Why do marathon runners get the runs?

This article was originally published on November 2, 2017.

In 2014, a woman visited a California doctor’s office complaining of a year and a half of watery diarrhea. She seemed healthy—she hadn’t lost weight and was in excellent shape. In fact, she had started running marathons two years prior, and typically ran about 20 miles every weekend. She also mentioned that she had noticed a correlation between her long runs and the uncomfortable bowel movements, which seemed to become less formed and more frequent as her intense training months dragged on. Her doctors advised…