Professor Richard D. Feinman says;
It is true that your brain needs glucose, but glucose can be supplied by the process of gluconeogenesis; that is, glucose can be made from other things, notably protein. This is a normal process: when you wake up in the morning, between thirty and seventy percent of your blood glucose comes from gluconeogenesis. There is no requirement for dietary glucose.

Source: Why You Don’t Want the “Experts” to Tell You What to Eat