Metabolic States
High-Yield Summary
- Three metabolic states, driven by nutrient availability and hormone balance: absorptive (~0-4 hrs, insulin dominant, anabolism/storage), postabsorptive (~4-24 hrs, glucagon dominant, mobilization/glucose preservation), prolonged fasting (24+ hrs, glucagon/epinephrine dominant, fat-based metabolism/protein sparing).
- Absorptive: liver builds glycogen then fat (VLDL export); muscle stores glycogen (local use only) and takes up amino acids; adipose stores triglycerides.
- Postabsorptive: liver runs glycogenolysis then gluconeogenesis; muscle shifts to fatty acid oxidation; adipose runs lipolysis (fatty acids to blood, glycerol to liver).
- Prolonged fasting: liver ramps up ketogenesis; muscle protein breakdown SLOWS (sparing protein) once ketone bodies are available; brain adapts to use ketone bodies (up to ~2/3 of its energy).
- Red blood cells always require glucose, in every state — no mitochondria, can't use fatty acids or ketone bodies.
Key Terms
- Absorptive state
- 0-4 hrs post-meal; insulin dominant; anabolism and storage.
- Postabsorptive state
- 4-24 hrs post-meal; glucagon dominant; mobilization and glucose preservation.
- Prolonged fasting
- 24+ hrs; glucagon/epinephrine dominant, insulin very low; fat-based metabolism, protein sparing.
The Three Metabolic States
| State | Timeline / Dominant Hormone / Theme |
|---|---|
| Absorptive | ~0-4 hrs / Insulin / Anabolism and storage |
| Postabsorptive | ~4-24 hrs / Glucagon (± epinephrine) / Mobilization, glucose preservation |
| Prolonged fasting | 24+ hrs / Glucagon, epinephrine / Fat-based metabolism, protein sparing |
Tissue Behavior Across States
| Tissue | Absorptive → Postabsorptive → Prolonged Fasting |
|---|---|
| Liver | Glycogen then fat synthesis → Glycogenolysis then gluconeogenesis → Ketogenesis dominant |
| Skeletal muscle | Glucose uptake, glycogen/protein synthesis → Fatty acid oxidation → Reduced protein breakdown (ketone-sparing) |
| Adipose tissue | Triglyceride storage → Lipolysis begins → Lipolysis remains highly active |
| Brain | Glucose → Glucose → Adapts to ketone bodies (~2/3 of energy) |
| Red blood cells | Glucose (always, all three states) |
Common MCAT Trap
- Muscle glycogen is for LOCAL use only, in every state — muscle can never release glucose to the blood (no glucose-6-phosphatase).
- The brain's shift to ketone bodies is a PROLONGED-fasting adaptation, not an early-fasting one — in postabsorptive state it still relies on glucose.
- Red blood cells are the one tissue with zero flexibility — always glucose-dependent, in all three states, no exceptions.
Quick Recall
What is the dominant hormone in the absorptive state?
When does the liver switch from glycogenolysis to gluconeogenesis?
What percentage of its energy can the brain get from ketone bodies during prolonged starvation?