Endurance athletes can have the perfect training block… strong fitness, dialed-in pacing, and feel totally ready to crush a race. But if you don’t know how to train your gut, race day can fall apart fast.
I share a case like this in my new book about a well-trained endurance athlete who did everything “right.” They had the fitness, the plan, and the goal of hitting 60–90g of carbs per hour on race day.
But in training? They barely practiced fueling.
Race day came, they tried to execute the plan, and within hours, their gut shut down. They were hit with nausea, urgency, and an inability to take in fuel.
So they stopped eating. Then stopped drinking. From there, everything went downhill… fast.
This wasn’t random. It wasn’t bad luck. And it wasn’t that their gut was “broken.”
It was a gut that had never been trained for what it was being asked to do.
Because here’s the reality: Race-day GI issues are rarely random… they’re predictable.
In this blog post, I’m breaking down how to train your gut to handle carbs, prevent nausea, and fuel effectively during endurance events.
Why GI Distress Is the #1 Performance Killer
We often think endurance performance is limited by fitness. So many miles, grueling terrain, extreme temperatures, etc.
But in longer events, it’s often limited most by fuel tolerance.
When the gut breaks down:
- Carbohydrate absorption drops
- Energy availability crashes
- Pacing falls apart
- DNF risk increases
In more severe cases:
- Nausea prevents intake entirely
- Diarrhea or urgency forces stops
- Dehydration compounds the problem
At that point, it’s not about your legs. It’s about what your gut can actually handle.
A Fueling Pattern I See All the Time
An athlete plans to take in 60–90g of carbs per hour on race day. That’s appropriate for their event.
But in training?
They’ve been:
- Skipping fueling on long runs
- Taking in minimal carbs
- Avoiding gels because they “feel heavy”
Then race day arrives, and suddenly they’re asking their gut to handle more fuel than it’s ever practiced under stress.
The result? Overload.
Symptoms start, fueling becomes inconsistent, then it stops altogether.
And here’s the kicker: Stopping fuel and fluids mid-race usually makes things worse.
It increases stress, worsens absorption, and accelerates performance decline.
Important note: If an athlete experiences blood in the stool, that requires urgent medical evaluation. That’s not a fueling issue to troubleshoot. That’s a “seek care now” situation.
What’s Actually Happening in the Body
To understand how to train your gut, you need to understand why it struggles in the first place.
Here’s what’s actually happening in your body:
Blood Flow Shifts Away from the Gut
During exercise, blood is redirected to working muscles.
The gut gets less support, making digestion and absorption less efficient.
High Intensity Slows Digestion
The harder the effort, the slower gastric emptying becomes.
This is why athletes experience:
- Sloshing
- Fullness
- Nausea
Especially when intake exceeds tolerance.
Stressors Stack Together
Most race-day GI issues aren’t caused by one thing.
They’re a combination of:
- High carb intake (especially concentrated sources)
- Dehydration
- Heat
- Anxiety
- Inconsistent fueling habits
Without preparation, the gut gets overwhelmed.
How to Train Your Gut (The Missing Piece)
Here’s the thing most athletes need to realize: your gut is trainable, just like your legs.
You wouldn’t race at a pace you’ve never practiced before, and fueling should be no different.
1. Start With What You Can Tolerate
If your goal is 60–90g/hour, you don’t start there.
You start with what your gut can currently handle without symptoms.
That might be:
- 30g/hour
- 40g/hour
- Or even less
And that’s okay.
2. Build Tolerance Gradually
This is where most athletes get it wrong.
They jump straight to race-day targets instead of building toward them.
Instead:
- Increase carbs in small increments
- Practice during long sessions
- Stay consistent week to week
This is how you train your gut to absorb more.
3. Practice More Than Just “Grams Per Hour”
When learning how to train your gut, it’s not just about quantity.
You need to practice:
- Type (gels, drinks, solids)
- Texture (thick vs diluted)
- Timing (early vs delayed fueling)
- Temperature (especially in hot conditions)
Race day should feel familiar to your gut, not like an experiment.
4. Separate Variables When Troubleshooting
If something isn’t working, don’t change everything at once.
Break it down:
- Carbohydrates
- Fluids
- Electrolytes
Adjust one variable at a time so you can identify the true issue.
How to Build Up to Carbohydrate Targets
Yes, carbohydrate intake matters, but tolerance matters more.
Instead of forcing a target:
- Match intake to your current capacity
- Progress over time
- Reassess based on symptoms and performance
The goal isn’t hitting a certain number. It’s building a system your gut can sustain.
What to Do After a Race-Day GI Blow-Up
One bad race-day GI experience can lead to a lot of second-guessing.
Most athletes respond by:
- Eating less
- Avoiding certain fuels
- Playing it “safe”
But this often backfires.
Rebuild Instead of Restrict
If you want to improve tolerance, you need exposure.
Start with:
- Easy-to-tolerate carbohydrate sources
- Lower doses during training
- Consistent hydration
Then gradually rebuild. Your gut adapts, but only if you give it the chance.
The Bottom Line
If fueling consistently leads to nausea, bloating, or GI distress on race day, it’s probably not just the fuel that’s causing problems.
The problem is often a gut that hasn’t been trained to handle intake under physiological stress.
When you learn how to train your gut, everything changes:
- Fueling becomes more consistent
- Symptoms become more manageable
- Performance becomes more reliable
Because in endurance sports, it’s not just about fitness. It’s about what your body can tolerate and actually use.
It’s about having a solid fueling strategy, practicing it, and making adjustments as needed.
A More Effective Way to Approach GI Issues in Athletes
If you’re working with endurance athletes, you already know how common GI complaints are.
What’s less obvious, and often missed, is what’s actually driving them.
In many cases, it’s not a primary gut issue. It’s low energy availability.
Inside my REDs Informed Provider Certification Program® and REDs Performance Mastermind, I teach you how to:
- Assess GI symptoms through a REDs and fueling lens
- Identify within-day energy gaps that disrupt gut function
- Use proven frameworks, templates, and case reviews to guide your decisions
My goal is to help you support athletes in staying fueled, strong, and performing at their best—without relying on guesswork.