Gut Training for Endurance Athletes: How to Train Your Stomach Before Race Day

One gel on a long run doesn't tell you whether six will sit well on race day.

It tells you about one gel. Useful information, but a fairly small rehearsal for a marathon.

Gut training means practicing the food and fluid you expect to use during exercise so you can learn what you tolerate and, potentially, improve that tolerance. My starting point for runners is a familiar gel, a counted amount, and a session where you can pay attention to what happens. You don't need to turn Tuesday's run into an eating contest.

So how do you build from that first packet to a race plan?

Practice the food where you plan to use it

A gel eaten at your desk hasn't passed the running test.

Exercise changes the conditions in which your gut works. Reduced gut blood flow, mechanical movement, food choices, and drink concentration can all contribute to symptoms, and the mix differs between athletes (de Oliveira and colleagues). That gives you a reason to rehearse at effort without assuming every cramp means you need more gut training.

For a runner, practice while running. For a triathlete, test the bike setup and what you can tolerate on the run afterward. A bottle that works beautifully on the trainer still needs a brick-session audition.

And test the breakfast too. The pre-race breakfast guide helps you settle the portion and timing before you start changing everything that happens after it.

What the research tells us

There is encouraging human evidence. In a small randomized study of 25 endurance runners, two weeks of repeated carbohydrate feeding during exercise reduced gastrointestinal symptoms and improved a subsequent running distance test compared with placebo (Costa et al., 2017).

That's a reason to practice. It doesn't promise a particular improvement in your marathon time.

The physiology needs a little care. The 2025 Sports Dietitians Australia and Ultra Sports Science position statement describes gut training as understudied. Improved comfort doesn't prove faster stomach emptying, and proposed changes in intestinal transport rely heavily on animal or indirect evidence. Practice can help without every proposed mechanism being established in exercising humans.

A 2023 systematic review by Martinez and colleagues found potential benefits for discomfort and some symptoms, alongside limited studies and quality concerns. The two included studies measuring gastric emptying during exercise found no significant change. Participants were predominantly male, which limits certainty for female athletes.

How long does gut training take? Some studies found benefits after two weeks, but the review couldn't establish the best duration or frequency. Eight weeks gives you room to experiment; it isn't a validated biological deadline. And if the race is next weekend, I'd keep the familiar intake you've tolerated rather than cram a training block into three breakfasts.

Choose a target for the race you're doing

You need something to practice toward, but the biggest number on a podcast isn't automatically your number.

Jeukendrup's carbohydrate guidance describes around 60g/hr for exercise lasting 2-3 hours and up to 90g/hr for longer efforts, using multiple carbohydrate sources at higher intakes. Duration, intensity, and tolerance matter. These are reference points for race fueling, not instructions to start at 90 tomorrow.

The same review expresses these during-exercise targets in grams per hour rather than scaling them directly to body weight. That doesn't make body size irrelevant to tolerance. Sex or size alone can't tell you which amount and formulation will work, so a smaller runner, including a woman, shouldn't automatically halve the target. Individual practice still matters.

If you're comfortable at 30g/hr, work from there. If you've repeatedly handled 60g/hr and that fits your event, practicing the same amount in more relevant conditions may be more useful than chasing another increase. The glucose-to-fructose guide explains the fuel-composition decision when targets rise.

Need a starting plan to rehearse? Build your carb, fluid, and sodium timeline in about 60 seconds: Use the free EnduranceOS planner.

Build the practice, then check the arithmetic

Here's how I'd organize it across the weeks available. This is an illustrative progression, not a tested protocol or a requirement to increase every week.

If you've never fueled a run, make the first session a product test: record what you took and how it felt before attempting the two-hour example below.

Stage	What to practice	What earns the next step
Find a baseline	A familiar product and an amount you already tolerate	You can repeat it and describe how it felt
Change one thing	A modest increase in carbs, or a different serving interval	The change works on more than one comparable session
Rehearse the race	Planned intake, breakfast, carrying setup, and relevant effort	You can follow the whole routine without forcing it
Settle the plan	Repeat what worked and pack a familiar backup	Fewer decisions left for race morning

No compulsory weekly increase. If symptoms appear, repeat a tolerable setup or step back and investigate, rather than deciding that suffering proves adaptation.

What does a counted session look like? Suppose you're already comfortable with gels and choose 40g/hr for a two-hour practice run. With hypothetical gels labeled at 20g each, take one at 0:15, 0:45, 1:15, and 1:45. Four packets supply 80g across two hours.

Using 30g gels on that same schedule supplies 120g, or 60g/hr. Same watch alerts, a different experiment.

If you also consume a measured drink serving containing 20g during the first example, the total becomes 100g, averaging 50g/hr. Count it before blaming the gel. Combining formats can fit a plan; it simply changes the dose you're testing. Follow product directions and keep fluid needs separate from the carb calculation.

For other durations and packet sizes, the marathon gel-count guide handles the arithmetic. And if gel texture is the problem, the gels, chews, and drink-mix comparison gives you other formats to try.

Write down enough to repeat the good run

“Stomach was weird” won't help much next Saturday.

Record the product and grams, serving times, fluid consumed, breakfast timing, effort, conditions, and when any symptoms started. Keep it brief. A note saying “80g over two hours, familiar breakfast, cool morning, mild fullness after second gel” gives you something concrete to compare.

Then change one part. Swapping breakfast, gel brand, bottle strength, and pace together is a process of denial and error.

Shorter sessions are useful too. You can try opening a GU or Maurten packet with sweaty hands, check a flavor, or practice drinking while moving without making every experiment part of your longest run. That is practical rehearsal, not proof that one gel on a 45-minute jog increases absorption.

But you still need longer sessions to test repeated servings. The long-run fueling guide covers how that fits a marathon block. You don't need added fuel on every easy run solely to collect gut-training reps.

Know when another practice session isn't the answer

Persistent symptoms deserve more than another increase in carbs.

If the same problem keeps returning, or symptoms occur outside exercise, discuss it with a sports dietitian or clinician. The stomach-problems guide covers common triggers, but a blog can't diagnose what is happening to you.

And don't force water to make a gel target work. NATA's fluid-replacement guidance recommends individual plans, avoiding weight gain from overdrinking, and drinking to thirst as a safer fallback when sweat rate is unknown. It also warns that dehydration and overhydration symptoms can overlap. Repeated vomiting, confusion, or a severe headache calls for medical help, not another packet.

My practical goal would be a routine you can repeat: a sensible target, counted servings, useful notes, and enough rehearsals to know where the problems are. Race week shouldn't be the first time you discover that the packet won't open or the fourth serving tastes unbearable.

The EnduranceOS planner gives you a carb, fluid, and sodium plan to take into training. It takes about 60 seconds, and it's free. Use the plan, record what happens, and adjust from there.

This article is educational, not individual medical advice. If you have diabetes, gastrointestinal disease, a condition affecting fluid balance, or recurring symptoms, work with a qualified clinician or sports dietitian.