Your brain (and a few other tissues) must have a small amount of glucose to function, even when it is mainly running on ketones. However, it does not need you to eat carbs in order to provide glucose. Even in the absence of all carbohydrates, if protein is being eaten then your body can synthesize glucose from the protein--a process called gluconeogenesis. And if protein is not being consumed, or the person is in a total fast, then the body will cannibalize its own muscle tissue in order to break it down for the necessary glucose.

I think this is probably one of the reasons Atkins said that the Fat Fast was only to be used for a very few days, to break stalls.

Emily