Expansion
Insulin dominates the fed state; glucagon and catecholamines dominate fasting
Expansion
Fed state (insulin dominant)
- Glucose uptake, glycogenesis, lipogenesis, protein synthesis
Fasting timeline
| Time | Dominant source |
|---|---|
| 0 to 4 hours | Absorbed glucose |
| 4 to 24 hours | Glycogenolysis (liver glycogen, about 100 g) |
| 24 hours to days | Gluconeogenesis from lactate, glycerol and amino acids |
| Days to weeks | Ketogenesis increasingly, sparing protein |
| Prolonged | Ketones supply up to two-thirds of brain fuel |
The brain normally requires about 120 g of glucose a day and cannot use fatty acids, which is why gluconeogenesis is obligatory early. Ketone adaptation is the crucial survival mechanism: by allowing the brain to use ketones, it reduces the need to catabolise muscle protein, extending survival from weeks to months.
Red cells and the renal medulla remain glucose dependent throughout, since they lack mitochondria or operate anaerobically.
Muscle glycogen cannot contribute directly to blood glucose, because muscle lacks glucose-6-phosphatase; it contributes indirectly through lactate via the Cori cycle.