It’s one of those ironies of life: You are doing something healthy and you still get sick. In this case, the step toward better health is endurance running. And the ailment is diarrhea, occasionally a side effect of what’s happening to your digestive system as you log the miles. Luckily, though, you can usually recover within a day or two.
If you’re wondering how common it is, though, browse the blogs and online forums for active people and habitual runners and you’ll see that “runner’s diarrhea” is not taken lightly -- even for those humorously referring to it as “runner’s diarrhea.” In fact, those engaged in the conversation are seeking answers to a problem that can plague them while training, as well as during and after a big race.
https://www.empowher.com/diarrhea/content/runners-diarrhea-can-occur-athletes