated with balancing out appropriate insulin changes and carbohydrate adjustments for exercise (Ebeling 1995, Roberts 2002, Ramalho 2006). Nonetheless, a recent study showed that children with T1D exercising more than two times weekly significantly improved their A1C levels and lipid profiles, suggesting that being regularly active for long periods (60 min at a time or more) is likely beneficial and should be recommended (Aouadi 2011).

      In addition to the potential health benefits of regular activity on blood pressure, lipid levels, body weight, and risk of complications (Moy 1993, Laaksonen 2000, Costacou 2007, Herbst 2007, Heyman 2007, Bishop 2009, Conway 2009, Trigona 2010, D’Hooge 2011, Maahs 2011), requirements for lower insulin doses that usually result from regular exercise participation are indicative of improved insulin sensitivity and—regardless of changes in glycemic levels—significantly reduce risk of all cardiovascular conditions (Trigona 2010, Seeger 2011). Youth with T1D present early signs of atherosclerosis, as well as low physical activity levels and levels of cardiorespiratory fitness, but endothelial function can be enhanced by engaging >60 min of daily moderate- to vigorous-intensity physical activity (Trigona 2010). Moreover, physical activity can offset some of the potentially negative impact of engaging in more sedentary pursuits: the glycemic impact of greater computer use on metabolic control in individuals with T1D is apparently not related to age, diabetes duration, television watching, or computer use, but rather independently and negatively related to the weekly hours spent on physical exercise (Benevento 2010).

      Regular exercise even promotes longevity in people with T1D. One study reported that the 7-year mortality rate in T1D is 50% lower in individuals doing the equivalent of ~7 h a week of brisk walking (i.e., expenditure of ~2,000 kilocalories weekly) compared with <1,000 kilocalories (Moy 1993). The estimated increase in longevity resulting from regular exercise is ~10 years, which exactly counterbalances the number of years that diabetes potentially can shorten a person’s life.

       Cardiorespiratory Fitness

      Some, but not all, individuals with T1D have the same aerobic capacity as similar-age people without diabetes. For instance, despite having a lower anaerobic threshold and lung capacity, the aerobic capacity in people with T1D doing programmed exercise has been reported to be similar to that of normal athletes (Komatsu 2010). It appears, however, that poor glycemic control impairs pulmonary, cardiac, and vascular responses to exercise (Baldi 2010b). Highly trained individuals with T1D in one study were able to achieve the same cardiopulmonary exercise responses as trained subjects without diabetes, but these responses were reduced by poor glycemic control (i.e., A1C >7.0%) (Baldi 2010a).

      Mitochondrial oxidative capacity also depends on glycemic control in untrained women with T1D, although it may not be lower than in untrained healthy women (Item 2011). Others have reported that both sex and diabetes control are significantly associated with cardiorespiratory fitness, with both women and anyone with poorer glycemic control exhibiting a lower level of fitness. At this point, it is not fully clear whether reduced fitness in children and others with T1D is attributable to lower physical activity levels or to physiological changes resulting from diabetes itself (Williams 2011), although exercise training in children improves their overall physical fitness (Seeger 2011).

      AEROBIC EXERCISE RESPONSES IN T1D

       Physiological Responses to Moderate Physical Activity

      Fuel metabolism. Several-fold increments in hormone concentrations contribute to the maintenance of fuel and fluid homeostasis during exercise (Meinders 1988, Koivisto 1992, Galassetti 2001, Kishore 2006, Diabetes Research in Children Network Study Group 2009). When individuals with T1D exercise with normal blood glucose levels, their substrate oxidation is similar to healthy individuals who experience a shift toward lipid oxidation during extended exercise. When exercising in a hyperglycemic state, however, fuel metabolism in T1D is dominated by carbohydrate oxidation, although muscular glycogen is not spared by the greater use of blood glucose (Jenni 2008). Exercise done with higher circulating insulin levels or during insulin peak times also increases blood glucose use (and the potential for hypoglycemia to develop) without sparing muscle glycogen (Chokkalingam 2007a). Some of the additional blood glucose uptake during high-insulin conditions