Gut Training: How to Stop GI Distress on Long Runs

Why do long runs trigger GI distress in the first place?

Endurance running places a specific, measurable strain on the digestive system. In their systematic review, Costa, Snipe, Kitic and Gibson (2017) defined 'exercise-induced gastrointestinal syndrome' and identified a threshold: exercise of about two hours or longer at roughly 60 percent of VO2max is where significant gut perturbations reliably appear. During prolonged effort, blood is redistributed away from the gut towards working muscles and skin. That reduced splanchnic flow can injure the intestinal lining, increase permeability so bacterial endotoxins leak into circulation, delay gastric emptying and impair carbohydrate absorption.

The review also noted that running, with its mechanical jostling, and heat stress both worsen these effects compared with cycling in cool conditions. This mechanistic picture matters because it explains why symptoms cluster in marathons, ultras and hot races rather than short, easy sessions. Gut training aims to counteract exactly these processes, chiefly the malabsorption and tolerance problems, rather than eliminate the underlying physiology.

Does gut training actually reduce symptoms?

The foundational evidence is a randomised controlled trial by Costa et al. (2017), which compared a carbohydrate gel, whole-food carbohydrate and a placebo across two weeks of repetitive in-exercise feeding. Both carbohydrate groups reduced GI symptoms by around 60 to 63 percent versus placebo, and the gel group additionally lowered carbohydrate malabsorption, raised blood glucose availability and improved running performance. A companion trial by Miall et al. (2018) confirmed that repeating a two-hour running carbohydrate challenge daily for two weeks reduced symptoms and malabsorption, measured through breath hydrogen.

The best synthesis comes from Martinez et al. (2023), who pooled eight gut-training and feeding-challenge studies. They reported a roughly 47 percent fall in gut discomfort and a 45 to 54 percent reduction in carbohydrate malabsorption. Honest caveat: the same review found effects on gastric emptying, intestinal integrity and immune markers were inconclusive, and the total number of participants across studies is small. The tolerance benefit is the clearest finding.

How should you structure a gut-training block?

The protocols that produced results were consistent and unglamorous. Costa et al. (2017) and Miall et al. (2018) both used a two-week block of repeated carbohydrate feeding during running sessions lasting around two hours, delivering carbohydrate at rates comparable to race-day targets. The principle is progressive exposure: the gut adapts its transport capacity and tolerance to whatever you habitually ask it to absorb during exercise.

A practical translation is to schedule your gut-training block in the final two to three weeks before a key long-distance race, when you are already doing long runs. Use the exact products, concentrations and feeding intervals you intend to race with, rather than switching brands on the day. Start towards the lower end of your carbohydrate target and build up. Because the studied window was only two weeks, there is no strong evidence on how quickly adaptations fade, so it is sensible to keep practising fuelling regularly rather than treating it as a one-off.

Why do multiple transportable carbohydrates matter?

Unabsorbed carbohydrate sitting in the gut draws in water and ferments, which contributes to cramping, bloating and diarrhoea. Reducing that residue is central to comfortable fuelling. Currell and Jeukendrup (2008) showed why the type of carbohydrate matters: a 2:1 glucose-to-fructose drink delivering 1.8 grams per minute produced an 8 percent faster time trial than glucose alone in trained cyclists, and 19 percent faster than water.

The mechanism is that glucose and fructose use separate intestinal transporters, SGLT1 and GLUT5. Using both routes together raises the ceiling on how much carbohydrate the gut can absorb and oxidise, leaving less unabsorbed to cause distress. Notably, the successful gut-training trials by Costa et al. (2017) used a 2:1 glucose:fructose blend, so multiple transportable carbohydrates and gut training work in tandem. When choosing gels and drinks, check the label for a glucose-plus-fructose or maltodextrin-plus-fructose formulation rather than a single-source product.

Can a low-FODMAP diet help alongside gut training?

Diet in the day or two before a run also influences gut load. FODMAPs are fermentable carbohydrates found in foods such as onions, wheat, legumes, certain fruits and dairy, and they can draw water into the bowel and ferment. Wiffin et al. (2019) tested this in 16 recreational runners using a crossover design: a seven-day low-FODMAP diet under free-living conditions reduced exercise-related GI symptoms and improved perceived ability to exercise compared with a high-FODMAP diet.

