General Characteristics of the Absorptive State
Metabolic changes during this period are driven by a sharp shift in the hormonal profile: insulin secretion increases, while glucagon secretion decreases, establishing a high insulin-to-glucagon ratio. The body shifts into a mode of active nutrient assimilation and storage.
In adipose tissue, hormone-sensitive TAG lipase is converted into its dephosphorylated (inactive) form. As a result, lipolysis (fat breakdown) is completely inhibited.
Substrate Utilization Pathways in Organs
The distribution of energy substrate fluxes depends on the functional specialization of specific tissues:
- Liver: Glucose is directed toward glycogen biosynthesis or undergoes glycolysis to pyruvate and acetyl-CoA. Acetyl-CoA either enters the tricarboxylic acid (TCA) cycle to generate ATP and CO₂ or is used for triacylglycerol (TAG) synthesis. The resulting TAGs are packaged into very-low-density lipoproteins (VLDLs) and secreted into the blood.
- Adipose tissue (adipocytes): The primary function is energy storage as TAGs. Insulin accelerates cellular glucose uptake via GLUT-4 receptors. Glycolysis generates acetyl-CoA (a substrate for fatty acids) and glycerol-3-phosphate (required for esterification). Concurrently, the pentose phosphate pathway is stimulated to produce NADPH.
- Muscle tissue: Glucose is oxidized in the TCA cycle to meet energy demands or stored as glycogen. Amino acids are actively incorporated into protein biosynthesis.
- Other tissues: Also take up amino acids to synthesize endogenous proteins and essential derivatives, while obtaining energy via substrate oxidation in the TCA cycle.
Specifics of Lipid Metabolism in Adipocytes
De novo fatty acid synthesis in human adipose tissue occurs at a high rate exclusively in a specific situation: upon the resumption of feeding after prolonged starvation.
Under normal dietary rhythms, adipocytes primarily utilize preformed fatty acids for TAG synthesis. These are extracted from the blood, where they circulate as part of chylomicrons (dietary fats) and VLDLs (hepatic fats). The release of fatty acids from these complexes is mediated by lipoprotein lipase (LPL).
Amino Acid Metabolism in the Liver
Amino acids are absorbed in the intestine and delivered to the liver via the portal vein. The liver distributes them along three main pathways:
- Biosynthesis: Production of endogenous and export proteins, as well as other nitrogen-containing compounds.
- Export: Excess amino acids are directed into the systemic circulation to meet the needs of other organs.
- Deamination: Activated when amino acids are in excess. The cleaved nitrogen is eliminated via the urea cycle. The carbon skeletons enter the common catabolic pathway (oxidized to CO₂ and H₂O) or serve as precursors for glucose and lipid synthesis.
Transition to the Postabsorptive State
The transition from the absorptive to the postabsorptive state occurs between meals. During this time, external nutrient intake ceases entirely, requiring a global metabolic reorganization to maintain blood glucose levels and energy supply.
हार्मोन hormone synthesis and secretion rates change during this shift. These alterations can be:
- Adaptive process: A normal response to changes in physiological activity and food deprivation.
- Pathological process: Occurring as a result of disorders directly within endocrine glands or their regulatory systems.