24.5|Metabolic States of the Body

Metabolic States of the Body

Overview of Metabolic States

• Three metabolic states are defined, allowing the body to manage energy supply effectively.

• Key abilities by the end of this section:

  • Describe each of the three metabolic states.

  • Explain the absorptive state processes.

  • Explain the postabsorptive state processes.

  • Describe glucose processing during starvation.

Importance of Energy Supply

• The body needs a continuous supply of glucose, particularly for brain function.

• Food intake occurs periodically, but energy demands persist, necessitating energy storage for later use.

• Mechanisms are in place for energy storage and mobilization during fasting/starvation periods to avoid constant eating.

Absorptive State (Fed State)

• Occurs after a meal when digestion and nutrient absorption take place.

• Characterized by:

  • Anabolism surpassing catabolism.

• Digestion process:

  • Begins with food intake, breaking food down into constituent parts.

  • Carbohydrate digestion starts in the mouth, whereas proteins and fats are processed in the stomach and small intestine.

• Nutrient absorption:

  • Nutrients are transported across the intestinal wall into the bloodstream (sugars, amino acids) or lymphatic system (fats).

  • Transported to the liver, adipose tissue, or muscle cells for energy use or storage.

• Duration:

  • The absorptive state can last up to 4 hours, depending on nutrient intake.

• Role of insulin:

  • Triggered by increased blood glucose levels from food ingestion, released by pancreatic beta cells.

  • Functions:

  • Facilitates blood glucose absorption by liver hepatocytes and muscle/adipose cells.

  • Converts glucose into glucose-6-phosphate inside cells, establishing a concentration gradient for glucose entry from blood to cells.

  • Stimulates storage of glucose as glycogen in liver and muscle cells for future energy needs.

  • Promotes protein synthesis in muscles, with muscle protein available for catabolism during starvation.

• Energy usage:

  • Immediately utilizes newly ingested dietary fats and sugars.

  • Excess glucose stored as glycogen or fat in adipose tissue and triglycerides.

Figure 24.21 - Absorptive State

• Visual representation summarizing metabolic processes during the absorptive state.

Postabsorptive State (Fasting State)

• Occurs when food has been digested, absorbed, and stored.

• Commonly experienced overnight or when meals are skipped.

• Key characteristics:

  • Initially relies on stored glycogen as glucose levels drop due to cellular use.

  • Insulin levels decrease concurrently with blood glucose levels.

  • Maintains normal blood glucose range (80–120 mg/dL) in response to hunger.

• Role of glucagon:

  • Released from pancreatic alpha cells in response to falling glucose levels.

  • Functions:

  • Inhibits glycogen synthesis in the liver.

  • Stimulates the breakdown of stored glycogen into glucose, which is sent to peripheral tissues and the brain, raising blood glucose levels.

  • Initiates gluconeogenesis in the liver to replace glucose used by tissues.

• Energy dynamics:

  • Peripheral tissues prefer glucose; after prolonged fasting, gluconeogenesis continues to replenish depleted liver glycogen stores.

  • Any excess glucose absorbed by the liver is converted to triglycerides and fatty acids for long-term storage.

Figure 24.22 - Postabsorptive State

• Visual representation summarizing metabolic processes during the postabsorptive state.

Starvation

• Prolonged lack of nourishment induces a 'survival mode'.

• Energy management priorities include:

  • Provisioning glucose or fuel for the brain.

  • Conservation of amino acids for proteins.

• Keton bodies emerge as an alternative energy source for the brain and glucose-dependent organs, preserving protein integrity.

• Glucose dynamics in starvation:

  • Glucose levels are significantly low; glycolysis ceases in cells capable of utilizing alternative fuels.

  • Muscles use fatty acids in lieu of glucose.

• Alternative processes:

  • Fatty acids convert to acetyl-CoA to generate ATP via the Krebs cycle.

  • Pyruvate, lactate, and alanine are exported to the liver for gluconeogenesis, rather than entering the Krebs cycle.

• As starvation prolongs, glycerol from fatty acids aids gluconeogenesis.

• Post several days of starvation:

  • Ketone bodies become a major energy source, reducing reliance on protein breakdown.

  • Fatty acid and triglyceride stores convert into ketones, preserving protein for gluconeogenesis.

  • Once fat stores deplete, muscle proteins are broken down for glucose synthesis.

• Overall survival during starvation is contingent upon body fat and protein stores.