Why does running make me go to the bathroom?

I’m training for a half-marathon but have noticed an odd setback: Every time I run more than a few miles, I get diarrhoea! Why does this happen, and can I prevent it? My athletic patients often describe having to pause mid-run to answer an urgent call of nature . Compared with those in other endurance sports , runners seem to experience worse gastrointestinal symptoms – and younger female runners are at higher risk. Moderate to intense exercise has several effects on the gut , including a painful “stitch” in the side of the abdomen, nausea, diarrhoea and even bleeding, whether microscopically or in plain sight.

The latter happened to Yohann Diniz, a French racewalker, at the 2016 Rio Olympics. Diniz was the favourite to win the men’s 50-kilometre event. About halfway through the race, he began to appear ill.

Diniz then had a bowel accident and, even scarier, it seemed to be bloody. Diniz collapsed, but with some help, got back up and finished in eighth place. (Diniz’s story has a brighter sequel: He continued to compete afterward and remains a world record holder.

) The effect of exercise on the gut.