Every year, tens of thousands of runners cross the finish line of the London Marathon having conquered something that never appears in any training plan: the desperate, mid-race urge to find a toilet. That’s right, we’ve heard that miles 18-22 are the brutal miles where runner’s trots have been known to strike.
What exactly is runner's trots?
Runner's diarrhoea is actually a mix of symptoms: urgent, sometimes explosive diarrhoea; cramping abdominal discomfort; and a heightened urge to open your bowels. It's brought on by the whole-body effects of sustained running and the mechanical stresses it places on the digestive system.
In most cases, it's not dangerous, just deeply inconvenient, occasionally mortifying, and often performance-sapping.
The science: what happens to your gut when you run a marathon?
During sustained exercise, your body makes a ruthless set of decisions about where to send blood. Your working muscles, particularly your legs, become the priority. To fuel them, blood flow is diverted away from the digestive system.
The common culprits of runner’s trots
1. Reduced gut blood flow
Blood diverted to working muscles starves the digestive system of oxygen, causing inflammation and increased gut permeability.
2. Dehydration
Staying hydrated is one of the most evidence-backed interventions for reducing GI distress.
3. High-carbohydrate gels and drinks
Concentrated carbohydrate solutions draw extra water into the gut and can accelerate transit, making diarrhoea more likely, especially if your gut hasn't been trained to handle them.
4. Caffeine
A popular pre-race pick-me-up, caffeine also has a laxative effect in many people. Timing and dose matter enormously.
5. Pre-race anxiety
The gut-brain axis means psychological stress directly influences gut motility. Race nerves alone can trigger symptoms before a single mile is run.
What can you actually do about it?
1. Train your gut like your legs
Practice consuming race-day carbohydrates and gels on long training runs. The gut is adaptable, research shows that regular in-run carbohydrate consumption significantly improves GI tolerance over time.
2. Start your microbiome work early
A recent study conducted with Symprove at St Mary’s University London, showed marathon runners felt the difference after just 6 weeks – supporting their training and on race day.
3. Stay well hydrated
Dehydration is one of the most significant drivers of gut damage during running. Sip consistently rather than gulping.
4. Time your pre-race meal carefully
Aim to finish eating at least 2–4 hours before the start. Keep it familiar, low in fat and fibre, and nothing you haven't tested in training.
5. Know your trigger foods
Cut high-fibre foods, fatty meals, dairy and high-fructose fruits in the 24 hours before a long run. Even foods that feel fine day-to-day can cause problems when the gut is under stress.
The bottom line
Runner's trots are a near-universal feature of distance running, the gut's entirely predictable response to being asked to keep working while 26.2 miles of blood-hungry muscle demands priority attention.
But they're also not simply something to be endured. The science is clear that the health of your gut microbiome matters not just for what happens on marathon day, but for how resilient your gut is across the weeks and months of training that get you there. A diverse, well-supported gut microbiome is better placed to handle the disruptions of long-distance running, recover more quickly, and keep the lining of the gut intact when the going gets tough.
As London Marathon 2026 preparations begin for the record number of hopeful applicants, there's one piece of training that tends to be left off the plan: looking after your gut. Start that part of the work now, your mile 18 self will thank you.
To learn more about other gut health topics, visit Symprove’s Gut Hub.
