Biochemistry
When food intake ceases, the body enters the starvation response. Initially, the body's glycogen stores are used up in about 24 hours. The level of insulin in circulation is low and the level of glucagon is very high. The main means of energy production is lipolysis. Gluconeogenesis converts glycerol into glucose and the Cori cycle converts lactate into usable glucose. Two systems of energy enter the gluconeogenesis: proteolysis provides alanine and lactate produced from pyruvate, while acetyl CoA produces dissolved nutrients (Ketone bodies), which can be detected in urine and are used by the brain as a source of energy.
In terms of insulin resistance, starvation conditions make more glucose available to the brain.
Read more about this topic: Starvation