In Too Afraid to Ask, we’re answering food-related questions that may or may not give you goosebumps. Today: Why does coffee make you poop?
For some people, coffee’s siren song is what coaxes them out of bed. For others, that morning brew gets things going on a different front: It means an urgent need to poop is on the way. But why? We launched a very serious investigation to find out.
While a cup of joe impacts drinkers in vastly different ways, there’s plenty of research indicating that the need to go (like, right now) is a common side effect of America’s favorite daily habit. A 2018 review of various clinical trials found that coffee helped, er, relieve patients who had undergone constipation-causing abdominal surgeries, says Desiree Nielsen, RD, a plant-based dietitian and multiple cookbook author based in Vancouver, Canada.
But experts aren’t exactly sure what’s behind this morning combo deal. “It’s one of the many parts of normal digestive function that remains a mystery,” says Kyle Staller, MD, MPH, a gastroenterologist at Massachusetts General Hospital and an assistant professor of medicine at Harvard Medical School. Coffee’s effect on digestion is so well-known, he explains, that scientists probably don’t feel pressed to study the phenomenon.
Pinpointing why coffee might make you poop is a surprisingly hefty logistical challenge too. “Coffee alone contains over a thousand different active compounds,” says Nielsen. And as we know from coffee and sleep—some people are wired after a few sips while others can nod off after a late night espresso—humans react differently to chemicals. Finding the culprit (or culprits) at play here is a complex whodunnit. Here’s what we do know.
Why does coffee make you poop?
It’s not clear why coffee makes some of us need to leg it to the toilet, or how much most people need to drink in order to feel the digestive effect. But experts are pretty sure the urge to go is mediated by the brain. Considering how quickly some people poop after coffee (within as little as four minutes), it’s likely that the reaction is happening well before coffee hits the colon (basically, the body’s departure lounge).
Your morning joe sloshing against the stomach lining—which, in this airport analogy, you can think of as the security check before your poop heads to the departure lounge—seems to “stimulate the production of two hormones, gastrin and cholecystokinin.” Both trigger bowel movements “via a gut-brain connection called the gastrocolic reflex,” says Nielsen. These muscular contractions pass stool from one end of the colon to the rectum, says Staller. Or, in other words: Flight attendants, please prepare for takeoff.
Does the type of coffee you drink make a difference?
The data is even more sparse here, but if coffee generally makes you want to poop, chances are most variations will have a similar effect.
Caffeinated vs. decaf coffee
Both varieties seem to encourage bowel movements (despite what its name implies, decaf coffee actually still contains some caffeine), though one 2022 study “suggested that regular coffee gives more of a kick to the colon than decaf,” says Staller. Older research also found that caffeinated coffee seemed to trigger more gastrin release than decaf. These studies indicate that caffeine might be a culprit, but like a tween shoplifting watermelon lip balm from CVS, it likely isn’t acting alone.
Hot vs. iced coffee
Although there’s no data comparing the effects of hot coffee to iced coffee on bowel movements, “we do know that it is not temperature alone that is the secret to coffee’s bowel-stimulating ways,” says Staller. One paper published in 2020, for instance, found that warm coffee helped relieve constipation in patients who had undergone laparoscopic gynecological surgery faster than hot water. And a 2014 study on rats found that colon activity was slower when they were given cold water versus room temperature water.
Black vs. coffee with milk
For some coffee fans, your creamer of choice might also be worth considering as a catalyst behind that bathroom dash, says Vanessa Rissetto, MS, RDN, a registered dietitian and the cofounder and CEO of Culina Health. Some people add cow’s milk to their coffee, “which they may [unknowingly] be intolerant to,” causing gas, bloating, diarrhea, and “a strong urge to go.”
Is coffee the only reason you might need to poop in the morning?
There are plenty of factors that might explain your dashes to the bathroom: For one, colonic activity naturally spikes after we wake up, says Nielsen. “So for some people, the connection between coffee and bowel movements might just be that we do both first thing in the morning.”