Energy Use During Exercise, Fasting

During Exercise

During Fasting 
Fasting (early, between meals)
glucagon-mediated, glycogen primary source 
- Liver glycogenolysis
       muscle glycogen can only be used as an energy source for muscle cells because muscle cells 
       lack glucose-6-phosphatase 
- Some gluconeogenesis
       fully activated after glycogen is depleted 

Starvation (1-3 days)
glycogen depleted, fatty acids major source 
- Liver gluconeogenesis
      from lactate + alanine, odd chain free fatty acids 
- Fatty acid oxidation 
      by muscle and liver 

Starvation (>3 days)
acetyl CoA accumulates, exceeds capacity of TCA cycle; ketone bodies and
protein major source
- Ketone bodies 
      ketones can be used by brain, heart, and skeletal muscle, but not by RBCs as these lack 
      mitochondria) 
- Protein degradation for gluconeogenesis