Hepatic glycogen. Liver glycogen is affected by glycemic control. In nondiabetic individuals, increases in hepatic glucose production during exercise are almost entirely the result of increased hepatic glycogenolysis; in contrast, in moderately controlled people with T1D, their increased rates of glucose production both at rest and during exercise are primarily accounted for by increased gluconeogenesis (Petersen 2004). In fact, poorly controlled individuals have a marked reduction in both hepatic glycogen synthesis and breakdown, which can be improved, but not normalized, by short-term restoration of normal levels of insulin and blood glucose (Bischof 2001). High levels of insulin and blood glucose during moderate exercise, however, do not suppress hepatic glycogen concentrations (Chokkalingam 2007b).
Specific Responses to High-Intensity Training
Moderate-intensity exercise has a greater potential to result in blood glucose reductions, but undertaking more vigorous activity actually may cause a deterioration in metabolic control not fully explained by hormonal responses of glucagon or catecholamines (Mitchell 1988). One study compared the blood glucose responses to intermittent high-intensity exercise and moderate-intensity exercise in individuals with T1D (Guelfi 2005a). The high-intensity protocol consisted of 30 min of continuous moderate exercise interspersed with 4-s sprints every 2 min to simulate the activity patterns of team sports, whereas the other involved the same length of moderate activity alone. Although both exercise protocols resulted in a decline in blood glucose, the glycemic decrement was greater with the moderate activity done by itself, despite the performance of a greater amount of total work with intermittent high-intensity protocol. For 1 h, blood glucose levels remained higher following the higher-intensity workout and were associated with elevated levels of lactate, catecholamines, and growth hormone during early recovery from exercise, with no differences in free insulin, glucagon, cortisol, or free fatty acids between the activities.
Thus, intermittent high-intensity exercise does not increase the risk of early postexercise hypoglycemia in individuals with T1D (Guelfi 2005b), which likely is related to a lesser decline in glycemia during exercise (due to greater hepatic glucose output) and attenuated blood glucose uptake during exercise and early recovery (Guelfi 2007). Such training also has been shown to enhanced muscle oxidative metabolism in young adults with T1D, which may have clinically important health benefits (Harmer 2008). It also may protect against nocturnal hypoglycemia in athletes with T1D (Iscoe 2011).
Case in Point: Continued
MF reveals that she used to be on many sports teams in high school: cross-country in fall, swimming in winter, and soccer in spring. She was not talented enough to get recruited to play any sports competitively in college, but she still enjoys many different types of activities. During further discussion, MF admits that she had trouble controlling her blood glucose levels in high school once she developed T1D and that she cut way back on her training because workouts often caused her blood glucose to go too low. In fact, her biggest fear related to starting an exercise training program again is the likelihood of more frequent hypoglycemia. She currently has hypoglycemia 1–2 times/week, and she is able to treat it as soon as symptoms develop, which for her is around a blood glucose level of 55–60 mg/dl. In high school, however, she had a couple of really bad overnight lows that required medical assistance to treat.
Additional Questions to Consider
1. What steps can MF take to prevent hypoglycemia both during and after exercise, given that her concerns about developing it are valid ones?
2. Can treating hypoglycemia have any impact on the possibility of MF reaching her stated body weight goals?
(Continued)
MAKING APPROPRIATE EXERCISE-RELATED REGIMEN CHANGES
A better understanding of the physiology of exercise with regard to fuel mobilization and other metabolic changes can suggest which problems may arise in managing diabetes during physical activity and sports participation. Any exercisers with diabetes need to be advised on appropriate diet and insulin management to maximize performance and reduce fatigue (Gallen 2011). With an appropriate adjustment of insulin dose and diet, people with T1D can even participate in competitive events and sports (Koivisto 1992). General management of blood glucose levels during and after exercise, hypoglycemia prevention and treatment, hyperglycemia and dehydration, and balancing insulin use with physical activity are all critical topics associated with exercise prescription for this group, and they are more fully discussed in chapters 11–14 of this book.
Self-Monitoring of Blood Glucose
Blood glucose responses to physical activity can vary with each exercise session, depending on its duration and intensity, and diabetes regimens (namely, insulin doses and food intake) will require adjustments to keep blood glucose levels balanced. Moderate aerobic exercise usually causes blood glucose levels to drop rapidly, while higher-intensity work may cause them to rise, thereby making glycemic control challenging. Accordingly, people who engage in any physical activity will need to use self-monitoring of blood glucose (SMBG) to better understand their glucose responses to a specific exercise bout and how to make appropriate regimen changes that effectively manage blood glucose levels, keeping in mind that both exercise adaptations and blood glucose responses are specific to the type of training.
Use of continuous monitoring. A more recent technology, continuous glucose monitoring (CGM), provides frequent interstitial glucose readings over a 24 h period and may be particularly helpful for individuals with T1D attempting to establish their usual glycemic responses, trends of hypoglycemia and hyperglycemia, and exercise effects (Riddell 2009, Maran 2010). Short-term use of CGM with alarms, together with appropriate instructions for users, reduces the incidence and duration of hypoglycemia, but only to a limited extent, in part because it tends to overestimate blood glucose levels in the low range and readings lag behind real-time values by 15–20 min (Davey 2010). Having the capacity to know their glucose levels and the direction of change during exercise increases self-efficacy in people who are prone to hypoglycemia and hyperglycemia (Riddell 2009).
Carbohydrate Intake
Sports and other carbohydrate drinks. Ingestion of carbohydrate-based drinks (or other sources of carbohydrate) is likely needed to prevent hypoglycemia both during and following physical activity in a T1D population, but the amount varies with the type of activity, exercise duration, and timing of activity relative to insulin dosing and doses. In one study, mean blood glucose levels were reportedly higher during 60 min of moderate treadmill walking in diabetic individuals consuming a glucose polymer sports drink group compared with placebo, and the same drink prevented the onset of postexercise hypoglycemia without causing or contributing to hyperglycemia (Tamis-Jortberg 1996). For 60 min of moderate cycling undertaken 3 h after breakfast (and the last dose of rapid-acting insulin), 40 g of a liquid glucose supplement, ingested 15 min prior exercise, is likely enough to maintain safe blood glucose levels in anyone on a basal-bolus regimen (e.g., Humulin N and lispro) without requiring changes in insulin doses (Dubé 2005). Ingestion of either whole milk or sports drinks designed for quick or long-lasting nutrient replenishment also has been used effectively to avoid late-onset, postexercise hypoglycemia (Hernandez 2000). For anyone using an insulin pump and engaging in moderate or vigorous activity without altering basal rates, ingestion of sugary drinks during exercise, reduction of the overnight basal rate, reduction of the predinner insulin bolus, or a bedtime snack should be considered to prevent hypoglycemia during and following exercise and overnight (Delvecchio 2009).
Daily carbohydrate intake. Endurance-training athletes usually are encouraged to take in a greater proportion of their total daily calories as carbohydrates, in addition to carbohydrate load before events (Burke 1999, American Dietetic Association 2009). This approach, however, may not be the best one for individuals with T1D.