Not true in the slightest bit. If “our brain starts shutting down the body,” how would those on the ketogenic diet survive?
We start producing ketone bodies which our brain can use for fuel. Some argue a ketogenic diet is healthier than a carb heavy diet.
And others would vigorously disagree.
I'm speaking as a runner and my comment about shutting down referred specifically to "hitting the wall", because that is exactly what happens. I burn between 2000 - 2500 calories running a half marathon. My muscles need glycogen to function. To have to convert protein to glucose once my supply is depleted isn't efficient. And yes, once glycogen supplies are depleted, your brain will start to shut things off if you don't replenish.