Starvation Adaptation

Early Starvation (0-24h) - Fuel Scramble

• Goal: Maintain blood glucose (brain, RBCs).
• Hormones: ↓ Insulin, ↑ Glucagon, ↑ Cortisol, ↑ Epinephrine. (Insulin/Glucagon ratio ↓).
• Fuel Sources & Pathways:
  • Liver Glycogenolysis: Primary source for ~12-18h (depletes by ~24h).
  • Gluconeogenesis (GNG): Liver (later kidney). Substrates: Lactate, Alanine, Glycerol.
  • Lipolysis (Adipose): ↑ FFAs (fuel for muscle/liver) + Glycerol (GNG).
  • Muscle Protein Catabolism: → Alanine (GNG).
• Organ Fuel:
  • Brain: Glucose.
  • Muscle: Switches to FFAs.
  • RBCs: Glucose (obligate).

⭐ Liver glycogen is the main glucose source for the first ~12-18h of fasting.

Mid-Starvation (2-24 days) - Fat Burning Fiesta

• Duration: 2-24 days; shift to sustained fat catabolism.
• Hormones: ↓↓↓Insulin, ↑↑Glucagon, ↑Cortisol, ↑Growth Hormone.
• Adipose Tissue: Dominant lipolysis (Hormone-Sensitive Lipase activation). Releases fatty acids (FAs) & glycerol.
• Liver Metabolism:
  • ↑↑ FA β-oxidation → abundant Acetyl-CoA.
  • ↑↑ Ketogenesis: Acetyl-CoA → ketone bodies (β-hydroxybutyrate, acetoacetate) for brain/muscle.
  • Gluconeogenesis persists (glycerol, lactate, amino acids).
• Peripheral Tissue Fuel Use:
  • Brain: Adapts to ketone bodies (up to 70% energy), sparing glucose.
  • Muscle: Utilizes FAs & ketone bodies; ↓glucose uptake; significant protein sparing.
• Key Outcome: Urea nitrogen excretion ↓, reflecting protein conservation.

⭐ The brain's ability to switch to ketone bodies as its primary fuel source during prolonged fasting is crucial for sparing muscle protein.

Prolonged Starvation (>24 days) - Protein Predicament

• Fat reserves critically low/exhausted; protein becomes the main endogenous fuel.
• Accelerated proteolysis: structural (muscle) and functional (enzymes, albumin, immunoglobulins) proteins catabolized.
  • Results in severe muscle wasting (cachexia).
  • ↓ Serum albumin → oncotic edema.
  • Compromised immunity (↓ immunoglobulins).
• ↑ Urinary nitrogen (urea) excretion signifies rapid protein breakdown.
• Brain still utilizes ketone bodies, but gluconeogenesis from amino acids is vital for glucose supply.
• Leads to multi-organ dysfunction (respiratory, cardiac) and eventual death.

⭐ The body sacrifices crucial functional proteins once fat stores are depleted; death typically occurs when ~50% of body protein is lost.

Hormonal Regulation - Starvation Conductors

• Primary Goal: Maintain glucose supply to brain & vital organs; conserve protein.
• Key Players & Shifts:
  • ↓ Insulin:
    • Hallmark of starvation.
    • ↓ Glucose uptake (muscle, adipose).
    • ↓ Anabolism (glycogenesis, lipogenesis, protein synthesis).
    • ↑ Catabolism.
  • ↑ Glucagon:
    • Dominant catabolic signal.
    • ↑ Hepatic glucose output (glycogenolysis, then gluconeogenesis).
    • ↑ Lipolysis & ketogenesis.
  • ↑ Cortisol:
    • Permissive for glucagon & catecholamines.
    • ↑ Proteolysis (muscle → amino acids for gluconeogenesis).
    • ↑ Lipolysis, ↑ Gluconeogenesis.
  • ↑ Growth Hormone (GH):
    • ↑ Lipolysis (FFA release).
    • ↓ Peripheral glucose utilization.
    • Protein-sparing effect.
  • ↓ Thyroid Hormones (T3):
    • ↓ T4 to T3 conversion.
    • ↓ Basal Metabolic Rate (BMR) → energy conservation.
    • ↑ Reverse T3 (rT3).

⭐ The Insulin:Glucagon ratio is the most critical determinant of metabolic state. A low ratio signals starvation, promoting catabolism and inhibiting anabolism across tissues to ensure glucose availability for the brain and red blood cells.

High-Yield Points - ⚡ Biggest Takeaways

• Early starvation: Relies on glycogenolysis, then gluconeogenesis (alanine, lactate, glycerol).
• Prolonged starvation: Marked by ↑ lipolysis, ↑ fatty acid oxidation, and ↑ hepatic ketogenesis.
• Brain adaptation: Shifts from glucose to ketone bodies (β-hydroxybutyrate > acetoacetate) as primary fuel.
• Protein conservation: Muscle protein breakdown ↓ as brain uses ketones, sparing glucose.
• Key hormones: Insulin ↓, glucagon ↑, cortisol ↑.
• Energy conservation: Basal Metabolic Rate (BMR) ↓.
• Fuel hierarchy: Glucose → Fatty acids → Ketone bodies become crucial.