Even under extreme starvation, the brain relies on glucose for about one-third of its energy.

“George Cahill demonstrated in those very famous fasting studies Circa you know 1960s 1970s that even in the presence of glucose the brain was still taking up significant beta hydroxy beate so if I'm not if I'm not misremembering this these subjects were fasted for a very long period of time I mean these were 40-day water-only fasts so these individuals had beta hydroxy beate levels of four to five Millo which actually exceeded glucose concentration by this point glucose concentration would have been about 3 Millo in steady state”

“one of the more interesting studies he did which it's possible he did while you were even a student there was the 40-day starvation study”

“So you'd have inpatient subjects given nothing but water and minerals for 40 days. And it was done to basically demonstrate what the steady-state fasting levels of glucose, insulin, BHB, and acetoacetate would be.”