Fasting suppresses insulin and mobilizes fatty acids for fuel.

“So, when you stop eating, right, you suppress the hormone insulin, you mobilize fatty acids for fuel. The brain's not a good, it can't use the longchain fatty acids that are stored in your atapost tissue. So through beta oxidation of fatty acids in the liver uh beta o accelerated beta oxidation in the context of insulin suppression generates these molecules beta hydroxybutyrate and acettoacetate and then they spill into circulation and uh they become largely responsible for preserving brain energy metabolism in the context of uh energy deprivation or carbohydrate restriction.”