80-90% of brain mass are not neurons, but the "supporting" tissue, taking care of things like storing nutrients supply. It is entirely possible to loose that extra mass and still be fully functioning.
OTOH if the supporting tissue id not excessive, what are the circumstances when it's important? Will this person have hard time holding breath without fainting? Fasting without fainting? Act well under a high prolonged intellectual load?
You'll reach thermal and energy constraints.
Or, only half-joking here, you'll fall prey of a terrible depression and withdraw from all human company.