[brunov]

No, the brain can only use ketone bodies after days of carbohydrate deprivation. You can't achieve ketosis overnight by skipping breakfast.

If there's not enough glycogen in the liver and/or muscles and you are not getting any sugar from food, the body starts degrading your bodies' protein to convert glucogenic aminoacids to glucose, and thus maintain blood glucose levels and avoid fainting.

[Evgeny]

If there's not enough glycogen in the liver and/or muscles and you are not getting any sugar from food, the body starts degrading your bodies' protein to convert glucogenic aminoacids to glucose

But if you have dietary glucogenic aminoacids, they will be converted to glucose and not your body protein.

I would adjust your comment above: "and you are not getting any sugar or glucogenic aminoacids from food"

[brunov]

You are right, of course.