The body naturally shifts to burning more fat when carbohydrate is restricted to conserve glucose.

“You can run your ass off for hours, and if you're only burning carbohydrate, you just get on the glycogen treadmill. And so then, what people do to counter that is they say, "Oh, I'm going to remove all the carbohydrate and shift my body into ketosis so I'm burning fat all the time. So that when they're running, they are burning fat. But, again, it's not necessary because it turns out, when we become more restrictive, your body is smart. And when we become more restricted, you naturally start shifting to more and more and more fat because your body knows to reserve glucose...conserve it. It wants to.”