Muscles hold enough energy stores for a short burst of activity. After that, they depend on increased blood supply to deliver oxygen, blood sugar and other nutrients to manufacture more energy. Your body burns the sugar in your blood, and then calls for your liver to supply stored glucose to keep up with energy demands. This causes fluctuations in your blood sugar when you exercise.
As you warm up, your muscles start to call for nutrients to manufacture energy. Glucose carried in your blood and delivered to the muscles is an energy supply, as are free fatty acids, a type of lipid carried in blood that provide energy when glucose is low. Using energy during exercise helps balance high blood sugar and provide fuel at the same time. As blood flow to your muscles increases, the energy supplies increase as well. Your muscle cells send signals to start burning glucose, and more of it is delivered to the cells. This lowers your blood sugar levels.
Sugars from the foods you eat are stored in your liver and in other tissues in a form called glycogen. When your body requires more sugar than is available in your blood, it starts to convert stored sugars to a usable form, releasing them into the blood. Blood sugar levels in your blood increase as muscles and oter tissues call for release of energy into your bloodstream. When glycogen provides fuel for your muscles, your blood sugar fluctuates up and down as it's used.
Elevated Blood Sugar
If your blood sugar is high when you begin to exercise, it can climb higher. This is because your body does not recognize the glucose in your blood, and calls for your liver to break down more glycogen. If your blood sugar is high before exercise, you should wait until it is within normal range before you exercise, according to Johns Hopkins Medicine. If you're diabetic, check your blood glucose before exercise and consider eating some high fiber, complex carbohydrates as a pre-exercise snack, because these promote a slow and steady rise in blood glucose that can sustain you during exercise.
Post Exercise Blood Sugar
Your body uses all available and stored sugars when you exercise. It is possible to burn off all the stored glycogen. When this happens, you might feel dizzy, your vision gets poor and, if the problem is severe, you might become unconscious. This is because your brain requires glucose to work and, if the level becomes too low, you could pass out. When your exercise session is complete, your blood sugar levels tend to stay low for a while, as the stores are rebuilt in your liver and muscles.