This positions short-term FODMAP reduction as a complementary strategy, not a replacement for gut training. Two caveats are worth stating. The study was small and short, and low-FODMAP eating is restrictive and not intended for long-term use, so it is best trialled briefly around key sessions or races. Because responses vary, the sensible approach is to test a low-FODMAP day before a hard long run in training and judge whether it helps you specifically before committing to it on race day.

Why does GI tolerance vary so much between runners?

Averages hide large individual differences, and this is the most important practical caveat in the whole area. Pfeiffer et al. (2009) studied carbohydrate gels during 16-kilometre runs at delivery rates up to 1.4 grams per minute, comparing glucose-only against glucose-fructose gels. Mean symptom scores stayed low on average, which sounds reassuring, but roughly 10 to 20 percent of runners still reported serious GI problems.

That spread tells you group data cannot predict your own gut. Some runners tolerate high carbohydrate rates comfortably; others struggle at modest intakes regardless of formulation. The consequence is that gut training is as much about personal experimentation as about following a protocol. Use training runs to establish your own tolerated carbohydrate rate, product type and timing, and log what happens. If you are among the minority who react badly even to well-designed fuelling, that is a signal to build up more gradually and to trial complementary tactics such as FODMAP reduction rather than forcing a standard target.

What does the evidence not yet settle?

It is worth being clear about the limits. Martinez et al. (2023) concluded that while symptom and malabsorption improvements are reasonably consistent, effects on gastric emptying, intestinal integrity and systemic immune markers were inconclusive across the eight studies reviewed. The literature is also small, dominated by a single research group, and mostly conducted in trained or endurance-experienced runners under controlled conditions rather than in the chaos of an actual race.

Several practical questions remain open. There is little data on how long adaptations persist once you stop practising, on the optimal carbohydrate dose for gut training specifically, or on how these findings transfer to ultra-distance events and extreme heat. None of this undermines the core recommendation, which rests on genuine randomised trials, but it does mean claims should stay measured. Treat gut training as a well-supported way to reduce symptoms and improve fuel absorption, not a guaranteed cure for every runner's stomach.

What is gut training for runners?

Gut training is the deliberate practice of taking on carbohydrate during exercise so the digestive system adapts to absorb and tolerate it better. Costa et al. (2017) showed that two weeks of repeated in-run carbohydrate feeding cut gastrointestinal symptoms by around 60 percent versus placebo, while improving carbohydrate absorption and running performance. In practice it means rehearsing your race-day fuelling on long training runs.

How long does it take to train your gut?

The trials that produced measurable benefits used a two-week block of repeated carbohydrate feeding during roughly two-hour running sessions (Costa et al., 2017; Miall et al., 2018). A sensible approach is to run this block in the final two to three weeks before a key race. Because the studied window was only two weeks, keep practising fuelling regularly rather than treating it as one-off preparation.

Why do glucose and fructose together reduce stomach problems?

Glucose and fructose use separate intestinal transporters, SGLT1 and GLUT5, so combining them raises total carbohydrate absorption and leaves less unabsorbed carbohydrate to ferment and cause distress. Currell and Jeukendrup (2008) found a 2:1 glucose:fructose drink was 8 percent faster than glucose alone. The successful gut-training trials also used a 2:1 blend, so check labels for dual-source carbohydrate.

Does a low-FODMAP diet help with running GI distress?

It can, as a short-term complement to gut training. Wiffin et al. (2019) found a seven-day low-FODMAP diet reduced exercise-related GI symptoms and improved perceived exercise ability in 16 recreational runners versus a high-FODMAP diet. The study was small and short, and low-FODMAP eating is restrictive, so trial it briefly around key sessions rather than adopting it long term.

Why does my stomach still hurt when others fuel fine?

Individual variation is large. Pfeiffer et al. (2009) found average symptom scores during running stayed low, yet roughly 10 to 20 percent of runners still reported serious GI problems at the same carbohydrate rates. Group averages cannot predict your gut, so use training runs to find your own tolerated carbohydrate rate, product and timing, and build up gradually if you react badly.

When does exercise start causing gut problems?

Costa, Snipe, Kitic and Gibson (2017) identified a threshold of roughly two hours of exercise at about 60 percent of VO2max, where significant gut perturbations such as intestinal injury, increased permeability, delayed gastric emptying and malabsorption appear. Running and heat stress make these worse than cycling in cool conditions, which is why symptoms cluster in marathons, ultras and hot races rather than short, easy sessions.