General characteristics of the absorptive state
Changes in metabolism during this period are driven by a sharp shift in the hormonal background: insulin secretion increases while glucagon secretion decreases. A high insulin-glucagon ratio is established. The body enters a mode of active nutrient assimilation and deposition.
In adipose tissue during this time, hormone-sensitive triacylglycerol (TAG) lipase transitions into a dephosphorylated (inactive) form. As a result, the process of lipolysis (fat breakdown) is completely inhibited.
Pathways of substrate utilization in organs
The distribution of energy substrate fluxes depends on the functional characteristics of specific tissues:
- Liver: Glucose is directed toward glycogen biosynthesis or undergoes glycolysis to pyruvate and acetyl-CoA. The latter either is burned in the tricarboxylic acid (TCA) cycle with the production of ATP and CO₂ or is used for triacylglycerol (TAG) synthesis. The formed TAGs are packaged into very-low-density lipoproteins (VLDL) and secreted into the blood.
- Adipose tissue (adipocytes): The main function is energy storage in the form of TAGs. Insulin accelerates glucose transport into cells via GLUT-4 receptors. During glycolysis, glucose forms acetyl-CoA (a substrate for fatty acids) and glycerol-3-phosphate (required for esterification). Simultaneously, the pentose phosphate pathway is stimulated to produce NADPH.
- Muscle tissue: Glucose is oxidized in the TCA cycle to meet energy demands or is stored as glycogen. Amino acids are actively incorporated into protein biosynthesis.
- Other tissues: They also take up amino acids to synthesize their own proteins and essential derivatives, while obtaining energy through the oxidation of substrates in the TCA cycle.
Specifics of lipid metabolism in adipocytes
De novo fatty acid synthesis in human adipose tissue proceeds at a high rate exclusively in a specific situation: upon the resumption of feeding after preceding prolonged starvation.
Under a normal eating rhythm, adipocytes predominantly use preformed fatty acids for TAG synthesis. These are extracted from the blood, where they circulate as part of chylomicrons (dietary fats) and VLDL (fats from the liver). The release of fatty acids from these complexes is mediated by the enzyme 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: Creation of endogenous and export proteins, as well as other nitrogen-containing compounds.
- Export: Excess amino acid pool is directed into the systemic circulation to meet the needs of other organs.
- Deamination: Activated when amino acids are in excess. The cleaved nitrogen is removed via the urea cycle. The nitrogen-free residues (carbon skeletons) enter the common catabolic pathway (oxidized to CO₂ and H₂O) or serve as material for the synthesis of glucose and fats.
Transition to the postabsorptive state
The shift from the absorptive to the postabsorptive state occurs between meals. At this time, the external supply of nutrients ceases completely, requiring a global metabolic reorganization of the body to maintain glucose levels and energy supply.
The rate of hormone synthesis and secretion changes accordingly. Such shifts can be of a twofold nature:
- Adaptive process: a normal response to changes in physiological activity and the absence of food.
- Pathological process: occurs as a result of disorders directly within the endocrine glands or their regulatory systems.
Briefly: answers to topical questions
Which biochemical processes in the liver ensure the maintenance of blood glucose levels during the postabsorptive period?
The maintenance of blood glucose levels during the postabsorptive period is ensured by the breakdown of stored carbohydrate and the synthesis of glucose from non-carbohydrate precursors. The following biochemical pathways take place in the liver:
- Glycogenolysis — mobilization and breakdown of liver glycogen, proceeding with the participation of the enzyme glycogen phosphorylase.
- Gluconeogenesis — de novo glucose synthesis, the key enzymes of which during this period are PEPCK (phosphoenolpyruvate carboxykinase) and fructose-1,6-bisphosphatase.
Why is the pentose phosphate pathway activated in adipose tissue during the absorptive period?
During the absorptive period, the pentose phosphate pathway in adipose tissue is activated to produce coenzymes required for lipogenesis. This metabolic pathway accomplishes the following tasks:
- Ensures the reduction of NADP⁺ to NADPH.
- The generated NADPH acts as a reducing equivalent necessary for de novo fatty acid biosynthesis.
What substrates do muscles use to obtain energy during the postabsorptive period?
During the postabsorptive period, muscle tissue switches to alternative energy sources. Muscles use the following as energy substrates:
- Fatty acids.
- Ketone bodies.
Additionally, during prolonged fasting, proteolysis is activated in muscles under the action of cortisol.
Class and exam questions
What are the sources of fatty acids for TAG synthesis under normal nutrition?
Normally, preformed fatty acids derived from chylomicrons and VLDL via the action of LPL are used. De novo synthesis is activated only after fasting.
How does insulin affect glucose uptake into adipose tissue?
Insulin significantly accelerates glucose transport into adipocytes by stimulating the translocation of GLUT-4 transporters to the cell membrane.
What is the fate of nitrogen-free amino acid residues in the liver?
After deamination, their carbon skeletons enter the common catabolic pathway for oxidation to carbon dioxide and water, or are utilized for the synthesis of fats and glucose.
What else to study on this topic
- Mechanism of activation and operation of GLUT-4 transporters
- Features of the pentose phosphate pathway and NADPH production
- Urea cycle and nitrogen disposal from amino acids
- Differences between VLDL and chylomicrons
- Regulation of hormone-sensitive TAG lipase
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