In prolonged starvation the main energy source of brain is:
STARVE FEED CYCLE
Divided into 5 stages:

Well-fed state (1–4 hours after food)
Early fasting (4–16 hours after food)
Fasting (16–48 hours after food)
Starvation (2–3 days without food)
Prolonged starvation (> 5 days without food).
Glucose
Ketone bodies
Fructose
Fatty acids