Exercise is recommended for everyone with diabetes. In people with type 1 diabetes exercise provides many benefits—improved cardiovascular health, a psychological lift, stronger bones, stronger muscles and more restful sleep. It evens aids in weight loss and improved insulin sensitivity. In fact, there have been studies that demonstrate that people with type 1 who exercise have lower mortality rates. But one thing it hasn’t been proven to do is decrease A1C levels.
When people with type 2 diabetes engaged in regular exercise, their A1C levels drop. The research looking at exercise benefits in terms of glucose control for people with type 1 isn’t as sanguine. The reason there isn’t always a positive impact on A1C has to do with the ever present risk of hypoglycemia and hyperglycemia whenever a person with type 1 diabetes exercises.
Both exercise-related hypo- and hyperglycemia occur in people with type 1 diabetes because the physiology of exercise works a bit differently. When we exercise we need more glucose to fuel our muscles. So in people without diabetes, glucagon is activated and causes the liver and muscles to breakdown glycogen in a process called glucogenolysis. At the same time, the liver starts to replenish its glucose stores from amino acid backbones through gluconeogenesis, so it can have backup glucose for extended periods of activity. Concurrently the level of insulin declines, weighing the hormonal milieu away from energy storage to energy generation.
In people with type 1 diabetes, insulin levels, injected exogenously, don’t decline. This blunts glucogenolysis and continues to enhance glucose uptake by the muscles. While the muscles need glucose, too-rapid consumption depletes the glucose in the blood stream. If production of glucose by the liver cannot keep up with demand, hypoglycemia occurs. In people without diabetes the body provides a fail-safe when blood glucose levels decline. The stress hormones, a growth hormone and cortisol are secreted and restore glucose levels into the normal range. In people with type 1 diabetes, repeated hypoglycemic reactions can dull the response of these hormones, especially if repeated insulin reactions occur during exercise. These physiological changes mean that exercise-induced hypoglycemia can be a problem for more than 24 hours post exercise.
Along with hypoglycemia, the threat of hyperglycemia can also make exercising more of a risk for people with type 1 diabetes. Certain exercises are glucose intensive. High intensity exercise, such as weight lifting, drives the rate of glucose production soaring. In people without diabetes this is compensated for by a boost in insulin production, which is lacking in people with type 1 diabetes. Generally, in people in good metabolic control, this condition is transitory. As exercise increases muscle sensitivity, rapid uptake of glucose by the muscles restores normal glycemia in 1 to 2 hours. Exercise in the presence of ketones and elevated glucose levels can spiral hyperglycemia as a severe lack of insulin fosters ever greater amounts of glucose creation.
Both hypoglycemia and hyperglycemia make exercise challenging for people with diabetes. Many episodes of hypoglycemia can be prevented by checking blood glucose frequently and having a plan to adjust insulin doses and consume adequate carbohydrate. (You can find some tips for exercise safety here). Avoiding exercise when blood glucose is elevated and ketones are present will help prevent exercise induced ketoacidosis.
If you have questions about exercising when you have diabetes, make an appointment with a Joslin Diabetes Center exercise physiologist. Click here for more information.