Initially
- Glygoen stores are used from liver and muscle
- Gluconeogensis and cori cycle
- Deplete after 24-48 hours Prolonged
- Switch to protein breakdown
- Fat oxidation
- Fatty acids converted to ketones in the liver
- Decreased BMR due to decrease T3
Overall
- Fuel shift
- Body switches from carbohydrate to fat and ketone metabolism
- Glycogen is rapidly depleted
- Gluconeogenesis and lipolysis sustain glucose and energy needs
- Protein catabolism
- Early breakdown of skeletal muscle to support gluconeogenesis
- Later protein sparing as brain adapts to ketone metabolism
- Hormonal changes
- ↓ Insulin
- ↑ Glucagon, cortisol, growth hormone, catecholamines
- ↓ T3 (euthyroid sick syndrome)
- Metabolic changes
- ↑ Ketogenesis from free fatty acids
- ↓ Resting energy expenditure with time
- Glucose used primarily by RBCs and renal medulla
- Consequences
- Progressive loss of fat and lean mass
- Hypoalbuminaemia and edema
- Immune dysfunction
- High risk of refeeding syndrome if nutrition abruptly reintroduced