Many people with type 1 diabetes and other insulin users are engaged in physically demanding exercise programs. These two things, Insulin and Exercise can be a deadly combination for our patients. This week Dr. Sheri Colberg, Ph.D., FACSM brings us part 2 of a series on Working with Diabetic Athletes – Physiological Responses to Different Types of Activities
Working with Diabetic Athletes: Part 2
Physiological Responses to Different Types of Activities
By Sheri Colberg, Ph.D., FACSM
During physical activity, energy expenditure by skeletal muscles increases substantially, supported by enhanced utilization of plasma glucose and free fatty acids, as well as glycogen and triglycerides stored within skeletal muscle. The intensity and duration of exercise are the primary determinants of the relative use of these metabolic fuels. For low-intensity exercise like slow walking, particularly if sustained for long intervals, the predominate fuel is circulating plasma free fatty acids, with relatively slow depletion of muscle glycogen. During prolonged lower-intensity activities, though, blood glucose use can still become quite significant as muscle glycogen stores become depleted over time, increasing the risk of hypoglycemia in diabetic exercisers.
During moderate intensity exercise, blood glucose use is usually increased three- to four-fold above rest, and lipid mobilization and oxidation are also enhanced. Both the duration of exercise and the training state of the athlete are determinants of the relative use of lipids, with longer duration of exercise and higher fitness favoring increased reliance upon fat, although the greatest utilization of triglyceride stores in muscle occurs during recovery from such activities. For short-duration activity, carbohydrate intake alone can effectively control blood glucose levels, but for more prolonged exercise sessions, most athletes with diabetes will need to reduce insulin doses as well (if taken). In some cases, type 2 diabetic athletes not taking insulin may alternately need to reduce doses of certain sulfonylureas (e.g., Diabinese, DiaBeta, Micronase, and Glynase, in particular) or Byetta to avoid hypoglycemia or sluggishness. Although aerobic activities use a fuel mix of muscle glycogen, blood glucose, and lipids, during higher-intensity, prolonged activities like running, carbohydrate is the body’s fuel of choice, and near depletion of both muscle glycogen and blood glucose is inevitable if the activity is sustained. Higher-intensity, repeated interval training also results in significant depletion of muscle glycogen and a higher risk of hypoglycemia.
During brief high-intensity exercise, the muscles rely on their glycogen stores and to a lesser extent plasma glucose, but relatively few lipids. Shorter, intense activities such as sprinting or power lifting are mostly anaerobic activities that cause muscles to rapidly use high energy phosphate compounds, and, if lasting longer than ten seconds, intramuscular stores of glycogen. High intensity exercise causes a strong sympathetic nervous system response, which can be further exacerbated by the mental stress of competition, resulting in the release of epinephrine and other hormones that raise blood glucose levels. Such activities generally cause a transient hyperglycemic state that may require additional insulin to correct. Uncorrected, blood glucose levels may remain elevated for two to three hours afterwards, even if blood glucose levels were normal prior to exercise. Athletes, however, must guard against hypoglycemia resulting from ongoing muscle glycogen replacement in muscles post-exercise, which is largely insulin independent until glycogen levels increase, particularly during the “window of opportunity” from 30 minutes up to two hours after physical activity.
Athletes with diabetes who train regularly, on the whole, exhibit a heightened sensitivity to insulin, which allows blood glucose to enter muscle cells more efficiently both acutely and chronically with exercise. Acute changes likely result from heightened muscle glycogen repletion following physical exertion. However, one study reported that following a competitive marathon, normoglycemic athletes with type 1 diabetes had unchanged insulin sensitivity on the morning after the event despite significant glycogen depletion. Under such conditions, it is likely that enhanced lipid oxidation following exhaustive exercise, which occurs normally even in nondiabetic individuals, combined with some degree of muscular damage to create a transient state of insulin resistance.
Chronic changes in insulin sensitivity, on the other hand, are attributed to adaptive changes in muscle tissue resulting in enhanced insulin-mediated glucose transport by insulin-sensitive glucose transporter (GLUT4) proteins and lower hepatic glucose output. Aerobic training also results in an increase in the proportion of lipids used during low- or moderate-intensity activity. Using lipids more effectively spares some muscle glycogen and blood glucose and allows for better glycemic control during activities. These changes in fuel utilization in response to training, though, will result in a need for smaller compensatory adjustments to carbohydrate or insulin intake to maintain glycemic control compared to the period of time prior to training. Therefore, training adaptations lower overall insulin needs, regardless of the insulin regimen used. Exercise must be consistent, though, as this heightened state of insulin action begins to decline after only one to two days of inactivity.
Similar to aerobic activities, resistance training also enhances insulin sensitivity and blood glucose utilization. Skeletal muscle is a metabolically active tissue that takes up more glucose (through non-oxidative disposal, or glycogen synthesis) than many other body tissues; therefore, athletes with diabetes who gain muscle mass from resistance work also have lower overall insulin requirements and must lower their insulin doses both acutely and chronically.
In the next column, I’ll discuss more about nutritional concerns for athletes with diabetes. For more information about participation of diabetic exercisers in a variety of sports and recreational physical activity (along with real-life athlete examples), please consult The Diabetic Athlete: Prescriptions for Exercise and Sports (Human Kinetics, 2001) by Sheri Colberg. A fully revised and expanded 2nd Edition of this book will be available in Fall 2008.