When the brain is fed by β-hydroxybutyrate, it has actually very little need for glucose. George Cahill (author of “Starvation in Man”) asserted that the brain needs glucose, but Benjamin Bikman asserts that that has never been demonstrated. In any case, the erythrocytes (red blood corpuscles) clearly do need glucose, since they can’t metabolise fatty acids or ketone bodies, so the body does have a need for a minimal amount of glucose.
That said, however, the amount of circulating glucose, if the body is allowed to manage it on a low-carbohydrate diet is not more than 4-5 grams total. Stephen Phinney has told the story in a few of his lectures of friends of his who were studying people on long-term fasts in the late nineteen-sixties, who one afternoon decided to see what happened if they applied a hyperinsulinaemic eugylcaemic clamp to their subjects. They drove the subjects’ serum glucose down to levels that would normally cause coma and death, but because of their ketone levels they experienced no symptoms of hypoglycaemia.
Granted, such an experiment was dangerous and would not be tolerated by any sane ethical review board today, but it does illustrate that the rules are different on keto. We have had similar anecdotal reports on these forums from members whose serum glucose dropped into the 30’s or 40’s with no ill effect. However, I’d say that anyone experiencing the slightest symptom from such a low serum glucose should seek help immediately.