Good coffee, even decaffeinated, according to a disconcerting study, can really send you to the toilet. Listeners to Marc Maron’s WTF podcast have long been forced to think about this gut-trembling phenomenon over years of reading the comedian’s scatological ads, but modern medical science has yet to really come up with an established explanation as to why coffee causes so many people to drop two. A medical article from 2025 described it, I think hyperbolically, as “the subject of an ongoing debate around the world.”
Microbiologists and nutrition researchers at the University of Copenhagen have now taken that question straight to the source, asking 50 Danish adults to swallow a wireless research capsule, called “SmartPill”, to study the effects of coffee in their intestines. The team’s little gastrointestinal explorers discovered that regular coffee drinkers generally had “increased contractions in the colon,” as the new study published in Gut Microbiology puts it. That extra agitation, they theorize, might just be what makes coffee drinkers so ready to poop.
Something else also happened to the more serious coffee drinkers: They had higher acidity in their small intestines and lower pH levels than more casual coffee drinkers.
“But perhaps more importantly, frequent coffee consumption appears to affect the physiology of the gut itself, in other words how it functions,” said the study’s first author, Francesca Bucci, a doctoral student specializing in nutrition at the University of Copenhagen, in a statement.
Gastronomic tourism
During each of their fantastical journeys, the researchers’ swallowable probes took temperature measurements, recorded pH levels, and tracked pressure changes in the form of gastrointestinal contractions. These devices also recorded the total time it took them to travel through the entire gastrointestinal tract.
Bucci and his colleagues also analyzed the bacterial content of their test subjects’ feces. Participants were divided into “frequent” coffee drinkers, who drank between one and five cups on at least three of the eight days of the experiment, and “rare” coffee drinkers, who fell below this threshold. An “observed richness” in bacterial biodiversity was found in stool samples from frequent coffee drinkers, even though “fecal bacterial load did not differ between the two groups,” the researchers wrote.
The working theory, according to Bucci and coauthors, is that this bacterial biodiversity is evidence of the “mixing and thus greater interactions of foods and microbes” that can be expected inside a colon agitated by these increased coffee-induced contractions.
Coffee-Inspired Medical Interventions
The team, which also included scientists from Belgium and Singapore, cautioned that more studies would be needed to determine the extent to which factors such as the number of colon contractions, bacterial richness and pH might contribute to what you are free to call the “coffee poop effect.”
“It is important to note that this study was not initially intended to investigate the effects of coffee consumption,” Bucci and co-authors emphasized. But that said, study co-author Henrik Munch Roager, head of nutrition, microbiome and metabolomics research at the University of Copenhagen, said he hopes the suggestive digestive biochemistry identified here could soon inspire real solutions for real people in real gastrointestinal distress.
“A large number of people experience constipation and irregular bowel movements,” Roager noted in a statement. “If we can understand how coffee affects digestion and intestinal contractions, it may be possible to use this knowledge to stimulate intestinal motility naturally.”
Gn Health
