At some point during training, just about every runner deals with an upset stomach or GI issues. If you’re currently training for a race and trying to figure out why fueling isn’t going as planned, here are some gut training tips for marathon runners.

gut training for runners
image from @eatforendurance

Maybe you’ve been told you need to train your gut before race day. Or maybe you’ve blamed a gel for an upset stomach and decided that fueling just doesn’t work for you.

But what does gut training actually mean? And do you really have to spend every long run forcing down gels to make it work?

We chatted with Claire Shorenstein, MS, CSSD, a board-certified sports dietitian, distance runner, and host of Eat For Endurance, to answer those questions and clear up some of the biggest misconceptions about gut training.

While we only covered a few of the topics below, you can listen to the full episode on the Tread Lightly Running Podcast.

What is Gut Training?

“If a gel upset my stomach, the gel must be the problem.”

As coaches, we’ve heard that from runners more times than we can count. But the gel usually isn’t the real problem.

Just like building endurance or getting faster, your gut is trainable.

There’s no single gel, electrolyte, or chew that’s perfect for everyone. The goal is to find the combination your body tolerates best, and give yourself enough time to practice it before race day.

Claire shared that gut training comes down to three things.

  • Stomach expansion – Getting comfortable holding more fluid and fuel without that sloshy, overly full feeling
  • Gastric emptying – Helping your stomach move food and fluid into your intestines more efficiently
  • Carbohydrate absorption – Teaching your body to actually absorb and use the carbs you’re taking in

gut training

Your symptoms can also give you clues about what’s going on.

Upper GI symptoms, like nausea, burping, or feeling full, usually point toward gastric emptying.

Lower GI symptoms, like bloating, cramping, or diarrhea, are more likely related to absorption.

Before you write off fueling, ask yourself a few questions:

  • Did you increase your carb intake too quickly?
  • Did you take a gel without enough water/electrolytes?
  • Have you only practiced fueling on easy runs and then just expected it to work at race pace?
  • Were you dehydrated or racing in the heat?
  • Could there be an underlying GI issue unrelated to sports nutrition?

Sometimes the answer really is that a certain gel doesn’t agree with your stomach. But more often than not, it’s the fueling strategy, not the gel itself, that needs a little tweaking.

How Long Does Gut Training Take?

There’s no specific timeline or magic number for how long it takes to train your gut.

Some research has shown improvements in as little as 5-10 days, but those protocols are pretty aggressive and not realistic for most runners.

Instead, Claire typically recommends thinking in terms of 2-3 months. This usually also aligns with most training cycles depending on your goals.

The key is to make gradual changes rather than jump from 30 grams of carbs per hour to 60-90 grams overnight.

Here are a few of the tips she uses with her runners:

  • Increase carbs by only 10-15 grams per hour at a time
  • Increase fluids alongside carbs since you’re training stomach volume too
  • Make sure your everyday diet contains enough carbs
  • Practice fueling more than just during long runs, even your speed workouts
  • Don’t freak out during your taper. You’re not going to lose all the work you’ve done training your gut in a few days

Why Fueling Feels Different During a Run

The moment you start running, your body begins prioritizing your working muscles.

Blood flow shifts away from your digestive system and toward your legs, making digestion naturally less efficient. It’s another reason we say you need to let your body digest your pre-run meal or snack.

Add in constant bouncing that comes with running, and it’s easy to understand why foods high in fat, protein, or fiber tend to sit in your stomach longer.

Simple carbs and fluids move through the system much more easily, which is exactly why they’re the foundation of most race-day fueling strategies.

The goal of gut training is to teach your body to handle fuel while you’re running, not while you’re sitting on the couch.

That’s why Claire recommends practicing under conditions that look as much like race day as possible, using the same fuel, at a similar effort, and even in similar weather when you can.

Your Everyday Diet Matters Too

Gut training doesn’t just happen during your long runs or workouts. 

If you’re consistently underfueling or eating a low-carb diet, it may make it harder for your body to absorb the carbs while running.

Research suggests that regularly eating enough carbs may actually increase the number of glucose transporters (SGLT1) in your gut, helping improve carbohydrate absorption over time.

What does this mean? Race day fueling starts long before race week.

Eating enough carbs throughout your training cycle helps build the gut adaptations you’re working toward with every practice of fueling.

As a bonus, it can also make carb loading before your race feel a whole lot less intimidating since your body is already used to eating a higher-carb diet.

Racing in the Heat and Gut Training

Training and racing in the heat makes everything feel harder, including fueling.

Claire explained that as your body works to keep you cool, blood flow is redirected toward your skin and working muscles.

That leaves less available for digestion, which is one reason stomach issues become much more common during hot runs and races.

Dehydration and insufficient sodium can make those GI issues even worse. So if a fueling strategy works perfectly on a cool autumn or spring morning but falls apart during a hot race, don’t automatically blame the gel.

fueling on race day

The conditions may be the culprit.

That’s why it’s so important to practice your fueling during warmer runs whenever you can.

It gives you a chance to figure out how much fluid, sodium, and carbs your body can comfortably handle before race day.

You can also control some of the variables. Keep your fluids cold, wear an ice bandana, tuck ice into your sports bra or hat, or use whatever cooling strategies you have available to help lower your body temperature.

One final piece of advice from Claire: expect race day to be hot.

So many races over the past few years have seen warmer-than-expected conditions. If you prepare for the heat instead of hoping for cool weather, you’ll be much better equipped if race day ends up warmer than planned.

This barely scratches the surface of all the information we can share with you about gut training for runners. Head over to the Tread Lightly Running Podcast to listen to the full episode.

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