Exercise and Diabetes. Sheri R. Colberg

carbohydrate intake for 3 weeks in adult athletes with T1D was associated with deterioration in glycemic control, increased insulin requirements, decreased muscle glycogen content, and reduced exercise performance. Thus, consuming a high-carbohydrate diet when doing sport-specific training is not advisable, at least not when compensatory insulin changes are not made and the resulting glycemic control is worse (McKewen 1999). For most individuals, intake of at least 40% of calories from carbohydrates (without overdoing it), along with adequate protein and total daily calories, is more likely to result in optimal glycogen storage (American Dietetic Association 2009).

       Insulin Adjustments

      Making adjustments to insulin doses and timing of doses can lower the risk of hypoglycemia during and following exercise. For example, consumption of a low–glycemic-index carbohydrate (75 g of isomaltulose) and taking an insulin dose reduced by 75% just 30 min before undertaking 45 min of moderate running improves pre- and postexercise blood glucose responses in T1D (West 2011). Similarly, a 75% reduction of insulin with a meal taken 2 h before 45 min of moderate running results in the greatest preservation of blood glucose during and following exercise (compared with 0%, 25%, and 50% dose reductions) and allows for minimal intake of extra food to maintain euglycemia during the 24 h following the activity (West 2010).

      Insulin regimen effects. The types of insulin and insulin regimens used can affect exercise-related adjustments. Insulin detemir is associated with less hypoglycemia than glargine in relatively well-controlled individuals with T1D both during and after exercise (Arutchelvam 2009). People treated with insulin detemir appear to improve their glycemic control with no increases in hypoglycemia, adverse events, or body weight (Marre 2009). Recently, it was reported that athletes with T1D who were treated with rapid-acting insulin analogs and participating in half-marathons required less insulin reductions compared to traditional guidelines, although they needed a significant quantity of carbohydrate supplements to avoid hypoglycemia during and after events (Murillo 2010). For children with T1D using an insulin pump, discontinuing basal insulin during exercise is an effective strategy for reducing hypoglycemia, but the risk of hyperglycemia is increased (Diabetes Research in Children Network Study Group 2006).

      Insulin pump use. Other evidence is related to glycemic control in adolescents with T1D on insulin pump therapy doing moderate- or higher-intensity physical activity with their insulin pumps switched on or off (and compared with glargine use) (Delvecchio 2009). Postexercise blood glucose levels were significantly increased with the pump off and were unchanged or lower with the pump on and when compared with glargine. The authors concluded that in that case, it is advisable to leave the basal rate on during activity, but doing so may require consumption of carbohydrate during the exercise, a lower overnight basal rate, less of an insulin bolus for dinner, and a possible bedtime snack to prevent nighttime hypoglycemia.

      Exercise timing. Finally, the time of day that exercise is done can also affect insulin requirements. One study compared cycling for 45 min moderately either 1 h after lunch with usual insulin doses or after an overnight fast without morning insulin (Biankin 2003). Reproducibility of the change in blood glucose levels during exercise after feeding in individuals on nonintensive insulin regimens was poor, but reproducibility was reasonable when fasting. Exercise does decrease the glycemic variability after a meal, meaning that blood glucose levels after exercise seek a reproducible “target.” Thus, the absolute glucose level after a typical bout of exercise in the fed state should be a good guide to carbohydrate or insulin adjustment on subsequent occasions.

       Prevention of Hypoglycemia

      Fear of hypoglycemia is the strongest barrier to regular physical activity in anyone with T1D, and information about and support for hypoglycemia management is critical (Brazeau 2008). Given the importance of this topic, it is more fully discussed in chapter 12. Various strategies and technologies have been developed to help detect and prevent hypoglycemia, including improved patient education, frequent SMBG, use of rapid-acting and basal insulin analogs, insulin pump therapy, exercise-related insulin modifications, and use of CGM (Realsen 2011).

