Gut Training — Training Dictionary

What it is

Gut Training is deliberate, progressive approach to conditioning the gastrointestinal system to tolerate and absorb the volume and type of nutrition (carbohydrates, fluids, electrolytes) required during long-duration or high-intensity endurance exercise such as cycling (or running and swimming).

A trained gut reduces gastrointestinal distress (cramps, nausea, bloating, diarrhea) and enables efficient fuel delivery under exercise stress, improving comfort and performance. Modern fueling can be a gamechanger for performance, but you need to practice how you eat to meet race demands.

Many of us have been there. You're on great pace... and then all of the sudden nature calls and you're off the trail in the woods bleeding time.

Why Gut Training Matters

Blood flow to the stomach and gut decreases during exercise, slowing digestion and increasing sensitivity.
Mechanical movement adds stress to the digestive system.
Without training, athletes often under-fuel or suffer GI issues that limit performance or cause DNFs.
The gut is adaptable: repeated exposure to carbs and fluid during training increases tolerance, gastric emptying speed, and absorption capacity.

What Happens When Gut Training

Gastric emptying: Repeated intake increases the stomach’s ability to tolerate volume and move fluid/food efficiently.
Gut permeability: Training reduces the risk of “leaky gut” under heat or high intensity.
Carbohydrate transporters: Regular carb intake upregulates intestinal transporters like SGLT1, improving glucose absorption.
Fuel utilization: A trained gut delivers more exogenous carbohydrate to the bloodstream, delaying fatigue. If you haven't experienced this yet, say goodbye to the bonk forever if you fuel appropriately.

Key Things to Keep In Mind

Progressive exposure Gradually increase carbohydrate intake during training sessions until you reach your race-day targets.
Specificity to race conditions Train your gut under similar pace, duration, temperature, and hydration demands as your goal event.
Use mixed carbohydrate sources Blends like glucose + fructose improve absorption and allow for higher carb intake (60–90g/h).
Include fluids and electrolytes Carbohydrate absorption is linked to hydration status; dehydration worsens gut tolerance.
Never try new fueling on race day Only use products and strategies practiced repeatedly in training.

Sample Progression

Many athletes find they can tolerate carbs without any training. Some need a bit of training, others need a full progression. If you're the latter here is a rough idea of what you can work through to build to the right carb load for you.

Weeks 1–2: Baseline & Light Training

Test current tolerance during a long run/ride.
Generally start around 20–30g carbs per hour.
Track symptoms: nausea, cramps, bloating, sloshing, urgency.

Weeks 3–4: Controlled Increase

Increase intake to 30–40g/h.
Keep fuel sources simple and consistent.
Add small hydration goals (e.g., 100–200 ml every 20 minutes).
You can accelerate this if your gut allows.

Weeks 5–6: Moderate Load

Move toward 40–60g/h depending on tolerance.
- This can be higher, depending on results of early weeks
Introduce mixes of carbs (e.g., glucose + fructose).
Increase electrolytes for hot conditions.

Weeks 7–8: Race Simulation

Target full fueling plan (60–90g/h for most endurance events).
- This may be higher for certain individuals
Practice exact products, timing, and fluid volumes you’ll use on race day.
Rehearse logistics: fueling intervals, drink station rhythm, gel timing.

Targets for Most Athletes

Carbohydrates: 60–90g per hour depending on duration/intensity.
Fluids: 400–800 ml per hour (context-dependent).
Sodium: 300–800 mg per hour (varies by sweat rate).

These are starting points; individual testing is essential. Note that if you are training or racing at altitude your fuel demands will be higher (you burn more when oxygen concentration is lower) but your gut will be able to handle less (this is an unfortunate reality especially for Leadville racer). Everyone's demands are a bit different so testing what works for you is critical.