Gut training for endurance athletes: how to fuel harder without paying for it
Your gut adapts to repeated exposure the same way muscle adapts to repeated load, and you can measure the change. That's the case for gut training, 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
To reach your muscles, carbohydrate has to cross the gut wall through specific transporters, and those transporters have a ceiling.
Glucose crosses via SGLT1. On its own, SGLT1 maxes out at around 60g of carbohydrate per hour. Push past that and the surplus sits in your gut, pulls water in, and you get the bloating, cramping and urgency every endurance athlete has had at least once.
Fructose crosses via a different transporter, GLUT5. Combine glucose and fructose at roughly 2:1 and you use both routes at once, which is how well-fuelled endurance athletes reach 90g an hour and above. It's why nearly every serious gel and drink now lists two sugar sources.
The interesting part is that transporter numbers can change.
The evidence that the gut adapts
Cox and colleagues put cyclists on a high-carbohydrate diet for 28 days and measured how much of the carbohydrate they took in during exercise was actually burned. That figure went up. Same athletes, same drink, more fuel getting through.
Asker Jeukendrup pulled the wider research together in a 2017 Sports Medicine review titled "Training the Gut for Athletes", which is still the reference point. The mechanisms he describes include more SGLT1, faster stomach emptying with repeated exposure, and fewer reported symptoms even where the physiology hasn't changed much. That last one matters more than people think. A lot of race-day gut trouble comes from 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 call exercise-induced gastrointestinal syndrome: during hard exercise blood moves away from the gut, the gut lining gets leakier and digestion slows. Heat makes it all worse. So the gut you're training is under real stress, and you need to progress it gradually, just like your running volume.
Does any of this apply to Hyrox?
Honest answer: less than it applies to a marathon, and more than most Hyrox athletes assume.
Most people finish a Hyrox in 60-100 minutes. Below about 75 minutes, carbs taken during the race make little difference to performance, and stopping for a gel between the sled push and the burpee broad jumps isn't a serious plan.
Where it matters for Hyrox is the pre-race window. You eat 2-3 hours out, sip something in the last hour, and then go straight into an all-out effort with your heart rate near threshold for up to 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 counts as gut training, even if you never take a gel mid-race.
It matters for your long sessions too. If you're doing 90-minute compromised running blocks or building an aerobic base with 2-hour rides, the gut work carries straight over.
Our carb loading guide for Hyrox covers the days before. This piece is about the system that has to process it.
How to train your gut, in order
Weeks 1-2: set a baseline. On your longest easy session, take 30g of carbohydrate an hour from a source you'll use when racing. Note anything: bloating, reflux, cramping, urgency. Write it down, because you won't remember it accurately in six weeks.
Weeks 3-4: raise the volume. Move to 45-60g an hour on long sessions only. Keep everything else the same so you know what caused what. Once you go past 60g, switch to a glucose and fructose blend.
Weeks 5-6: raise the intensity. Now fuel during harder work as well as easy volume. This is where most people find their real ceiling, because blood flow to the gut drops sharply as intensity climbs. If 60g an hour was comfortable at conversational pace and miserable at threshold, that gap is what you're training.
Throughout: sort the fluid. A carb concentration above roughly 8% slows stomach emptying and pulls water into the gut. Most gut disasters are a concentration problem dressed up as a fuelling problem. Drink enough alongside gels, and keep sodium in the mix so the fluid gets absorbed. Our Electrolyte+ (£19.99 for 20 servings) is made for that job, with 1500mg of electrolytes per stick.
Three mistakes that undo the work
Trying a new product on race day. It's obvious, and it's still the most common cause of race-day gut failure.
Training the gut only on easy days. The adaptation is partly intensity-specific, and race intensity is where things go wrong.
Ignoring fibre and fat in the pre-race meal. Both slow stomach emptying. Whatever your everyday diet looks like, the 3 hours before hard racing is the wrong time for a high-fibre breakfast. Test the alternative in training first.
What to take from this
Gut training works, it takes about 6 weeks, and the only cost is the discipline to practise fuelling on sessions where it feels unnecessary.
Start at 30g an hour. Build to whatever your event actually needs. Do it at race intensity at least twice before you race.
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