Gut training: 6-week protocol to tolerate 90 g/h on race day

What gut training is and why it works

Gut training is the repeated, progressive exposure to carbohydrate during exercise that drives the small intestine to expand its absorption capacity while the stomach loses its sensitivity to volume. Over 14 days of a structured protocol, GI distress drops by an average of 47%, with a parallel 45 to 54% reduction in carbohydrate malabsorption, according to the most recent systematic review in Sports Medicine.

The core idea is simple, and Asker Jeukendrup formalised it back in 2017 in the Sports Medicine review "Training the Gut for Athletes": just as you train the heart and the muscles, you can train the gut. The walls of the small intestine express dedicated transporters for each type of carbohydrate. Those transporters upregulate when they are repeatedly exposed to their substrate. Cox and colleagues showed in 2010 (Journal of Applied Physiology) that 28 days at 8.5 g/kg/day of carbohydrate increased exogenous carbohydrate oxidation during exercise compared with 5.3 g/kg/day. Capacity is not fixed; it is trainable.

For an amateur athlete who consistently blows up with cramps or nausea by km 25 of the marathon, the bottleneck is rarely the heart, the legs, or hydration. It is a gut that has not been trained to absorb 60 to 90 g/h of carbohydrate under effort, and the whole fuelling plan collapses as a result.

The three systems you adapt during the protocol

When you train the gut, you are training three things in parallel. Each one has its own mechanism, and the 6-week protocol stimulates all three. Understanding these mechanisms is what helps you see why escalating doses, specific ratios, and weekly frequency either make or break the adaptation.

How long does the gut take to adapt?

Two weeks is the minimum effective dose. The Tiller et al. (2023) systematic review pulled together 8 studies running between 4 and 28 days and concluded that 14 days of daily exposure to 60 to 90 g/h cuts GI distress by 47% and malabsorption by 45 to 54%. To reach a comfortable 90 g/h, 6 to 8 weeks at 3 sessions a week is the practical standard.

The difference between 2 and 6 weeks is how high you push the ceiling. At 14 days you adapt up to the threshold your existing genetic baseline allows. At 42 days you have actually upregulated transporter expression and dialled down the protective reflexes that would normally throttle absorption. Elite ultra-trail and Ironman athletes work on gut training year-round, with more intensive blocks during the 8 weeks before a target race.

Recreational athletes gain more than professionals in relative terms. Tiller et al. found that 56% of athletes tested in the lab at 70 g/h with no prior training exceeded the clinical threshold for malabsorption. After 14 days only 12% were still over the line. The window for improvement is widest for the people starting furthest back.

FuelRace 6x3 protocol: week-by-week table

This is the core protocol of this guide. 6 weeks, 3 gut training sessions per week, with a dose escalation backed by the literature. It applies to running, cycling, and triathlon. In each session, take the prescribed dose spread across the full hour (one feed every 20 to 25 minutes, with water), and never test anything new on race day. The ratio column tells you the right carbohydrate blend for that phase.

The starting dose is deliberately low. If you jump straight to 70 g/h with no base, you simply replicate the Tiller study where more than half the athletes finished in a state of malabsorption. Sessions can be long runs, long rides, brick sessions, or any continuous effort above 60 minutes. Short sprints and hard intervals will not do the job.

Which carbohydrates to use in your sessions

Below 60 g/h, single-source glucose or maltodextrin is enough. Above 60 g/h, you absolutely need a 2:1 (glucose:fructose) or 1:0.8 blend, because glucose on its own saturates the SGLT1 transporter. A dual-source blend recruits SGLT1 and GLUT5 simultaneously and lets trained athletes reach 90 to 120 g/h (Jentjens et al. 2004, Jeukendrup 2017).

In commercial terms, that means dual-source gels and drinks: Maurten 100/160, GU Roctane, SiS Beta Fuel 80, Precision Fuel & Hydration PF 30/90, Cars-Up Roctane, OTE Super Gel, Spring Energy Awesome Sauce. Read the label: if it lists "maltodextrin + fructose" or "glucose + fructose", you are on track. If it lists only "glucose", "maltodextrin", or "dextrose", it is single-source and you are capped at 60 g/h.

