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%