Learning from Asker Jeukendrup
The scientist who figured out how to absorb 90 grams of carbs per hour—and changed endurance sports forever
The Story
For decades, coaches told endurance athletes: "You can only absorb 60 grams of carbs per hour. Don't bother trying more—it'll just upset your stomach."
Asker Jeukendrup thought that sounded like a limit worth breaking.
Using radioactive tracers to track exactly where ingested carbs went in the body, he discovered something that changed everything: the 60g limit wasn't a human limit. It was a transporter limit.
Your gut has multiple doorways for absorbing carbs. If you use different types of carbs—glucose AND fructose—you can open multiple doorways simultaneously.
Result: athletes can now absorb 90+ grams per hour, nearly doubling their available fuel during races.
This discovery transformed how every Tour de France cyclist, Ironman triathlete, and marathon runner fuels their events.
Who is Asker Jeukendrup?
| Credential | Detail |
|---|---|
| Role | Professor of Exercise Metabolism; Founder of Mysportscience; Performance Manager Nutrition for Dutch Olympic Committee |
| Known For | "Multiple transportable carbohydrates" — using glucose + fructose to exceed absorption limits |
| Teams/Athletes | Team Jumbo-Visma (cycling), PSV Eindhoven, Red Bull Athlete Performance Center |
| Background | PhD from Maastricht University; former Global Director at Gatorade Sports Science Institute |
Jeukendrup has consulted for more Grand Tour cycling teams than anyone alive. When elite endurance athletes need to fuel 5+ hour races, they use his protocols.
What ISP Students Learn
Lesson 1: Know Your Limits—Then Find Ways Around Them
The "60g per hour" limit wasn't wrong. It was incomplete.
Jeukendrup's tracer studies revealed that the intestinal transporter for glucose (called SGLT1) saturates at about 60g/hour. Eat more glucose than that, and it just sits in your gut, pulling in water and causing bloating.
But fructose uses a different transporter (GLUT5).
The breakthrough: Combine glucose and fructose (2:1 ratio), and you can absorb both simultaneously—pushing total absorption to 90g/hour or higher.
"You're not limited by your body. You're limited by your knowledge of your body."
What this means for young athletes: Don't accept limits just because "everyone knows" them. Ask: "What's actually limiting me?"
Lesson 2: Train Your Gut Like You Train Your Muscles
Jeukendrup discovered something that sounds crazy: your gut adapts to training.
Athletes who practice eating 90g/hour during training can absorb it comfortably. Athletes who never practice can't.
The gut responds to "progressive overload" just like muscles:
- Week 1-2: Practice with 45g/hour
- Week 3-4: Increase to 60g/hour
- Week 5-6: Push toward 90g/hour
By race day, your gut is trained to handle the fuel you need.
"The gut is an athletic organ. It can be conditioned."
What this means for young athletes: If you want to perform well on race day, you have to practice your nutrition during training—not just your physical skills.
Lesson 3: Match Your Fuel to Your Duration
Not every workout needs the same fueling. Jeukendrup created a framework based on how long you're exercising:
| Duration | Carb Intake | Why |
|---|---|---|
| < 30 min | None needed | Glycogen stores are plenty |
| 30-75 min | Mouth rinse only | Brain senses carbs; performance improves without even swallowing |
| 1-2 hours | 30g/hour | Maintains blood sugar |
| 2-3 hours | 60g/hour | Approaching glycogen limits |
| 3+ hours | 90g/hour | Need maximum fuel delivery |
The key insight: a soccer player and an Ironman triathlete have completely different needs. One-size-fits-all doesn't work.
What this means for young athletes: Think about what your sport demands. A basketball player doesn't need to eat like a marathon runner.
Lesson 4: Never Try Anything New on Race Day
Jeukendrup's most practical advice is also his simplest:
"The most critical rule is to train with the exact nutrition strategy planned for race day."
Your gut needs to be familiar with what you're asking it to do. New gels, new drinks, new timing—all of these can cause disasters when stress is high and blood flow to your gut is reduced.
What this means for young athletes: Figure out your race nutrition weeks before the race. Practice it. Then execute what you've practiced.
Key Takeaways
| Lesson | One-Liner |
|---|---|
| Break limits intelligently | The 60g limit was real—but there was a workaround |
| Train your gut | Digestion adapts to practice, just like muscles |
| Match fuel to duration | Different events need different strategies |
| Never experiment on race day | Practice your nutrition plan in training |
How This Shows Up at ISP
Jeukendrup's framework is foundational to the Bio Skill Tree:
- Fueling challenges teach students to match carb intake to training duration
- "Train the gut" appears in pre-competition preparation education
- The 2:1 glucose:fructose ratio is taught for endurance events
- Race-day nutrition planning is part of competition preparation
When ISP students learn about fueling for performance, they learn the same science that powers Tour de France teams.
Learn More
"The gut is not a limitation. It's an opportunity."