such thing as an essential dietary carbohydrate

However, there is no such thing as an essential dietary carbohydrate - your body is perfectly capable of making as much glucose as it needs. In fact, in the absence of dietary carbohydrate, the brain runs so efficiently off of ketone bodies that you can drastically deplete glucose levels of people in deep ketosis and it won’t affect them. A 1972 paper describes an ethically questionable study where overweight patients fasted for 2 months, putting them in deep ketosis meaning they were producing a very large amount of ketone bodies. Insulin was then infused into these patients until their blood glucose went as low as low as 9mg per deciliter -

a level that should cause coma or death. However, the patients didn’t really react to this. There was no particular change in cognition, nor did their body react to the drastic drop in blood sugar. "The only people in the room who had palpations and sweating and anxiety were the physicians. The people who had the blood sugar levels that should be associated with coma or death were completely unphased. Had completely normal mentation. It is clear that the human brain is perfectly happy with beta hydroxybutyrate as not just its majority fuel, but essentially its sole fuel."