Starvation

[Ref: PK1:p321-322]

During starvation
--> Glycogen stored in liver (70-100g) and muscle (400g) is rapidly used up within 24hours
--> Then FFA released by adipose tissues are used

In the first 24hours

• Glucose is produced predominately by liver glycogenolysis
  * Low levels of insulin
  * Small amount produced from lactate and glycerol by liver and kidney
• Small amount of ketones produced

After the first 24 hours

• Glucose is almost entired produced by gluconeogenesis from amino acids, glycerol, and lactate (from RBCs)
  * Increase in glucagon in the first 24-48 hours
  * Alanine is the most amino acid for gluconeogenesis
• Cortisol and epinephrine also increase
  --> Increased production of FFA
  --> Increased ketone bodies (acetoacetate, beta-hydroxybutyrate, acetone)

NB:

• Growth hormone also increases over the first 24-48 hours
• Epinephrine also increase glycogenolysis in liver and muscle
  * [WG21:p362]
• Growth hormone increase glycogenolysis in liver only
• Cori cycle:
  * Lactate formed in tissues (e.g. renal medulla, RBC, muscles)
  --> Diffuse to liver
  --> Converted back to glucose
• Glucagon reaches peak at about 4 days

Prolonged fasting

• Ketone bodies become the fuel for brain and nervous tissues
• Other tissues revert to using FFA as fuel
• Gluconeogenesis is reduced
• Glucagon return to pre-fasting level at about 10 days
  --> Gluconeogenesis rate reduced
• Insulin concentration stays low
• Plasma cortisol and epinephrine remain increased
• Protein breakdown was 75g/day initially
  --> 20g/day by third week, due to ketone formation
• BMR reduce by 30%