If you exercise too much, you will burn up these reserves yoo quickly and get very exhausted and the reserves will end up being pulled from other sources, such as the muscle.
Exercise moderately, make sure you are putting in as much as you are getting rid of, regarding your nutrition.
If you feel very exhausted like a mack truck hit you, then it is time to go off the fast, especially if it lasts for more than one day.A sign of all reserves utilized and not enough left to sustain you through the fast safely.