Energy stores and starvation
Vivian Imbriotis | June 22, 2026
Energy stores in adult
- 500g of glycogen (100g in liver + 400g in skeletal muscle) @ 4kcal/g = 2000kcal
- Adipose tissue - highly variable amount. Stores lipids as triglycerides. 14kg @ 9kcal/g = 126,000kcal
- Protein - 15% body mass = 10kg for 70kg adult.
- Of this, 1% is labile = 100g @ 4kcal/g = 400kcal
- 99% is structural. Up to ~25% of structural protein can be catabolized before fatality \(\to\) 2.5kg @ 4kcal/g = 10,000kcal
In starvation
- There is relative / absolute inadequacy of energy supply
- the body must catabolise endogenous stores
- the body conserves energy and minimises protein loss
- glucose must be produced as CNS and RBC are obligate glucose consumers
Starvation phases
- (<24 hours) Glycogenolytic phase
- Initially, food in GI tract still digested
- Then, once GI tract exhausted, fall in insulin / increase in glucagon
- Triggers glycogenolysis in liver (\(\to\)glucose for whole body) and skeletal muscle (\(\to\)G6P for local use, muscle lacks glucose-6-phosphatase)
- Minimal protein catabolism
- (1-3 days) Gluconeogenic phase
- Glucagon climbs further. Catecholamines and cortisol increase.
- Exhaustion of glycogen stores \(\to\) gluconeogenesis, mostly hepatic (some renal)
- Glycerol and FFAs liberated from adipocytes
- FFAs \(\to\) \(\beta\)-oxidation \(\to\) acetyl-CoA
- Acetyl-CoA \(\xrightarrow{liver}) some ketones
- Glycerol \(\xrightarrow{liver}\) gluconeogenesis
- Lactate (from RBC Embden-Meyerhof) \(\xrightarrow{liver}\) gluconeogenesis
- Amino acids from labile stores \(\xrightarrow{liver}\) gluconeogenesis
- (3 days - ) Ketogenic phase
- Insulin is suppressed. Further increases in catecholamines and cortisol.
- Ketones become prominent energy source. CNS receives ~75% of energy from ketones.
- Structual proteins catabolised for gluconeogenesis
- \(\downarrow\)Metabolic rate, \(\downarrow\)thyroid hormone
- Loss of water soluble vitamins (C, B complex)