Postabsorptive

Postabsorptive is a metabolic state that follows the period after a meal when nutrients have been absorbed from the digestive tract. It starts several hours after eating and lasts until the next intake of food. In this phase, the body shifts from storing energy to mobilizing stored fuels to maintain blood glucose and overall energy supply.

Hormonal changes drive the transition. Insulin levels fall, while glucagon, epinephrine, cortisol, and growth hormone rise. This hormonal milieu promotes catabolic pathways: hepatic glycogenolysis releases glucose; gluconeogenesis from amino acids, lactate, and glycerol begins to sustain glucose production. Adipose tissue increases lipolysis, releasing fatty acids and glycerol for energy and for use in gluconeogenesis. Fatty acid oxidation rises in various tissues, and the liver contributes to energy needs by producing ketone bodies during prolonged fasting.

ATP generation during the postabsorptive state relies increasingly on fatty acids, with glucose sparing for tissues most dependent on it, such as the brain during longer fasts when ketone bodies become a significant energy source. Ketogenesis in the liver provides alternative fuels that support physiological functions when glucose is limited.

This state contrasts with the absorptive (fed) state, in which insulin predominates and most tissues take up glucose, amino acids, and lipids for anabolic storage. The postabsorptive state is essential for maintaining blood glucose and energy balance between meals.