Why do my blood sugar levels rise during exercise?

If you go for a run early in the morning before breakfast, your blood sugar can rise. This is because your body doesn't have enough energy (carbohydrates) in its reserves. As a result, your liver will start producing glucose on its own to give you enough energy to finish your run. The same goes for swimming, cycling, walking, and all other forms of (intensive) exercise. Even vacuuming before eating can cause a rise. To prevent this, it's best to eat something before exercising, like a piece of fruit. Or just exercise after breakfast!