I stand corrected, but there are limitations:

From:Steven B. Harris
Newsgroups: sci.med.nursing,sci.med
Subject: Re: very low blood sugar level

"All cells - even the brain - can also use ketones, intermediates in the oxidation of fats. But there's a limit (about 50%) as to what extent the brain can switch from sugars to ketones, even in starvation."

"And maybe most important, it takes days for the brain to switch to using ketonbodies in the first place. Wich excludes them from saving the brain in any acute episodes."

"Yes, but even weeks of starvation and maximal brain switch to ketone metabolism will not save you from blackout and brain damage if your blood glucose falls to very low levels. They key to the danger is that the changeover is not ever complete, not that it takes too long."