Welcome! Oh also if you wanna learn more about how your body signals itself to use more of one or the other fuel between carbs and fat, look up insulin and glucagon and the interactions between them in the two dimensional space of their various levels. Glycogen stores, fat, and protein hanging out in, I believe your liver? can be used to pull fuel into your blood in addition to the food in your digestive system. Your glucagon and insulin levels both respond to and manage levels of each of these fuel sources. High glucagon is overall catabolic, while high insulin is overall anabolic. There are organ groups that will prevent catabolism, like say cannibalization of your muscles, even with high glucagon during fasting, unless your body needs to use it as a fuel source more than it needs it for its usual purpose, in this case if you use your muscles at all while fasting, you will keep your muscles because your body is like oh, I need that for moving around in the universe.