What does the brain run on during a long fast? · Grey Matter

At first on glucose, released from the liver's glycogen and then made from amino acids and glycerol.

What does the brain run on during a long fast?

At first on glucose, released from the liver's glycogen and then made from amino acids and glycerol. As the fast continues, the liver converts fat into ketone bodies (beta-hydroxybutyrate and acetoacetate), their blood level rises, and the brain takes them up and burns them in its mitochondria. In the classic measurements of Owen and colleagues (1967), after five to six weeks of starvation ketones supplied about 60 % of the brain's energy, and up to about two thirds by other estimates. This spares muscle protein that would otherwise be broken down to make glucose, while the brain keeps a smaller but necessary glucose supply.