Runner's stomach: causes, gut training and electrolytes

Cramping, nausea, an urgent dash to a portable toilet: "runner's stomach" (or "runner's trots") is one of the most common reasons endurance races go wrong. The good news is that the gut is adaptable. With the right preparation, most runners can reduce symptoms substantially.

How common is it?

A review by de Oliveira, Burini and Jeukendrup (2014) found that, generally, 30 to 50% of athletes experience gastrointestinal complaints during exercise, with endurance athletes most affected. Most symptoms are mild and harmless, but the authors noted rare serious problems such as bleeding from the stomach or bowel and ischaemic bowel.

Symptoms fall into two groups:

• Upper gut: nausea, reflux, belching, bloating, stomach cramps, vomiting.
• Lower gut: urge to go, flatulence, loose stools, diarrhoea, lower abdominal cramps.

Why running upsets your gut

The same review identified three main causes:

1. Physiological. During intense exercise, especially when dehydrated, blood flow to the gut falls sharply as it is redirected to muscles and skin. This is believed to be one of the main drivers of symptoms.
2. Mechanical. Running jostles the gut, one reason running seems to disturb it more than other exercise modes.
3. Nutritional. Fibre, fat, protein, concentrated drinks and some sugars before or during exercise can slow gastric emptying or draw water into the bowel.

A systematic review by Costa et al. (2017) called this "exercise-induced gastrointestinal syndrome". As intensity and duration rise, markers of gut injury, gut permeability and endotoxin in the blood all increase, while gastric emptying and absorption worsen. Heat and running make it worse, and exercise of 2 hours or more at 60% of VO2max appeared to be the point where significant disturbances showed up, regardless of fitness. More on heat in exercise and gut health.

Fixes that work

Strategy	What to do	Evidence
Gut training	Practise race fuelling on long runs for 2+ weeks	Small trials, consistent
Multiple carbohydrate types	Use glucose plus fructose products at high intakes	Proposed, limited evidence
Hydration and electrolytes	Start hydrated, replace sweat losses, avoid big deficits	Mechanism plus observation
Low-fibre pre-race meal	Cut fibre and fat for 1 to 2 days and on race morning	Guidelines, practice
Short-term low FODMAP	Try for a few days under guidance if symptoms persist	One small crossover trial
Heat acclimation and pacing	Acclimatise, start conservatively in heat	Indirect

Gut training

In Costa et al., 2017 (Appl Physiol Nutr Metab), endurance runners did a two-hour run while taking 30 g of carbohydrate every 20 minutes, before and after two weeks of daily carbohydrate intake during training. Repeated gut challenges reduced gut symptoms and lowered breath hydrogen, a sign that less carbohydrate was being malabsorbed.

Jeukendrup (2017) reviewed the wider evidence: gastric emptying and stomach comfort can be trained, and a high-carbohydrate diet increases intestinal glucose transporters, allowing more carbohydrate absorption during exercise.

In practice, use your race gels, drinks and timings on long runs and zone 2 sessions (the easy bulk of 80/20 training), building up over several weeks. Never try a new product on race day.

Electrolytes and fluids for a running stomach

Because dehydration lowers gut blood flow further, staying hydrated is one of the simplest protections. Practical points:

• Start the race well hydrated and drink to a plan you have tested in training.
• Replace sodium in long or hot events. Heavy sweaters lose a lot. See our electrolytes guide.
• Take gels with water. Very concentrated carbohydrate without fluid slows gastric emptying.
• Do not overdrink. Drinking far more than you sweat risks low blood sodium (hyponatraemia).

Low FODMAP for runners

FODMAPs are fermentable carbohydrates found in foods like onions, garlic, beans, wheat and some fruits. In a small crossover trial, Lis et al. (2018) put 11 runners with exercise-related gut symptoms on a low or high FODMAP diet for six days. On the low FODMAP diet, daily symptoms such as loose stools and diarrhoea fell, although symptoms during the exercise sessions themselves did not differ. Treat it as a short-term pre-race tool, ideally with a dietitian, not a permanent diet. Fibre timing is covered in fibre and gut health for endurance athletes.

What about the microbiome?

Your gut bacteria may play a part in how you tolerate exercise, but evidence is young and mostly associative. See gut microbiome and athletic performance. Some probiotic strains have been studied for gut symptoms in athletes, with mixed results; see our probiotics guide.

Race-day checklist

1. Low-fibre, low-fat meal 2 to 4 hours before the start.
2. Caffeine only if tested in training (see caffeine and nitrate).
3. Pace sensibly: running far above your lactate threshold early increases gut stress.
4. Adjust for heat; heat training helps you adapt beforehand.
5. Use only fuel and drinks you have practised with.

See a doctor if you notice blood in your stool, black or tarry stools, severe or persistent pain, symptoms that also occur at rest, or unexplained weight loss. These are not normal runner's stomach.

Sources

• de Oliveira EP, Burini RC, Jeukendrup A. Gastrointestinal complaints during exercise: prevalence, etiology, and nutritional recommendations. Sports Med, 2014.
• Costa RJS, Snipe RMJ, Kitic CM, Gibson PR. Systematic review: exercise-induced gastrointestinal syndrome, implications for health and intestinal disease. Aliment Pharmacol Ther, 2017.
• Costa RJS, et al. Gut-training: the impact of two weeks repetitive gut-challenge during exercise on gastrointestinal status, glucose availability, fuel kinetics, and running performance. Appl Physiol Nutr Metab, 2017.
• Jeukendrup AE. Training the gut for athletes. Sports Med, 2017.
• Lis DM, et al. Low FODMAP: a preliminary strategy to reduce gastrointestinal distress in athletes. Med Sci Sports Exerc, 2018.