That’s false. Carbs are not a requirement for human body. Proof of that are native populations living far north, exclusively off of animal food sources.

Human body prefers ketones as a source of energy. But glycogen is faster.

My simplified mental model is like this. Ketones are normal gasoline. And glycogen is racing fuel (ethanol). If you run a stock car on racing fuel it may start and drive for a while but it’s damaging the engine and creates unnecessary wear.

“Ketones are normal gasoline” is a very fringe view.

Ketone supplements have been available for a while, and nobody was able to show any benefit of using them. 100% snake oil.

If on the other hand you want to produce ketones endogenously, you basically have to put your body in a state of starvation. It’s not really gasoline then, more like burning the passenger seats as fuel so you can arrive at the destination.