Many runners are preparing for the London Marathon on 26 April – but one Harvard doctor explained the reason so many marathoners experience urgent need to use the toilet around the 18-mile mark
With thousands of runners gearing up for the 2026 London Marathon on 26 April, one gastroenterologist shed light on why so many participants find themselves desperately searching for the nearest toilet around mile 18 of the race.
Whether you’re a seasoned athlete or a newcomer to running, there’s every chance you’ve encountered what’s known as ‘runner’s trot’. This is the sudden, urgent need to rush to the nearest bathroom for a bowel movement during or after a run. Though it may seem odd to those unfamiliar with the term, it’s an incredibly common occurrence among those pushing themselves through intense exercise – such as running a marathon.
Harvard doctor and scientist Dr Trisha Pasricha, who also wrote the bestselling book ‘You’ve Been Pooping All Wrong’, offered a detailed explanation of what’s actually happening inside your gut while running, ultimately causing runners to make a desperate beeline for the nearest toilet at around mile 18.
“Why do so many marathoners suddenly need to use the bathroom in the middle of the race?” she posed in the caption of her post, before elaborating further in her accompanying video.
“When you run, blood flow to your gut can drop to 20% of its baseline,” she explained. “That is literally just your body choosing your legs over your intestines.”
She went on to clarify this explains why numerous runners struggle to keep food down when they reach mile 18. Athletes frequently begin feeling unwell and need to dash to the toilet as a result.
“Your gut is screaming that it is not getting enough blood,” Dr Pasricha said.
However, there are methods to anticipate this, particularly if you’re using a smartwatch or similar fitness device. This is because the warning signs typically appear when a runner’s heart rate reaches a specific level.
“So when you’re training, pull up that heart rate date and see ‘how high does it go before I start to get symptoms?'” she recommended. The information can then help adjust your training objectives to remain beneath that critical point.
Dr Pasricha also encouraged runners to maintain proper hydration, while avoiding sugary drinks. Instead, she suggested using electrolytes to enhance fluid intake.
“Honestly, I’d probably skip the coffee the morning of the race too,” she said, claiming it would ‘rev up your colon’, while emphasising the digestive system should remain as settled as possible before and throughout the event. This is especially important if you already find that a cup of coffee makes you need the toilet, with Dr Pasricha writing: “If you know you know.”
Numerous runners quickly flocked to the comments section under the clip to express their appreciation for the gut specialist’s explanation of the common issue.
“Ah, thank you. Another reason half marathons seem like a better option,” one person said. Someone else wrote: “Thanks for this information!”
