Glossary

Gut Training

Updated April 9, 2026

Many endurance athletes know their fueling targets on paper and still fail to absorb them when pace, heat, and stress rise. Gut training is the repeated practice of taking in carbohydrate and fluid during exercise until race-day feeding feels ordinary to the stomach and intestine. Gut Training for Race Nutrition lays out the full rehearsal process.

Tolerance changes before transport does

Gut training changes tolerance at more than one level. The stomach learns to handle larger volumes with less discomfort. The intestine gets repeated exposure to the carbohydrate mix you plan to use. The athlete also stops turning every race feed into a novelty event. Those changes matter because the gut is one of the main rate limiters for exogenous fuel delivery during long exercise.

Asker Jeukendrup summarized the transport problem clearly in 2014 and again in the GSSI review in 2017. When athletes ingest glucose-based carbohydrate above about 60 to 70 g per hour, exogenous carbohydrate oxidation usually plateaus at about 60 g per hour because SGLT1 transport becomes the bottleneck.¹ Mixed glucose-fructose feeding works better at higher intakes because fructose uses a different transporter. That is why high-end marathon, triathlon, and ultra plans often move toward multiple transportable carbohydrates once the target reaches 60 to 90 g per hour or more.

The stomach side adapts too. Gregory Lambert and colleagues studied seven runners across six 90-minute runs at 65% of VO2max while they drank a glucose-electrolyte solution at roughly sweat-rate-matched volumes.² Stomach comfort improved on runs 5 and 6 compared with run 2, even though gastric emptying itself did not clearly speed up. Rehearsing volume can improve comfort even before deeper physiological changes show up.

What two weeks of practice can change

The clearest direct data comes from the Monash group. Ricardo Costa and colleagues ran a two-week gut-training intervention in endurance runners and found that exercise-associated gastrointestinal symptoms fell by 60% in the carbohydrate supplement group and 63% in the carbohydrate food group when the athletes repeated a daily feeding challenge during exercise.³ The intervention also improved glucose availability during the gut challenge, which matters when late-race intake is supposed to support pace and clear the stomach cleanly.

Recent reviews support the same direction with more caution. Janez Mlinaric and Nina Mohorko concluded in a 2025 systematic review that gut-training protocols appear promising for lowering GI symptoms over time, while hydrogel products still lack evidence of clear superiority over standard carbohydrate products.⁴ Repeated exposure still matters more than chasing a special product.

Build the race script in training

The target should match the race demand. A 60-minute session does not need the same gut challenge as a marathon rehearsal. The most useful habit is to practice the exact products, concentration, sip pattern, and hourly dose you expect to use in competition, then progress only when the previous step is stable.

This is where hydration, sodium intake, and carbohydrate sources have to work as one system. Athletes often blame gels for symptoms that actually came from overconcentrated bottles, low fluid intake, a poor sodium plan, or a pace that was too aggressive for the feeding strategy. Endurance Athlete Fueling is useful here because the hourly carbohydrate target only works if the fluid and sodium plan can carry it.

Pre-session food still matters. Race-specific gut sessions usually work better when the pre-run or pre-ride meal is lower in fiber and built from tested carbohydrate staples. Daily soluble fiber supports health and steadier meal responses, but a high-fiber breakfast before a hard gut challenge often adds residue and fermentation load that make the session harder to read. The same logic carries into glycogen loading and the breakfast setup in How to Set Up a Race-Week Nutrition Plan.

Where rehearsal stops helping

Gut training improves tolerance to the inputs you practice. It does not rescue a race plan that ignores event duration, heat, or pacing. It does not turn every athlete into a 100 g per hour athlete. It also does not fix GI symptoms that occur outside exercise or come with blood in the stool, repeated vomiting, unexplained weight loss, or severe abdominal pain.

The other hard limit is copying a number without copying the process. A runner who practices almost no intake in long runs and then tries 90 g per hour on race day has not trained the gut at all. A cyclist who tolerates a bottle-based plan on the bike still needs run-specific practice if the race ends with impact and higher mechanical stress. The rehearsal dose, product mix, and pacing conditions have to resemble the event closely enough to teach the gut something real.