With thousands of runners preparing for the 2026 London Marathon on 26 April, a Harvard gastroenterologist has revealed why so many participants desperately need the toilet around the 18-mile mark. Known as 'runner's trot', this sudden, urgent need for a bowel movement during or after a run is common among athletes pushing through intense exercise like a marathon.
The Science Behind Runner's Trot
Dr Trisha Pasricha, a Harvard doctor and author of the bestselling book 'You've Been Pooping All Wrong', explained the biological mechanism causing this phenomenon. In a social media post, she asked: "Why do so many marathoners suddenly need to use the bathroom in the middle of the race?" Her accompanying video detailed that when running, blood flow to the gut can drop to 20% of its baseline. "That is literally just your body choosing your legs over your intestines," she said.
This reduced blood flow explains why many runners struggle to keep food down at mile 18 and feel the urgent need to dash to the toilet. "Your gut is screaming that it is not getting enough blood," Dr Pasricha added.
How to Predict and Prevent Symptoms
Dr Pasricha noted that symptoms often appear when a runner's heart rate reaches a specific level, which can be tracked using a smartwatch or fitness device. She recommended: "When you're training, pull up that heart rate data and see how high it goes before you start to get symptoms." This information can help adjust training goals to stay below that critical threshold.
She also advised maintaining proper hydration while avoiding sugary drinks, suggesting electrolytes to enhance fluid intake. "Honestly, I'd probably skip the coffee the morning of the race too," she said, as coffee can 'rev up your colon'. Keeping the digestive system settled before and during the event is crucial, especially for those who already find coffee triggers a need to use the toilet. "If you know you know," she added.
Runners React
Many runners appreciated the explanation in the comments. One said: "Ah, thank you. Another reason half marathons seem like a better option." Another wrote: "Thanks for this information!"
