>>11923464No one has actually talked about the science yet. The action of fasting produces a unique pattern in our physiology that makes good teleological sense. To understand it you need to understand the roles of the hormone insulin. To just sum it up, insulin is an anabolic hormone released by the pancreas in response to glucose and (to a lesser extent) amino acids in the blood, that functions to promotes energy storage. Energy storage can be by forming glycogen, or by forming fat.
When you start eating, your insulin goes high in response to the new glucose in your blood, and insulin acts to store the energy you just ate. When you stop eating, your insulin eventually declines, which promotes energy utilization. Glycogen is the easiest store to access and so for the first 12-24 hours of your fast, you will primarily be breaking down glycogen stored in the liver to supply a steady source of glucose to your blood, which you need some level of because the brain is an obligate user of glucose. After you have exhausted your glycogen stores, your brain still needs glucose to function so the liver makes glucose out of amino acids and glycerol. This will continue throughout the fast. After your glycogen stores are used and insulin is still low, you will start to produce ketone bodies. These are an alternative fuel that most of your tissues can use, including your brain (though its important to note that the brain still needs some glucose to function, as much as 2/3 of its energy requirement must be through glucose). Your liver synthesizes these ketones from fats mostly and exports them to the blood. This is what is known as the state of "ketosis". This will continue until you break your fast. Throughout this process, your adipose tissue releases fat (due to lack of insulin, increase in glucagon, epinephrine etc.) and most of your tissues can utilise this fat directly without needing them to be turned to ketones.
