Atlas MCAT Course
Systems & Physiology → Metabolic State Integration
Metabolic State Integration
How the body chooses what to burn, store, or spare.
01The idea
Up to now, you’ve learned individual pathways (glycolysis, PPP, β-oxidation, fatty acid synthesis), hormonal regulation, and tissue specialization. The biggest conceptual leap is this:
If you identify the state, the pathway directions become obvious.
THREAD · Signal vs. response
02Build the model
AThe two global metabolic states
At the highest level, metabolism toggles between two states. Everything else is a consequence.
BHormonal control: the state switch
The metabolic state is determined primarily by hormone ratios, not absolute levels.
| State | Insulin | Glucagon |
|---|---|---|
| Fed | High | Low |
| Fasted | Low | High |
CTissue roles: division of labor
Different tissues solve different problems.
| Tissue | Role |
|---|---|
| Liver: the coordinator | Buffers blood glucose; switches between storage and export; interfaces with hormones directly |
| Muscle: the consumer | Uses fuel for work; stores glycogen locally; does not export glucose |
| Adipose: the reservoir | Stores fat during abundance; releases fatty acids during scarcity |
DThe fed and fasted states, tissue by tissue
D.1The fed state: storage and biosynthesis
Hormonal context: high insulin, low glucagon.
| Tissue | What happens |
|---|---|
| Liver | Glycolysis active; PPP active → NADPH; fatty acid synthesis active; glucose stored or converted to fat |
| Muscle | Glucose uptake increased; glycogen synthesis; protein synthesis favored |
| Adipose | Fat uptake; triglyceride synthesis |
D.2The fasted state: mobilization and sparing
Hormonal context: low insulin, high glucagon (± stress hormones).
| Tissue | What happens |
|---|---|
| Liver | Glycogen breakdown (early); gluconeogenesis (later); β-oxidation supplies ATP; PPP suppressed except for redox needs |
| Muscle | Fatty acid oxidation; glucose sparing |
| Adipose | Lipolysis; fatty acid release |
EWhere each pathway fits (big picture)
| Pathway | Primary role | State |
|---|---|---|
| Glycolysis | Rapid ATP | Fed |
| PPP | NADPH + ribose | Fed / stress |
| FA synthesis | Energy storage | Fed |
| β-oxidation | Sustained ATP | Fasted |
| Gluconeogenesis | Glucose maintenance | Fasted |
FNADH vs. NADPH: final integration
NADH serves energy extraction (the ETC). NADPH serves biosynthesis and protection. In the fed state, NADPH demand is high, so PPP is active. In the fasted state, NADH demand is high, so β-oxidation is active.
This single distinction solves many redox questions.
GStress as a modifier (not a new state)
Stress hormones overlay both states, bias toward fuel availability, and can override insulin temporarily.
03See the connections
The lesson names these links itself. Read the map left to right.
State switch
- Insulin : glucagon ratio
- Stress hormones modify
Tissue roles
- Liver coordinates
- Muscle consumes
- Adipose stores and releases
Pathways follow
- Fed: glycolysis, PPP, FA synthesis
- Fasted: β-oxidation, gluconeogenesis
This completes the biochemistry block.
04Reason with it
Four questions. Answer in your head first, then choose. You’ll see the reasoning either way.
Think first.
A passage asks whether β-oxidation is active in the liver. What should you ask first?
Think first.
Insulin is high. What happens to fatty acid synthesis and β-oxidation?
Think first.
A fasting person faces strong oxidative stress. What happens to PPP?
Think first.
During fasting, do liver, muscle, and adipose all do the same thing?
05MCAT lens
Predict the direction
High insulin → fat synthesis ↑, β-oxidation ↓
Fasting → β-oxidation ↑, PPP ↓
Oxidative stress → PPP ↑ even during fasting
Muscle fatigue → glucose sparing logic
No memorization required.
06Common traps
Trap 1: Pathways turn on independently
Misconception
State governs all.
Correct model
Coordinated shifts.
Trap 2: All tissues respond the same
Misconception
Tissue specialization.
Correct model
Division of labor.
Trap 3: PPP is only fed-state
Misconception
Also stress-responsive.
Correct model
Demand-driven.
07Dependency check
Can you reconstruct this concept without looking at the page? You should now be able to reason through:
Why insulin favors storage
Check against 02 · D (fed state): in the fed state, excess carbon is stored, not burned.
Why fasting favors fat use
Check against 02 · D (fasted state): fat use spares glucose for the tissues that need it.
Why NADPH ≠ NADH
Check against 02 · F: building and protection vs energy extraction.
How liver, muscle, and adipose coordinate
Check against 02 · C and D: coordinator, consumer, reservoir.
If something feels unclear, the missing idea is usually thisMetabolism is state-based, hormone-driven, and tissue-specialized.
08Remember
REMEMBER · if you keep only a few things
- Identify the state; the pathways follow.
- State is set by the insulin : glucagon ratio.
- Fed: store excess carbon and reducing power.
- Fasted: preserve glucose for those who need it.
- NADPH builds and protects; NADH powers the ETC.
- Stress modifies the state; it doesn’t replace it.
09Forward hook
This completes the biochemistry block: metabolism is state-based, hormone-driven, and tissue-specialized. Physiology so far has focused on moving matter.
How do cells encode, transmit, and integrate information?
That leads to neurophysiology.
Next lessonSensation & Perception→