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How You Fuel Is How You Finish: Gut Training for Endurance Athletes

Writer: Alister GardnerAlister Gardner

When it comes to excelling in endurance sports like running and cycling, nutrition is just as crucial as training. For events lasting several hours, your ability to take in and absorb fuel efficiently can determine whether you finish strong or hit the wall.

This is where gut training comes in—a process that conditions your digestive system to handle higher carbohydrate intake without discomfort. In this article, we’ll break down why gut training matters, how much fuel you actually need, and how to gradually train your body to absorb more carbs for peak endurance performance.


Why Gut Training Is Essential for Endurance Athletes

Running long distances isn’t just about training your muscles, heart, and lungs—your gut needs training, too.

For workouts lasting under an hour, your body can typically rely on stored glycogen. However, for sessions or races lasting two hours or more, glycogen stores deplete, and your body needs an external energy source. If your gut isn’t trained to digest and absorb carbohydrates efficiently, you may experience:

Energy crashes ("bonking") 

GI distress (bloating, cramps, nausea) 

Inability to absorb enough fuel to sustain performance

The good news? You can train your gut just like you train your muscles—by gradually increasing carbohydrate intake during training sessions.



How Much Carbohydrate Do You Need?

Your carbohydrate needs depend on the duration and intensity of your activity:

Carbohydrate Intake Guidelines for Endurance Athletes

Duration

Recommended Carb Intake

Less than 1 hour

No fuel needed during effort (pre-run snack recommended)

1-2 hours

25-50g of carbs per hour

2-3 hours

50-75g of carbs per hour

4+ hours

75-100g of carbs per hour

Why train your gut?

  • At high intensity, your body burns through glycogen quickly, making carbohydrate intake essential.

  • Many athletes struggle to absorb more than 60g per hour without GI distress—gut training helps push that limit higher.

  • Training your gut allows you to avoid hitting the wall and maintain strong performance over long distances.



Glucose vs. Fructose: Why Dual-Source Carbs Matter

Not all carbohydrates are processed the same way. The two main types of sugar used in sports fueling—glucose and fructose—follow different absorption pathways:

Glucose: Absorbed directly in the small intestine and used by muscles for energy. The body can process up to 60g per hour. ✅ Fructose: Absorbed through a different transporter and processed by the liver. Adding fructose allows athletes to absorb up to 30g more per hour.

Why does this matter? By combining glucose and fructose (typically in a 2:1 or 1:1 ratio), athletes can increase carb absorption to 90-120g per hour—fueling performance while reducing gut overload.

🚀 Pro Tip: Many sports drinks, gels, and chews are designed with this glucose-fructose combo. Experiment to find what works best for you!



How to Train Your Gut: A 10-Week Progression Plan

Weeks 1-2: Get Started (30g per hour)

  • Begin by consuming one small carb source (e.g., 1 gel or chew) per hour.

  • Focus on how your stomach reacts and get used to fueling while running.

Weeks 3-5: Increase to 60g per hour

  • Take one gel or chew every 30 minutes.

  • Try different carb sources and start hydrating alongside your fuel.

Weeks 6-8: Push to 90g per hour

  • Fuel every 20 minutes.

  • Experiment with glucose-fructose combinations to improve absorption.

Weeks 9-10: Peak Fueling (100-120g per hour)

  • Take in fuel every 15 minutes.

  • Find your best fueling method (gels, chews, sports drinks, etc.).

🎯 Key Takeaway: By slowly increasing intake over time, your gut will adapt to handle more carbs efficiently—keeping you fueled for longer races and hard training sessions.



Final Thoughts: Gut Training = Performance Gains

Gut training isn’t just about avoiding stomach issues—it’s about unlocking your full endurance potential. By gradually increasing carbohydrate intake and incorporating a glucose-fructose fueling strategy, you’ll improve your energy availability, endurance, and race-day confidence.

The bottom line:

Train your gut just like you train your legs. 

Gradually increase carb intake to boost absorption. 

Use a dual-carb strategy (glucose + fructose) to maximize fuel efficiency.

🔹 Try this approach in training, and you'll be amazed at how much better you feel and perform on race day!


 
 
 

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