Learning from Luc Spriet
The scientist who mapped how your body chooses between burning fat and carbs—and why you can't always pick
The Story
Athletes love the idea of "fat burning." Burn fat instead of carbs! Access unlimited energy! Never bonk!
Luc Spriet, a professor at the University of Guelph, has spent his career explaining why it's not that simple.
Through meticulous research on the enzymes that regulate fuel use, Spriet discovered that at high intensities, your body physically cannot burn fat efficiently—not because you lack willpower, but because of molecular bottlenecks in your cells.
His work explains why "fat-burning zones" work at easy paces but fall apart when you need speed.
Who is Luc Spriet?
| Credential | Detail |
|---|---|
| Role | Professor, University of Guelph; Human Performance Lab |
| Known For | Metabolic flux control; PDH and CPT-1 regulation; "Train Low" periodization |
| Collaborations | Gatorade Sports Science Institute, IOC nutrition panels, Louise Burke |
| Focus | Understanding rate-limiting steps in human metabolism |
Spriet's philosophy: you can't optimize what you don't understand. His work maps the exact molecular checkpoints that determine fuel use.
What ISP Students Learn
Lesson 1: Fat Burning Has a Molecular Speed Limit
Inside your muscle cells, there's a "gatekeeper" enzyme called CPT-1 that controls whether fat can enter the mitochondria (your cellular power plants) to be burned.
At low-to-moderate intensities, CPT-1 works fine. Fat flows into the mitochondria.
At high intensities, two things happen:
- The cell becomes more acidic (from lactate buildup)
- Carnitine (needed to transport fat) gets "trapped"
This effectively locks the door to the mitochondria for fat. No matter how fat-adapted you are, the molecular machinery shuts down.
"The limitation is enzymatic and structural, not merely nutritional."
What this means for young athletes: You can't "burn fat" your way through a sprint. The chemistry doesn't allow it.
Lesson 2: Carbohydrates Have Their Own Gatekeeper
For carbohydrates, the key enzyme is PDH (Pyruvate Dehydrogenase)—the checkpoint where carbs enter the energy-producing cycle.
PDH is activated by:
- Calcium (released when muscles contract)
- Low energy status (signals that fuel is needed)
PDH is inhibited when:
- Energy supply is high (no need to burn more)
- Fat byproducts accumulate (Randle cycle)
This creates a "fine-tuning" system where your body matches fuel burning to fuel demand moment-by-moment.
What this means for young athletes: Your body is constantly adjusting fuel use based on intensity. It's not a light switch—it's a dial.
Lesson 3: Training Creates "Pulses" of Adaptation
Spriet's research on mitochondrial biogenesis (building more mitochondria) revealed that adaptation happens in bursts:
After a hard training session:
- PGC-1α (the "master switch" for mitochondria building) spikes 10x within 4 hours
- It returns to baseline within 24 hours
But the protein it creates sticks around. Each session adds a small increment of mitochondrial capacity.
This is why consistency matters more than any single session. Miss a day, and you miss a "pulse" of adaptation.
What this means for young athletes: Showing up consistently—even when sessions feel routine—is how you build fitness at the cellular level.
Lesson 4: "Train Low" Works—But Be Careful
Spriet supports strategic low-carb training ("Sleep Low") for endurance athletes:
- Evening: Hard interval session (with carbs)
- Overnight: Skip carbs before bed
- Morning: Easy aerobic session in the fasted/low-glycogen state
- After: Eat normally
This amplifies the signal for mitochondrial building because low glycogen triggers stronger adaptation signals.
But he warns: This is a tool, not a lifestyle.
- High-intensity sessions NEED carbs
- Chronic low carbs cause hormonal disruption (RED-S)
- Use "Train Low" strategically, not constantly
What this means for young athletes: Advanced fueling strategies exist, but they require careful application. Master the basics first.
Key Takeaways
| Lesson | One-Liner |
|---|---|
| CPT-1 is the fat gate | High intensity locks fat out of the mitochondria |
| PDH is the carb dial | Your body fine-tunes carb burning moment by moment |
| Adaptation comes in pulses | Each session triggers a temporary surge that builds over time |
| Train Low strategically | Low-carb training amplifies signals, but don't make it your default |
How This Shows Up at ISP
Spriet's metabolic research shapes energy education in the Bio Skill Tree:
- Students learn why different intensities require different fuels
- The "consistency over intensity" message is backed by his molecular research
- Advanced periodization concepts (for older students) include "Train Low" strategies
- We teach that you can't "hack" biology—you have to work with it
When ISP students learn about fueling for different intensities, they're learning the same science Spriet teaches to Olympic teams.
The Randle Cycle Simplified
Spriet's research clarified how fat and carbs interact:
| Intensity | What Happens |
|---|---|
| Low-Moderate | High fat availability suppresses carb use (glycogen sparing) |
| High | Carbs dominate—the fat pathway gets overwhelmed |
This explains why "fat adaptation" helps endurance but not sprinting. At high intensity, carbs always win.
Learn More
"You can't override your cellular machinery by wishing. You have to understand it—and work with it."