Why athletes get the runs of their lives during races – and what you can do to stop it
Runner’s diarrhoea affects up to 90% of endurance athletes. Learn the science behind it and practical steps to keep your gut on track.
A race, a toilet and a very public mishap
In Beijing this week Joanna Wietrzyk crossed the finish line of a Hyrox race covered in her own waste. The incident drew headlines, but it also shines a light on a problem that many endurance athletes experience in silence: runner’s diarrhoea, or "exercise‑induced gastrointestinal (GI) syndrome".
How common is the gut‑crash?
The Guardian reports that about 60 % of endurance athletes admit to some GI upset during training or competition, with the American Medical Society for Sports Medicine suggesting the figure could be as high as 90 %. Symptoms range from mild bloating to full‑blown loss of bowel control, and they appear most often in high‑intensity events lasting 90 minutes or more – marathons, ultramarathons, and Hyrox being prime examples.
Why intense exercise rattles the intestines
When you run, your body redirects a large proportion of blood away from the digestive tract and towards the working muscles and skin. This diversion reduces the gut’s ability to absorb nutrients and slows motility – the coordinated contractions that move food along. In hot conditions the skin demands even more blood, worsening the shortfall.
The mechanical impact of running – the repeated up‑and‑down jostling of the abdomen – adds a physical shake to the chemical stress. Professor John Hawley describes this as "mechanical jostling of the intestines", which can trigger cramping and accelerate transit, especially when the lower gut is already struggling to absorb the carbohydrates and electrolytes athletes ingest during the race.
A second, less obvious player is the gut‑brain axis. Stress hormones released during competition (adrenaline, cortisol) can alter gut permeability and pH, while the microbiome may react to sudden changes in substrate (e.g., gels, sports drinks). Ricardo Da Costa’s team notes that these neuro‑hormonal signals can amplify the sensation of urgency, even when the actual volume of stool is modest.
The trade‑off
In practice this means that the very strategies athletes use to fuel performance – high‑carb gels, frequent sipping of isotonic drinks – can become double‑edged swords. The gut is forced to process a large carbohydrate load while its blood supply is compromised, creating an environment where fluid shifts into the lumen and diarrhoea can erupt.
Options for protecting the gut – what works and what doesn’t?
| Strategy | What it does | Evidence strength | Practical considerations |
|---|---|---|---|
| Pre‑race dietary familiarisation – trial all foods, gels and drinks in training | Reduces surprise intolerances, allows gut adaptation | Moderate – multiple case‑series show fewer symptoms when athletes stick to tried foods | Requires at least 2‑3 weeks of planned nutrition trials |
| Gradual carbohydrate loading during training – practice drinking 30‑60 g · h⁻¹ in long runs | Trains the gut to absorb carbs while blood flow is limited | Moderate – Hawley reports improved tolerance in trained athletes | Must monitor for early signs of distress; increase load by ~5 g · h⁻¹ each week |
| Hydration strategy focused on electrolyte balance – use sodium‑rich drinks, avoid over‑drinking | Maintains plasma volume, limits osmotic pull of water into the gut | Moderate – dehydration is a known aggravator of GI upset | |
| Low‑fiber or gluten‑free trial runs – reduce bulk and fermentable substrates | May lower gas production and rapid transit | Low – evidence is mixed; some athletes report benefit, others see no change | |
| Heat‑management tactics – dress for the weather, use cooling vests, start early | Preserves blood flow to gut by limiting skin‑cooling demand | Strong – heat is a clear exacerbator of symptoms | |
| Probiotic or glutamine supplementation | Theoretically supports gut barrier function | Low – promising in laboratory settings but limited real‑world data |
The evidence suggests that the most reliable approach is a personalised, training‑based nutrition plan. "Treat your gut like any other performance system and train it accordingly," Henningsen advises.
Who should be most concerned?
Women appear to report GI distress slightly more often than men, possibly because hormonal fluctuations affect gut motility. However, high‑profile male athletes such as Davis Clarke and Yohann Diniz have also suffered publicly, showing that sex alone is not a protective factor.
Beginners entering their first marathon are at particular risk. Their training volume is often insufficient to develop gut tolerance, and they may experiment with new energy products on race day. Conversely, seasoned ultra‑endurance athletes who have systematically practiced nutrition in training tend to experience fewer and milder episodes.
From a health‑service perspective, the NHS currently treats most exercise‑related GI complaints as self‑limiting. Persistent or severe symptoms, however, merit a referral to a gastroenterology clinic – especially if they interfere with training or cause weight loss.
What the research still can’t answer
The Guardian article notes that the precise trigger for any individual episode is often unclear. While blood‑flow redistribution, mechanical jostling, heat, and food intolerance are all plausible, the relative contribution of each factor likely varies day‑to‑day.
Two gaps stand out:
- Microbiome influence – Studies have linked gut‑brain signalling to performance, but we lack robust trials showing that altering the microbiome (through probiotics or diet) reliably reduces GI distress in athletes.
- Long‑term health impact – Most reports focus on acute symptoms. There is little data on whether repeated bouts of runner’s diarrhoea affect gut integrity or nutrient status over months or years.
Until these questions are answered, the safest stance is to treat frequent or worsening symptoms as a sign to seek professional advice, rather than accepting them as an inevitable part of sport.
A simple plan for this week
- Log your last long run – Write down everything you ate and drank in the 24 hours before, the fluids taken during the run, the temperature, and any GI symptoms.
- Pick one food or gel you used and replace it with a version you have tolerated before (or omit it entirely) for your next training session.
- Add a pinch of salt to your water bottle if you normally use plain water, aiming for about 300‑500 mg of sodium per litre – this can help maintain plasma volume without over‑drinking.
- Dress for the weather – if the forecast is warm, choose lightweight, breathable clothing and consider a cooling headband.
- Schedule a 10‑minute cool‑down walk after your next long run. Gentle movement encourages the gut to return to normal motility without a sudden drop in blood pressure.
By turning the gut into a deliberate part of your training plan, you can dramatically reduce the odds of an unexpected sprint to the toilet – and keep the focus on the finish line.
