The brain does need some glucose to function, but the bulk of its energy need can be met by ketones. After adapting to this energy source the need for glucose i

The brain does need some glucose to function, but the bulk of its energy need can be met by ketones. After adapting to this energy source the need for glucose is low enough that it can be met entirely by gluconeogenesis (making glucose from proteins and fats). Humans can survive on a zero carbohydrate diet with no cognitive impairment.