Ketone bodies do just fine for the brain. I've been through a 10 day supervised water fast and worked all the time, no problems with the concentration or energy levels except for days 2 and 3. I've done plenty (hundreds) of 36hr fasts and if anything, my brain seems easier to focus on a specific task, or to work longer, while fasting.

Glucose is needed for some red blood cells though, and the body has ways of producing it in absence of food. Those are insignificant amounts.