When your body runs out of glucose in blood and glycogen in liver, it starts creating new one from protein and fat [1] for the few processes that need it, while

When your body runs out of glucose in blood and glycogen in liver, it starts creating new one from protein and fat [1] for the few processes that need it, while switching most metabolism (including brain) to use fat-derived Ketone bodies as fuel [2]. This normally happens when fasting, and is no big deal. It's how people survive days without food. Now there's something going on with how brain processes ketone bodies and glucose differently and there some research about how that affects things like Alzheimer's disease but I don't know that much about that. [1]: https://en.wikipedia.org/wiki/Gluconeogenesis https://en.wikipedia.org/wiki/Gluconeogenesis [2]: https://en.wikipedia.org/wiki/Ketosis https://en.wikipedia.org/wiki/Ketosis