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After a meal, insulin favors glucose use and storage, fat synthesis, and protein synthesis. As fasting begins, lower insulin and higher glucagon promote liver glycogen breakdown and release of fatty acids and glycerol from adipose tissue. With glycogen depletion, the liver makes glucose from lactate, glycerol, and alanine; muscle and red blood cells contribute substrates through lactate and alanine pathways. During prolonged starvation, fatty-acid oxidation supports hepatic energy needs while ketone production rises. The brain increasingly uses ketones, helping reduce reliance on muscle protein. The kidney also contributes to glucose production and acid buffering. These coordinated shifts illustrate how fuel priorities and organ roles change over time.