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 1 of a series on Working with Diabetic Athletes
Working with Diabetic Athletes: Part 1
Effects of Insulin on Exercise Glycemia
By Sheri Colberg, Ph.D., FACSM
Many people with type 1 diabetes and other insulin users are engaged in physically demanding exercise programs. Some of them participate in and excel at the top level of competitive sports, including swimming, golf, football, baseball, basketball, ice hockey, soccer, marathons, triathlons, and many others. To perform competitively, they need a greater understanding of the effects of circulating levels of insulin on exercise responses, the physiological response to different types of activities, and the effects of nutrition on performance. To assist these athletes in achieving optimal glycemic control and their athletic goals, practitioners themselves need to become educated in these areas.
Effects of Insulin on Exercise Glycemia
Unlike nondiabetic individuals whose bodies normally maintain blood glucose levels in a very tight range, athletes with type 1 diabetes must constantly adjust their insulin and food intake to balance blood glucose levels during physical activity. Any type of physical activity promotes the uptake of glucose into active muscle cells additively with insulin, but insulin and muscle contractions oppose each other’s effect on fuel availability from hepatic glucose output and lipid mobilization.
Therefore, the correct timing and dosage of insulin is the key to effective glucose management during exercise, and athletes and practitioners need to carefully consider the pharmacokinetics of injected insulin when attempting to achieve exercise glycemic control.
Athletes who rely on exogenous insulin frequently find themselves with a relative state of hyperinsulinemia that can result in hypoglycemia during exercise, early fatigue, and compromised performance. In athletes with diabetes, the normal, physiological decline in insulin seen in the non-diabetic athlete is difficult to achieve due to sustained absorption of exogenous insulin from subcutaneous depots. The combined blood glucose uptake by circulating insulin and muscle contractions can produce hypoglycemia, particularly when exercise occurs during the peak effects of short-acting insulin or rapid-acting insulin analogs. Under such conditions, glucose uptake into muscles is enhanced and the ability of the liver to produce glucose is suppressed, often resulting in the rapid onset of hypoglycemia. This situation can further be exacerbated by prior hypoglycemic episodes or prior bouts of exercise (both the previous day), which blunt the normal, glucose raising hormonal responses to moderate exercise in exercisers with type 1 diabetes even when blood glucose levels are normal.
Not surprisingly, then, glycemic control is easiest to maintain when insulin levels are relatively low, such as during morning exercise before any insulin is administered or more than 2-3 hours after the last dose of rapid-acting insulin.
However, almost any time that rapid-acting insulin is given prior to participation in extended exercise, doses must be reduced to prevent hypoglycemia. For example, in anticipation of moderate exercise, insulin doses may need to be reduced by 33 to 50%, with greater reductions required for longer-duration activities. Insulin pump users can more closely replicate a normal physiological decline in insulin levels by reducing or suspending administration of basal insulin as well, particularly if reduced prior to the start of exercise. If anticipated, users of basal insulins like Lantus or Levemir may also choose to reduce those doses either the evening before or morning of such physical activity.
Conversely, initiating vigorous exercise without sufficient circulating insulin can aggravate hyperglycemia. In fact, when insulin levels are too low, the rise in catecholamines (epinephrine and norepinephrine) elicited by exercise can trigger hyperglycemia or, if ketones are being produced, diabetic ketoacidosis. Thus, skipping normal insulin doses for moderate or greater food intake prior to exercise seldom works effectively for these reasons. Blood glucose levels should be monitored closely when the plan is to suspend basal insulin administration for more than 60 to 90 minutes to make sure that sufficient insulin is available during exercise.
Short-term hyperglycemia may or may not be limiting to athletes with diabetes, depending on whether ketones are present in moderate or higher amounts.
However, athletes have reported that they feel less sluggish and perform better when their starting blood glucose falls in a more normal range, such as 80 to 180 mg/dl depending on the sport. Longer-term elevations in glycemia can also negatively impact performance, though. In a study of younger individuals with long-standing type 1 diabetes, the maximal workload and oxygen uptake were markedly impaired in chronically hyperglycemic subjects compared with those with normal or near-normal glycated hemoglobin levels, who experienced lesser restrictions of lung volume, lung diffusing capacity, and cardiac performance (stroke index and membrane diffusing capacity) during exercise.
In addition to circulating insulin levels, myriad variables can impact glycemic responses to exercise as well. For instance, morning exercise lowers blood glucose levels less than the same bout of exercise done later in the day due to increased insulin resistance caused by early morning elevations in cortisol and growth hormone following an overnight fast. Thus, undertaking morning exercise usually requires a lesser reduction in insulin doses and, in some cases, a supplemental dose of insulin to lower blood glucose levels before or afterwards.
Late evening exercise is most likely to result in nocturnal hypoglycemia, but exercise earlier in the day can also result in later-onset hypoglycemia without precautionary measures, such as a bedtime snack or lower bedtime insulin doses. In addition, prior strenuous exercise can impair the normal glucoregulatory response to subsequent hypoglycemia or another bout of exercise, meaning that athletes with diabetes may be at higher risk for developing hypoglycemia when engaging in successive days of exercise training.
Variables Affecting Exercise Glycemic Control
• Starting blood glucose level
• Circulating insulin levels, determined by:
– Types of insulin(s) used
– Timing of prior insulin injection or bolus
– Insulin dosage given
• Type, intensity, and duration of activity
• Carbohydrate ingestion before and during activity
• Activity status (new or usual)
• Time of day of exercise
• Prior exercise
Chronic Exercise Training
• Type of training done (aerobic vs. anaerobic)
• Training status (specific to the activity)
• Enhancements in insulin sensitivity
• Reductions in overall insulin doses
• Change in fuel utilization (greater fat use)
• Dietary status (energy balance, fuel intake)
• Body composition (i.e., increased muscle mass)
• Frequency of glycogen-depleting exercise
• Attenuated hormonal response
In the next column, I’ll discuss the physiological responses to different types of physical activities. For more information about participation of diabetic exercisers in a variety of sports and recreational physical activities (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.