For sensitive stomachs above 70 g/h, hydrogels (Maurten, the SiS hydrogel) lower osmolarity inside the stomach and move into the small intestine faster. They are no silver bullet, but in athletes with a history of GI distress, they genuinely help. Hydrogels work by encapsulating carbohydrate in a pH-sensitive gel that only releases it once it reaches the intestine.

How to combine it with daily diet and the carb load

Gut training is not just about what you take in during the session itself. Cox et al. showed that a chronically high daily carbohydrate intake (8.5 g/kg/day) raises exogenous oxidation on its own. The everyday diet prepares the ground. On heavy days, aim for 6 to 10 g/kg/day. On easy days, 3 to 5 g/kg/day is enough. For a 70 kg athlete that works out at 420 to 700 g of carbohydrate on a hard day and 210 to 350 g on a light one.

Pair the gut training protocol with the pre-race carb load like this: 6 weeks out, you start the gut training. 7 days before the race, you cut training volume but keep 90 g/h in at least 2 short sessions. 48 to 72 hours out, you kick off the carb-loading protocol (8 to 10 g/kg/day, swapping out fat). On race day you execute the plan with the right dose on the right timeline. The two strategies are complementary: the carb load fills the glycogen tank, and gut training makes sure you can actually keep taking fuel in while you race.

Athletes preparing for trail and ultra-trail use gut training year-round, with 8-week blocks ahead of target races. Triathletes doing long-course racing fold the work into brick rides. For both, the FuelRace calculator adjusts the strategy automatically based on the tolerance profile you set in the wizard.

What results to expect in the first two weeks

Across 14 days, the literature shows consistent outcomes: GI distress drops by an average of 47% (Miall et al., 2018), upper GI symptoms (nausea, reflux, vomiting) fall 64 to 70%, lower GI symptoms (urgency, gas, cramps) fall 40 to 70%, and carbohydrate malabsorption drops 45 to 54%. Performance does not improve directly across those 2 weeks, but the platform for tolerating more fuel on race day is already in place.

What you should not expect in those first 2 weeks: improvements in markers of epithelial integrity such as I-FABP, claudin-3, or circulating endotoxin. These do not shift with gut training, only with risk management (managing heat, hydration, avoiding NSAIDs such as ibuprofen). You also should not expect gastric emptying itself to speed up. What changes is your perception of pressure and your absorption capacity, not the raw physical speed of the stomach.

If after 2 weeks you cannot feel any difference, the failure almost always comes down to frequency (fewer than 3 sessions per week), insufficient dose (you never broke past 40 g/h), or the wrong carbohydrate (single-source glucose above 60 g/h). Walk it back and audit the protocol.

Common mistakes that sabotage gut training

1. Skipping weeks to speed things up. Going from 40 to 80 g/h in 7 days guarantees distress and kills motivation. The optimal jump is 10 to 15 g/h per week.
2. Taking everything early in the session. The body prioritises blood flow to the working muscles. Spread your intake out, one dose every 20 to 25 minutes, with 100 to 200 ml of water per dose.
3. Mixing brands and profiles randomly. Every gel has its own osmolarity, ratio, and carrier. Train with the brand you are going to race with. Do not improvise on race day.
4. Ignoring hydration. Carbohydrate without water sits concentrated in the stomach, delays gastric emptying, and triggers cramps. Aim for at least 500 ml of water per hour during sessions.
5. Sticking with single-source glucose above 60 g/h. Glucose saturates SGLT1. With no fructose to feed GLUT5, the excess pools in the gut, ferments, and produces gas and diarrhoea.
6. Training fasted on Friday and force-feeding 90 g/h on Monday. Adaptation needs a steady stimulus. Either you train the gut consistently or you do not. No casual 90 g/h sessions out of nowhere.
7. Taking NSAIDs before or during sessions. Ibuprofen and diclofenac increase intestinal permeability 3 to 5 times during exercise. Switch to paracetamol if you absolutely need it, or drop it entirely.
8. Not simulating heat before a hot race. In the heat, splanchnic blood flow drops and absorption falls by 20 to 30%. Run at least 2 sessions during the hottest part of the day if a hot race is on the cards.