Phases of the Starvation Response.
Phases of the Starvation Response:
- Glycogenolytic (First 24 hours (early)): 70–100 g of glycogen stored in the liver and a further 400 g stored in the muscles is mobilised as an energy substrate;
- Gluconeogenic (early < 24 hours): Insulin levels fall in response to low glucose and amino acid levels, glucagon levels rise and lipolysis occurs in the liver. Glucose forms from glycogen due to lower insulin levels.
- Gluconeogenic (late, beyond 24 hours): After 24 hours all new glucose is derived from: amino acids (alanine being the most important), glycerol (from adipose tissue), lactate (from erythrocytes via the Cori cycle). This coincides with a rise in plasma glucagon concentration and continued insulin suppression. Increased catecholamines and cortisol mobilise fat stores, increasing plasma free fatty acids. Gluconeogenesis declines after 3–4 days as the body adjusts to mobilise energy from fat stores (fully adjusted by 2 weeks).
- Ketogenic (Up to 2 weeks and beyond): Ketone bodies gradually replace glucose as the fuel source for the brain and nervous system up to a maximum of 50%. Ketone body formation by the liver is maintained. Other tissues (cardiac and skeletal muscle) obtain energy from free fatty acids. In this phase gluconeogenesis is reduced as a protein-sparing mechanism. This occurs at 10 days via reduction in glucagon. Initial protein catabolism = 70 g / day, reducing to 20 g /day by week 3.
