Inside the Science of Gut Training: How to Turn Your Stomach into a Performance Tool

For many marathon runners, it’s not the legs that give up first, it’s the stomach.

Cramping, nausea, or that heavy, sloshing feeling can end a great race before the finish line.

The idea that you can train your gut to handle more fuel might sound strange, but early research suggests it’s possible. You can train your gut, just like your legs.

With practice, many runners learn to take in more carbs with fewer issues. The science is still growing, but the pattern is clear in studies and in real-world running: practice fueling in training, and race day goes better.

If you’re training for a spring marathon, now’s the perfect window to start. Building your fueling tolerance over the next few months means your stomach will be as ready as your legs by race season.

WHY GUT TRAINING MATTERS

Your gut is part of your performance system. It doesn’t only digest food. It moves fuel into your blood so your muscles can use it.

During runs, carbohydrates are your body’s fastest fuel. But your gut can only move so much energy into your bloodstream at once before it starts to complain.

Training the gut may increase how effectively your body absorbs carbohydrates.

In animal studies, repeated carbohydrate feeding appears to improve how gut transporters move fuel into the bloodstream within days to weeks.

A few human studies show similar trends after consistent practice.

That means gut training probably helps in two ways:

1. Physiological adaptation: your gut may become more efficient at moving carbs into your bloodstream.
2. Improved tolerance: your digestive system becomes more comfortable processing fuel while running.

Together, these may explain why runners who practice fueling often report fewer stomach issues and more steady energy, even though every runner responds differently.

In a small but landmark 2017 Monash University study, runners who practiced consuming 90 grams of carbohydrate per hour for two weeks reported fewer stomach problems and signs of better carbohydrate tolerance compared with those who didn’t.

Other studies and countless athletes report the same pattern: practice reduces discomfort.

As researcher Jamie Pugh puts it, “The gut can adapt—but we’re still learning how trainable it really is.”

HOW TO TRAIN YOUR GUT

Gut training isn’t a shortcut. It’s a skill you build over time.

The goal isn’t to reach a specific number, but to find the fueling rate your body can handle comfortably and consistently.

Here’s a simple three-level framework to help you progress safely.

Level 1: Novice (Building Phase)

• Target: 30 to 60 g of carbohydrate per hour.
• When to use it: 60 to 90-minute runs.
• Timing: Fuel every 20-30 minutes.
• Product fit: SiS GO Isotonic Gel (22 g)

Start with one or two fueling sessions each week.

GO Isotonic Gels are ideal early on; they’re pre-diluted to match body fluids, so they’re gentle on the stomach and easy to digest.

Stay here for 6–8 weeks before increasing your intake.

Level 2: Intermediate (Adaptation Phase)

• Target: 60 to 90 g per hour.
• When to use it: 1.5 to 2.5-hour-long runs.
• Timing: Fuel every 30 to 45 minutes.
• Product fit: SiS BETA Fuel Gel (40 g).

The BETA Fuel Gel combines glucose and fructose in a 1:0.8 ratio. This uses two different transport pathways in the gut, helping you absorb more energy with less discomfort.

Mild bloating early on is normal. With steady practice, most runners feel more comfortable within a few weeks.

Level 3: Advanced (Elite Race-Simulation Phase)

• Target: 90 to 120 g per hour. Most runners stop at 90.
• When to use it: 1.5 hours or longer. Race-pace segments.
• Timing: Fuel every 20-25 minutes
• Product fit: BETA Fuel Gel (40 g)

Take one gel every 20-25 minutes. This setup simulates your race-day plan and helps you see how your stomach handles full fueling loads.

Only progress to this phase in the last 4–6 weeks before your event, and only if your stomach feels ready.

WHAT THE RESEARCH ACTUALLY SHOWS

The science behind gut training is promising but still developing.

The Monash Study remains the main direct trial, and newer reviews show a similar pattern: practicing fueling may reduce stomach issues and improve tolerance, but responses vary widely.

Some runners adapt within weeks. Others need months. A few may never tolerate very high intakes, and that’s okay.

Gut training isn’t just about physiology; it’s also about confidence. When you’ve practiced your fueling, you trust your stomach on race day. That reassurance may help as much as any physical change.

Your goal is to find the intake that works for you.

REALISTIC FUELING TARGETS

For most marathoners, the performance sweet spot is 60-90 g of carbs per hour.

Going above 90 grams per hour is typically helpful for elite athletes or runners who’ve practiced extensively and know they can handle it comfortably.

If you currently fuel at 30 g/h, add about 10-15 g each week or two.

WHY SIS FUELING WORKS

• GO Isotonic Gel (22 g): Gentle, pre-diluted gel that’s easy on the stomach. Ideal for early gut training and shorter runs.
• BETA Fuel Gel (40 g): Built for high carbohydrate intake, using a 1:0.8 glucose-to-fructose ratio to help your body absorb more fuel at 90 g per hour and above without added stomach stress.

These two gels create a simple progression – from early comfort to race-day fueling confidence.

WHAT TO EXPECT AND HOW TO ADJUST

Adaptation takes time. Everyone’s timeline is different. Some runners feel better after two weeks, others take six or more.

A few may find they perform best at moderate intakes, and that’s perfectly fine.

If you feel bloated or nauseous:

• Stay at your current level for another week or two.
• Take smaller sips or space gels further apart.

If progress stalls:

• Reassess hydration, stress, and sleep-each affects digestion
• Remember that comfort is individual, not a competition

On race day:

• Stick to the plan you practiced
• Never test new products or timing mid-race

Gut training isn’t about chasing numbers. It’s about finding what works for you and trusting it.

Train your gut like you train your legs-with patience and practice.

The more you repeat your plan, the smoother race day feels.