Gut Training: How to Train Your Gut for 90 g of Carbs per Hour

Between 30 and 60 percent of endurance athletes report gastrointestinal problems in competition — in marathons and triathlons the digestive tract limits performance more often than the legs do. The good news: the gut is trainable. With targeted “train the gut” work you can raise your carbohydrate intake from 40 to 90 grams per hour — and that decides what happens at mile 22.

Why the gut limits race pace

Under load, the body prioritizes: blood flow to the digestive tract drops by up to 80 percent during intense endurance exercise, because the blood is needed in the working muscles. Yet that blood is exactly what the gut needs to absorb carbohydrates and move them along. If fuel sits unused in the gut, it draws water osmotically — the result is bloating, cramps, nausea or diarrhea.

The second limit is the transporters: glucose and galactose are absorbed via the SGLT1 transporter, whose capacity caps out at around 60 grams per hour. Only the combination with fructose, which reaches the bloodstream via its own transporter (GLUT5), raises the maximum oxidation rate to 90 grams per hour. That is why modern sports gels blend glucose and fructose at 2:1 or 1:0.8.

Crucially, neither limit is fate. Studies — above all from Asker Jeukendrup’s research group — show that the gut’s absorption capacity can be measurably improved by repeated exposure to carbohydrate intake under load. Transporter density increases, gastric emptying speeds up, and symptoms fade.

What exactly adapts

• Transporter capacity: The number of SGLT1 and GLUT5 transporters in the small intestine rises with regular carbohydrate exposure under load — differences are measurable after about ten fueled sessions.
• Gastric emptying: The stomach “learns” to pass even more concentrated solutions along quickly. If you never train with fuel, the gel quite literally sits in your stomach on race day.
• Symptom tolerance: Subjective comfort improves markedly. Bloating and cramps that occur at 60 g/h early on usually disappear completely after weeks of habituation.
• Fluid absorption: In parallel you train your drinking volume under load — important, because many athletes drink far less in races than they sweat.

The 4-week protocol

The progression works like any other training: apply a stimulus, adapt, increase. Schedule two to three fueled sessions per week — ideally the long run or long ride plus one tempo or threshold session, because high intensity is what reduces gut blood flow and therefore the most honest test bench.

• Week 1 — 30–40 g/h: One session of 75 minutes or more with intake. Start with 30 grams per hour as a drink (6–8 % carbohydrate concentration) or half a gel every 30 minutes. Goal: the body registers that food arrives under load.
• Week 2 — 45–60 g/h: Raise to 45–60 grams per hour, split into two or three portions. Now is the time to switch to a glucose-fructose product. Untrained athletes often still notice mild symptoms here — normal, not a setback.
• Week 3 — 60–75 g/h: The dose most age-groupers race with. Combine drink plus gel or chews. Increase fluid intake too: 400–600 ml per hour, more in the heat.
• Week 4 — 75–90 g/h: Race simulation: marathon pace or race threshold for 90–120 minutes with full intake. Only with a glucose-fructose blend — pure glucose products max out at 60 g/h. If this works without symptoms, you are race-ready.

This progression fits perfectly into the final four to six weeks before your A-race — that is where the long, race-specific key sessions sit anyway. If you have more time, stretch the protocol over eight weeks and maintain the level afterwards with one fueled session per week.

Practice: gel, drink or solid food?

• Mind the concentration: Sports drinks should contain 6–8 % carbohydrate (60–80 g per litre). Significantly more concentrated solutions delay gastric emptying. Always wash gels down with water, never with sports drink.
• Liquid vs. solid: Under high load, liquid and semi-solid forms (gels, chews) work more reliably than bars. Solid food has its place in training — on long, easy gravel rides, for example — but barely in a race at tempo intensity.
• Do not forget sodium: 300–600 mg of sodium per hour supports water absorption via the SGLT1 pathway and replaces sweat losses. Details in our article on electrolytes for runners.
• Unburden the gut the day before: Reduce fibre- and fat-rich food 24–36 hours before the start — leftovers in the gut are a frequent trigger of race-day problems. Loading the stores is handled by the carb-loading protocol.

Common mistakes

• Always training fasted. Fasted runs have their place for fat metabolism — but if you only ever train fasted, you never train the gut. Come race day, the exact adaptation that carries your fueling is missing.
• New products on race day. Gel brand, flavour, caffeine content — everything gets tested in at least two hard sessions beforehand. Also limit caffeinated gels: two or three per race are enough.
• Starting too late. The first intake belongs at 20–30 minutes, not 60. If you wait for hunger, you are already behind on energy balance — an early warning sign for “the wall” in the marathon.
• Only practising on flat, easy sessions. The gut must tolerate fuel at race pace. If you only fuel easy, you will meet the symptoms for the first time in competition — the worst possible moment.
• Forgetting to drink. 90 grams of carbs per hour only works with enough fluid. Below roughly 400 ml/h the solution in the gut becomes too concentrated and pulls water instead of delivering it.

How Peakora builds fueling into your training

In Peakora, fueling is not an afterthought but part of the plan: the fueling calculator sets target amounts of carbohydrate and fluid for every long session — matched to duration, intensity and your body weight. That way every key session in your marathon build automatically becomes a gut-training session too, instead of fueling practice being forgotten. You will also find the fueling targets in the context of your weekly plan, so nutrition, load and recovery stay aligned.

Frequently asked questions about gut training