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In early fasting, liver glycogenolysis is the main source of blood glucose, while gluconeogenesis begins using lactate, alanine, and glycerol. As fasting continues and glycogen reserves decline, gluconeogenesis and ketone production become more important. Adipose tissue releases fatty acids and glycerol; muscle initially supplies gluconeogenic amino acids, then reduces proteolysis as it uses fatty acids and ketones. The brain gradually increases its use of ketones, while red blood cells and the renal medulla continue to depend on glycolysis. The kidney also contributes to gluconeogenesis and helps buffer acid load. Hormonal changes coordinate these shifts, supporting glucose supply and protein sparing. The lesson also considers the plasma fuel sequence, nitrogen changes, acid–base balance, and refeeding risk.