      Later-onset hypoglycemia. A major concern following exercise is the later, delayed onset of hypoglycemia, especially overnight during sleep. Overnight hypoglycemia after afternoon exercise has been shown to be common in children with T1D (Tsalikian 2005). A biphasic increase in glucose requirements to maintain euglycemia after exercise has been reported, suggesting a unique pattern of early and delayed risk for nocturnal hypoglycemia after afternoon exercise (McMahon 2007). Anyone with T1D should be made aware of this possibility to enable them to make adjustments in their management plans before and after any physical activity to minimize or avoid late-onset hypoglycemia (MacDonald 1987, Hernandez 2000, Kalergis 2003, Alemzadeh 2005, Tsalikian 2005, Diabetes Research in Children Network Study Group 2006, McMahon 2007, Tamborlane 2007, Cooperberg 2008, Wilson 2008).

      Effects of sprints. A novel approach to hypoglycemia prevention during physical activity is related to the adrenergic effects of sprinting. After moderate-intensity exercise, young individuals with insulin-treated, complication-free T1D have engaged in a 10-s maximal sprint that acutely opposes a further fall in blood glucose compared with rest alone. The addition of the sprint after moderate-intensity exercise is a novel method to reduce the risk of hypoglycemia in active individuals (Bussau 2006). Along the same lines, a 10-s sprint performed immediately before moderate-intensity exercise also prevents blood glucose levels from falling during early recovery from moderate-intensity exercise in individuals with T1D (Bussau 2007). Doing intermittent, high-intensity training may result in a higher incidence of delayed nocturnal hypoglycemia (Maran 2010).

      EXERCISE PRESCRIPTION FOR ADULTS AND YOUTH WITH T1D

       Mode

      Any form of aerobic exercise that utilizes large muscle groups and causes sustained increases in heart rate is likely to be beneficial to cardiovascular risk management (and possibly glycemic control) in children, adolescents, and adults with T1D (Orchard 2003, Costacou 2007, Bishop 2009, Shivu 2010, Maahs 2011). Undertaking a variety of modes of physical activity is recommended for all individuals to optimize training effects and lower injury risk (Physical Activity Guidelines Advisory Committee 2008). Examples of acceptable aerobic activities include both weight-bearing and non–weight-bearing ones, such as walking, jogging, running, cycling, swimming, water aerobics, aquatic activities, conditioning machines, dancing, chair exercises, rowing, and cross-country skiing, among others.

      For children and adolescents ages 6 to 17 years, most of their physical activity should be either moderate- or vigorous-intensity aerobic activity, but it can include muscle-strengthening activities (even if unstructured and part of play, such as playing on playground equipment, climbing trees, and playing tug-of-war) and bone-strengthening activities like running, jumping rope, basketball, tennis, and hopscotch. Other more age-specific activities include skateboarding, rollerblading, martial arts, playing tag, wall rock climbing, gymnastics, soccer, and other competitive sports teams. Both adults and youth with T1D can safely and effectively participate in competitive athletic training and sports (Tuominen 1997a, 1997b; Cauza 2005; Graveling 2010; Murillo 2010).

       Intensity

      For adults, aerobic exercise should be moderate or vigorous intensity, corresponding to 40–89% of HRR (heart rate reserve; see chapter 4) (Garber 2011, Haskell 2007, Nelson 2007, Physical Activity Guidelines Advisory Committee 2008). Moderate-intensity exercise equates to 40–59% HRR, whereas vigorous-intensity exercise is 60–89% HRR (Garber 2011). A severely deconditioned person with T1D can start out at 30−39% HRR (“light”) and progress to more moderate levels.

      For children and adolescents, recommended aerobic activities should be either moderate or vigorous intensity as well (40–89% HRR), but they also should include vigorous-intensity physical activity (60–89% HRR) at least 3 days a week (Physical Activity Guidelines Advisory Committee 2008).

      A corresponding overall body rating of “somewhat hard” to “hard” is appropriate for most individuals with T1D