Exercise affects blood sugar so it is important to know how to monitor your glucose levels during workouts. Factors affecting blood glucose during exercise include insulin levels, training status, intensity, and duration.
Blood insulin determines how much carbohydrate and fat can be used as fuel. The right insulin level prevents fuel delivery problems. Blood glucose must be low enough for muscles to easily access internal stores of fat and glycogen, but high enough that glucose can still be transported into the muscle.
The glucose already present in the blood is the first fuel source used, followed quickly by the release of glucose from muscle and liver glycogen stores. The glycogen stored in the liver can be depleted after 20 to 30 minutes of strenuous exercise. Fat stores are about 2,000 times larger than glucose stores, making them nearly impossible to deplete. Blood insulin levels must fall before internal fat stores can be accessed for long periods of activity.
High insulin levels cause muscles to use glucose from the blood rather than glycogen stores. Less fat is released to replace the dropping glucose level, and blood sugar will drop rapidly unless carbs are eaten. Conversely, low insulin levels cause the release of excess glucose and free fatty acids into the blood. Glucose is less able to enter exercising muscles, which weakens performance and causes blood glucose levels to rise.
Being trained for a particular exercise will decrease the required glucose by as much as 25 percent. A physically fit person has larger glycogen stores to draw upon, because training builds glycogen stores in the muscles involved in a specific exercise. This usually dampens blood sugar fluctuations, providing more glucose for release during falling blood sugar, and allowing more glucose to be shifted into glycogen stores during rising blood sugar.
Glycogen stores grow after any new activity, in case the body does the same activity again. However, the glycogen stores developed begin to disappear when an exercise is missed for three days. When a routine exercise schedule is interrupted, the involved muscles will undergo more glycogen rebuilding upon a return to exercise. A larger than normal fall in blood glucose will also occur in the first few days of restarting the routine, so a larger reduction in boluses and basal rates is required.
Walking and running both burn the same amount of calories, but the source of those calories differs. During mild exercise like walking, about 80 percent of the calories consumed come from fat while 20 percent come from glucose. While running, about 80 percent of the calories come from glucose. Mild and strenuous exercise may consume a similar amount of calories, but the necessary carb intake and insulin reductions will differ. The higher percentage of glucose used during strenuous exercise is more likely to cause a drop in blood sugar, so a higher carb intake or larger reduction in insulin doses is required.
Longer activities consume more calories, and so are more likely to drop blood glucose levels unless the insulin level is too low. Whereas a 30 minute walk has little impact on blood glucose, a 60 minute walk may require extra carbs or a small insulin reduction.
For someone with a working pancreas, when exercise extends beyond 40 to 60 minutes, blood insulin gradually falls so the body can preserve limited glucose and glycogen stores and switch to use more fat for fuel. For someone with diabetes, basal rates and boluses often need to be lowered so the body can switch from using glucose to using fat. If the insulin level remains too high, most of the burned calories would have to be eaten to keep the blood glucose level from falling.
Exercise affects blood sugar
If you have diabetes make sure you are monitoring your blood sugar before and after exercising. Also remember that insulin levels, training status, intensity, and duration, can affect the level of glucose in your bloodstream. If you have specific questions about exercising with diabetes we recommend that you contact your doctor or diabetes educator.