Gut training for endurance athletes: how to fuel harder without paying for it
The gut adapts to repeated exposure in the same way muscle adapts to repeated load, and the adaptation is measurable. That's the case for gut training for endurance athletes, and it's better supported than most people racing this weekend realise.
Here's what the research shows, and how to build it into a block without wrecking a session to find out.
What actually limits carbohydrate during exercise
Carbohydrate doesn't reach your muscles by magic. It has to cross the intestinal wall through specific transporters, and those transporters have a ceiling.
Glucose crosses via SGLT1. On its own, SGLT1 saturates at around 60 grams of carbohydrate per hour. Push past that and the surplus sits in the gut, pulls water in, and you get the bloating, cramping and urgency every endurance athlete has experienced at least once.
Fructose crosses via a different transporter, GLUT5. Combine glucose and fructose in roughly a 2:1 ratio and you use both routes at once, which is how well-fuelled endurance athletes reach 90 grams per hour and above. This is why nearly every serious gel and drink on the market now lists two sugar sources rather than one.
Transporter density isn't fixed. That's the interesting part.
The evidence that the gut adapts
Cox and colleagues put cyclists through 28 days on a high-carbohydrate diet and measured how much of the carbohydrate they consumed during exercise was actually burned. Exogenous carbohydrate oxidation went up. Same athletes, same drink, more fuel getting through.
Asker Jeukendrup pulled the wider literature together in a 2017 Sports Medicine review titled "Training the Gut for Athletes", which is still the reference point for this. The mechanisms he describes include increased SGLT1 expression, faster gastric emptying with repeated exposure, and reduced symptom reporting even where the physiology hasn't changed much. That last one matters more than people give it credit for. A lot of race-day gut trouble is a gut meeting an unfamiliar volume of fluid and sugar for the first time under stress.
Source: Jeukendrup A., "Training the Gut for Athletes", Sports Medicine, 2017.
The counterweight is what Costa and colleagues describe as exercise-induced gastrointestinal syndrome: blood diverts away from the gut during hard exercise, the intestinal barrier gets leakier, motility slows. Heat makes all of it worse. So the gut you're training is also a gut under genuine stress, and progression needs to be gradual for the same reason your running volume does.
Does any of this apply to Hyrox?
Honest answer: less than it applies to a marathon, and more than most Hyrox athletes assume.
A Hyrox race takes most people between 60 and 100 minutes. Below about 75 minutes, in-race carbohydrate makes little difference to performance, and stopping to drink a gel between the sled push and the burpee broad jumps is not a serious plan.
Where it does matter for Hyrox is the pre-race window. You're eating 2 to 3 hours out, sipping something in the last hour, and then going straight into an all-out effort with a heart rate near threshold for an hour and a half. If your gut has never handled food at that intensity, race morning is a bad time to find out. Practising the pre-race meal is gut training, even if you never take a gel mid-race.
It also matters for your long sessions. If you're doing 90-minute compromised running blocks or building an aerobic base with 2-hour rides, the gut work carries over directly.
Our carb loading guide for Hyrox covers the days before. This piece is about the machinery that has to process it.
How to train your gut, in order
Weeks 1 to 2: establish a baseline. On your longest easy session, take 30 grams of carbohydrate per hour from a source you'll use in racing. Note anything: bloating, reflux, cramping, urgency. Write it down, because you won't remember accurately in six weeks.
Weeks 3 to 4: raise the volume. Move to 45 to 60 grams per hour on long sessions only. Keep everything else the same so you know what caused what. Use a glucose and fructose blend rather than glucose alone once you cross 60 grams.
Weeks 5 to 6: raise the intensity. Now fuel during harder work, not just easy volume. This is where most people find their real ceiling, because blood flow to the gut drops sharply as intensity climbs. If 60 grams per hour was comfortable at conversational pace and miserable at threshold, that gap is the thing you're training.
Throughout: fix the fluid. Carbohydrate concentration above roughly 8 per cent slows gastric emptying and pulls water into the gut. Most gut disasters are a concentration problem wearing a fuelling problem's clothing. Drink enough alongside gels, and keep sodium in the mix so the fluid actually gets absorbed rather than sloshing. Our Electrolyte+ (£19.99 for 20 servings) is built for that job, with a calcium-heavy formula and no artificial sweeteners.
Three mistakes that undo the work
Trying a new product on race day. Obvious, still the most common cause of race-day gut failure.
Training the gut only on easy days. The adaptation is somewhat intensity-specific, and race intensity is where it fails.
Ignoring fibre and fat in the pre-race meal. Both slow gastric emptying. Whatever your everyday diet looks like, the 3 hours before hard racing is the wrong window for a high-fibre breakfast. Test the alternative in training first.
What to take from this
Gut training for endurance athletes works, it takes about 6 weeks, and it costs nothing except the discipline to practise fuelling on sessions where it feels unnecessary.
Start at 30 grams per hour. Build to whatever your event actually demands. Do it at race intensity at least twice before you race.
Interval makes ready-to-drink supplements for athletes who train with intent, with every dose listed on the label. See the range.