Runner's gut refers to the nausea, cramping, bloating and urgent bathroom trips that show up on longer runs, and it's common enough that researchers have a name for the underlying condition: exercise-induced gastrointestinal syndrome. It's driven by blood shifting away from the digestive system during exercise, the mechanical jostling of running, and often made worse by over-concentrated drinks or poorly timed food. Most of it is manageable with a few practical changes.
What exactly is runner's gut?
Runner's gut isn't one specific symptom, it's a cluster of gastrointestinal problems that show up during or after sustained exercise: nausea, stomach cramping, bloating, reflux, an urgent need to find a bathroom, and in more severe cases, bleeding. Researchers describe this collectively as exercise-induced gastrointestinal syndrome, and it's a genuinely well-studied phenomenon in endurance sport, not just a case of nerves or bad luck (Costa et al., 2017).
It shows up more often on longer runs, in hot conditions, and at harder efforts, which points to what's actually causing it. It's also more common than most runners realise, which is worth knowing if you've quietly assumed you're the only one dealing with it.
Why does exercise cause gastrointestinal distress?
Three overlapping factors tend to drive it.
Blood redistribution. During exercise, blood flow shifts away from the gut and toward working muscles and skin, to support movement and cool the body. Digestion doesn't stop entirely, but it slows, and the gut has to work with a reduced blood supply. The harder and longer the effort, the more pronounced this redistribution becomes, which is a big part of why GI symptoms tend to escalate as a run goes on.
Mechanical jostling. Running involves repetitive impact that cycling or swimming doesn't. That jostling physically agitates the stomach and intestines, which is part of why runners tend to report more GI issues than athletes in lower-impact endurance sports doing similar durations and intensities.
Over-concentrated drinks and poor food timing. Highly concentrated sports drinks or gels taken without enough water can draw fluid into the gut rather than out of it, worsening bloating and cramping. Eating too much, too close to a run, or relying on high-fat or high-fibre food beforehand, adds further load onto a gut that's already under stress from the first two factors.
None of these three causes is something a runner did "wrong" in some moral sense. They're physiological responses to exercise, and the goal isn't to eliminate them entirely but to manage the factors within your control.
Why does the type of carbohydrate matter?
This is where fuel composition becomes genuinely relevant, not just a marketing detail. A single carbohydrate source, glucose on its own, is absorbed through one pathway in the gut, and that pathway has a ceiling of roughly 60 g per hour. Push more glucose than that through it and the excess tends to sit in the gut rather than being absorbed, which can contribute to bloating and discomfort.
Combining glucose and fructose changes this picture, because fructose is absorbed through a separate pathway. Using both at once allows more total carbohydrate to be absorbed without overloading either pathway, and research comparing single versus multiple transportable carbohydrates has found not just better absorption but better tolerance and performance outcomes with the glucose-fructose combination (Currell & Jeukendrup, 2008). Practically, that means a well-formulated glucose-fructose gel can deliver more usable carbohydrate with less gut stress than an equivalent amount of a single-sugar gel.
This is a core part of how Sunday Salt's gels are built. The maple syrup base naturally provides glucose and fructose in close to a 50:50 split, without maltodextrin, refined sugar, or artificial additives layered on top. It's not a trick to avoid GI issues altogether, nothing fully is, but matching fuel type to how the gut actually absorbs carbohydrate is one of the more evidence-backed ways to reduce the load on a stomach that's already working under stress.
Can the gut actually be trained?
Yes, and this is one of the more useful, under-used pieces of information for runners who deal with GI issues. The gut adapts to repeated exposure, much like muscles adapt to repeated training load. Runners who regularly practise taking in carbohydrate during exercise tend to develop better tolerance and absorption over time than runners who only try it occasionally or for the first time on race day (Jeukendrup, 2017).
This means the fix for a sensitive stomach on long runs often isn't avoidance, it's gradual, repeated exposure in low-stakes training runs. Start with small amounts of carbohydrate during moderate-length runs, build up the frequency and quantity over weeks, and pay attention to what your gut tolerates well versus what it doesn't. What works for another runner won't necessarily work for you, and what didn't work for you six months ago might work fine now that your gut has had more practice.
What practical fixes actually help?
A handful of changes tend to make the biggest difference for runners dealing with GI issues on long runs:
- Practise fuelling in training, never for the first time on race day. This applies to gels, drinks and solid food alike.
- Avoid high-fat, high-fibre meals in the hours before a long run. These slow digestion and leave more material in the gut while blood flow is being redirected elsewhere.
- Don't over-concentrate drinks. Follow mixing instructions on sports drinks and avoid stacking multiple concentrated products close together without enough water.
- Start fuelling early and steadily rather than in large amounts all at once. Small, regular carbohydrate intake tends to sit better than trying to catch up with a big dose after already feeling low on energy.
- Consider a glucose-fructose fuel source over a single-sugar one, particularly for runners who've had trouble with straight glucose gels or drinks in the past.
- Manage pre-run nerves where possible. Anxiety itself affects gut motility and blood flow, and can worsen symptoms independent of anything eaten.
Most runners find relief through a combination of these rather than any single fix, since the underlying causes, blood flow, mechanical stress and fuel choice, are all acting at once.
When is it worth seeing someone about it?
Occasional mild nausea or an urgent bathroom stop on a long run is common and usually manageable with the changes above. Persistent severe symptoms, blood in stool, significant weight loss, or GI distress that doesn't improve despite adjusting food, fluid and training habits are worth discussing with a doctor or sports dietitian, since they can rule out other causes unrelated to exercise.
Frequently asked questions
Is runner's gut dangerous? Mild to moderate symptoms, nausea, cramping, bloating, are uncomfortable but generally not dangerous and tend to resolve once the run ends. Severe or persistent symptoms, including bleeding, warrant medical attention rather than self-management.
Why do I only get GI issues on long runs, not short ones? Blood redistribution away from the gut and mechanical jostling both intensify with duration and intensity, so the cumulative stress on the digestive system is much lower on a 30 minute run than a two-hour one.
Do electrolyte drinks cause stomach issues? They can if too concentrated or introduced without enough plain water alongside them. Following mixing guidance and testing a formula in training before relying on it during a long run or race reduces this risk considerably.
Should I stop eating gels if they upset my stomach? Not necessarily. Try a different formulation first, particularly one using a glucose-fructose blend rather than a single sugar source, and reduce the dose before ruling gels out altogether. Many GI issues improve with gut training rather than avoidance.
References
- Costa, R. J. S., Snipe, R. M. J., Kitic, C. M., & Gibson, P. R. (2017). Systematic review: exercise-induced gastrointestinal syndrome. Alimentary Pharmacology & Therapeutics, 46(3), 246-265.
- Currell, K., & Jeukendrup, A. E. (2008). Superior endurance performance with ingestion of multiple transportable carbohydrates. Medicine & Science in Sports & Exercise, 40(2), 275-281.
- Jeukendrup, A. E. (2017). Training the gut for athletes. Sports Medicine, 47(Suppl 1), S101-S110.
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