Postabsorptive State Lecture Notes

Introduction

• Postabsorptive State Timing:
  • Occurs more than 4 hours after a meal.
  • Typically during the night or early in the morning.
• Key Goal:
  • Maintain consistent blood glucose levels between meals.
  • Ensure glucose availability for cells that primarily use glucose, like brain neurons.

Energy Sources in Postabsorptive State

• Fat Utilization:
  • Promoting the use of fats for energy to reserve glucose for essential cells.
• Sources of Glucose:
  • Glycogenolysis:
    • Occurs in the liver and skeletal muscle.
    • Conversion of glycogen to glucose, producing glucose-6-phosphate and generating an extra ATP.
  • Lipolysis:
    • Breakdown of triglycerides into glycerol and fatty acids in the liver and adipose tissue.
    • Glycerol used for gluconeogenesis to reverse-engineer glucose.
  • Protein Catabolism:
    • Breakdown of cellular proteins at a higher rate during prolonged fasting.
    • Important energy source when fasting.

Role of the Liver

• Central in glycogenolysis, gluconeogenesis, and protein catabolism.
• Regulates nutrient pools for other cells.

Hormonal Control

• Sympathetic Nervous System:
  • Dominates during the postabsorptive state.
• Insulin & Glucagon Dynamics:
  • High glucose triggers absorptive state.
  • Low glucose triggers postabsorptive state with reduced insulin.
  • Moderate alcohol intake can increase insulin efficiency.
  • Glucagon:
    • Antagonistic to insulin; referred to as a hyperglycemic hormone.
    • Stimulated by declining blood glucose.

Effects of Glucagon

• On Hepatocytes:
  • Promotes glycogenolysis and gluconeogenesis.
• On Adipose Cells:
  • Promotes lipolysis, increasing circulating fatty acids.
• On Muscle Tissues:
  • Promotes protein breakdown to amino acids for gluconeogenesis.

High Protein, Low Carb Meal Scenario

• Impact:
  • Triggers insulin and glucagon release.
  • If unbalanced, it can lead to hypoglycemia affecting the brain.
  • Glucagon helps stabilize glucose by promoting glycogenolysis and gluconeogenesis.

Adipose Tissue and Sympathetic Nervous System

• Stimulates lipid breakdown when glucose is low.
• Triggers:
  • Low plasma glucose, fight-or-flight response, exercise.
• Hormone:
  • Epinephrine (Adrenaline).

Additional Hormonal Influences

• Growth hormone, thyroxine, sex hormones, corticosteroids also impact metabolism and nutrient balance.