The brain can also use a different engery source: ketone bodies. They can be produced from fatty acids. The body starts doing this when glucose levels fall too

The brain can also use a different engery source: ketone bodies. They can be produced from fatty acids. The body starts doing this when glucose levels fall too low. You can function just fine for prolonged periods of time without ingesting any glucose sources. It does take the body a while to adjust and ramp up ketone production, which can lead to feeling drained/low energy and induces sugary-rich food cravings. But that mostly subsides after a few days. Nice side effect: it is quite common to get a certain sense of eurphoria and well-being after not getting sugar for a few days. The current hypothesis is that one of the ketone bodies (BHB) might have similar effects on the brain as GHB, which is a psychoactive drug. (they have a similar structure, namely they are isomers) Personally, I feel much more energetic and healthy when skipping carbs and fruits entirely. Even mentally! At least after the initial adjustment. Sadly it's hard to fall back into eating sugar all the time.