Metabolic States of the Body: The Postabsorptive State

The postabsorptive state typically begins about four hours after a meal and lasts until the next meal is consumed. During this period, the digestive system stops absorbing nutrients, and the body shifts to utilizing stored energy reserves to maintain stable blood glucose levels.

Initially, the liver breaks down stored glycogen to release glucose into the bloodstream. As glycogen depletes, the liver performs gluconeogenesis, synthesizing glucose from non-carbohydrate sources like glycerol and amino acids. This ensures continuous glucose supply for glucose-dependent tissues like the brain and red blood cells.

Triglycerides in adipose tissues are broken down through lipolysis into fatty acids and glycerol. Fatty acids are transported to tissues like muscle for oxidation and energy production, while glycerol enters the bloodstream and is used by the liver for gluconeogenesis.

The body employs a glucose-sparing strategy to conserve glucose for the brain and red blood cells, which rely predominantly on glucose for energy. Other tissues switch to alternative fuel sources like fatty acids from adipose tissues, and over time, ketone bodies produced from fatty acids in the liver.

In response to a drop in blood glucose, the pancreas releases glucagon, which stimulates glycogenolysis and promotes gluconeogenesis. These hormonal actions ensure a continuous supply of glucose into the bloodstream to maintain stable blood glucose levels during the postabsorptive period.

During prolonged fasting, tissue proteins are catabolized to release amino acids into the blood. The liver utilizes these amino acids as non-carbohydrate sources to produce glucose through gluconeogenesis, helping sustain blood glucose levels when glycogen stores are exhausted and energy demands persist.

The postabsorptive state is the initial phase of fasting lasting several hours after a meal. If fasting continues beyond this period, the body enters metabolic states of the body fasting and starvation, where ketone bodies become increasingly important as an energy source and protein breakdown accelerates.