About a third of people who eat coffee feel they have to poop afterwards. There was even clinical research at Royal Hampshire Hospital in 1990. The urge was demonstrable in some of the subjects: increased activity in the intestines were observed within minutes of drinking a cup of coffee. It used to be thought to be the Caffeine found in coffee, but it didn't turn out to be. It only became clear that a particular substance in coffee stimulates the production of the hormone gastrin, which in turn promotes bowel movement. It is also the case that you often drink your first cup of coffee the moment you just got up.

Why do many people have to poop after drinking coffee?