"During the first 10 hours of starvation, when the body utilizes any consumed food, stored glycogen becomes the main source of energy. When liver glycogen store

"During the first 10 hours of starvation, when the body utilizes any consumed food, stored glycogen becomes the main source of energy. When liver glycogen stores deplete, amino acids act as a source of energy. Eventually fatty acids and glycerol from adipose tissue serve as a major source of energy. If starvation continues for a week or more, fat derived β-hydroxybutyrate becomes the most abundant ketone in body while glucose production becomes very low. When fat stores are depleted rapid muscle degradation occurs and provides energy to the body through gluconeogenesis. Weight loss caused by starvation is initially intense and rapid but eventually becomes gradual. A study has shown that during the first week of starvation average weight loss is 0.9kg/day and reduces to 0.3kg/day by the third week." You should probably try reading the article, since the entire thing deals with the science of fasting and starvation and they talk about everything from intermittent fasting to multi-week starvation diets. 72 hour fasts, alternate day fasting, and IF provide the greatest return for the least physiological cost (in that order).