Picture this: it's Monday morning at the office and you've got to start the day off right with a little caffeine, so you pour yourself a nice hot cup of coffee, settle in at your desk, and get to work. But soon your guts start to rumble, things start moving, and you gotta poop. The phenomenon is so well-known that it's even plastered on mugs. "Coffee makes me poop" Around three in ten people find themselves on their way to the bathroom after their morning cup of joe and a small study done in 1990 found that it wasn't just that the people felt the urge to go; within a few minutes of drinking coffee, people who needed to poop also had an increase in motor activity in their colons so their guts were physically responding to the drink. Which brings us to the question: 'Why?' Some scientists think that it might be not the coffee at all, it could just be that coffee is the first thing you put in your stomach in the morning and as your stomach stretches, it triggers the gastrocolic reflex.
Basically, your guts wake up and start things moving. On the other hand, drinking a glass of water when you wake up doesn't necessarily send you to the bathroom, so there's probably more to it than that. Strangely enough, there's never been a large scale study investigating the mechanism of the post-coffee poop. But researchers have done smaller studies to test different possibilities. Caffeine could be part of what's causing the effect.
Since caffeine is a stimulant, it could be stimulating some things at the other end, as well. Except that studies have found that even decaf coffee can make you have to go. Which means it can't just be the caffeine's fault. Coffee is pretty acidic and acid in the stomach might be another part of the problem. When there's a lot of acid in your stomach, you produce more bile, Which can, in turn, cause diarrhea.
But that takes a while and the urge to use the restroom happens just a few minutes after drinking coffee. Research from the 1980s showed that drinking regular coffee and decaf coffee increases the release of a hormone called gastrin, which helps the stomach produce acid and encourages movement in the intestines But gastrin wasn't released when the subjects drank hot water or a stimulated coffee beverage that contained caffeine and sugar. So there is something about coffee specifically that leads to the production of this hormone, encouraging your guts to move things along a little faster than normal and interrupting morning meetings everywhere. We just don't know what that is. Thanks for asking and thanks especially to all of our patrons on Patreon who keep these answers coming. If you'd like to submit some questions to be answered, just go to patreon.com/scishow and don't forget to go to youtube.com/scishow and subscribe. You're reading a book in bed after a long day, you finish the chapter and go to close the book and: "Ow, paper cut!" Paper cuts are usually small and shallow, but they do tend to hurt a lot.
