Scientists Have a New Explanation for Why Coffee Makes You Poop

Good coffee—even decaf, according to one baffling study—can really send you racing to the bathroom. Listeners to Marc Maron’s WTF podcast were long forced to think about this bowel-shaking phenomenon over years of the comedian’s scatological ad reads, but modern medical science has yet to really land on a settled explanation for why coffee makes so many people need to drop a deuce. One 2025 medical paper described it, I think hyperbolically, as “the subject of ongoing debate worldwide.” Microbiologists and nutrition researchers at the University of Copenhagen have now taken this question directly to the source, asking 50 Danish adults to swallow a wireless research capsule, called the “SmartPill,” to investigate what coffee is doing inside their bowels. The team’s tiny gastrointestinal explorers found that regular coffee drinkers typically experienced “increased contractions in the colon,” as the new study, published in Gut Microbiology, put it. This added agitation, they theorize, might just be what makes coffee drinkers so ready to poop. Something else also happened to the more serious coffee achievers: They displayed higher acidity in their small intestine and pH levels lower than more infrequent coffee drinkers. “But perhaps more importantly, frequent coffee consumption appears to affect the physiology of the gut itself, in other words how the gut functions,” the study’s first author, Francesca Bucci, a PhD student focused on nutrition at the University of Copenhagen, said in a statement. Gastronomic tourism Along each of their individual fantastic voyages, the researchers’ swallowable probes took temperature measurements, recorded pH levels, and tracked pressure changes in the form of gastrointestinal contractions. These devices also logged the total time it took them to pass through the entire gastrointestinal tract. Bucci and her colleagues also analyzed bacterial content from the fecal matter of their test subjects. Participants were divided into “frequent” coffee drinkers, who drank between one and five cups on at least three of the eight total days of the experiment, and “rare” coffee drinkers, who fell below that threshold. An “observed richness” in bacterial biodiversity was seen in stool samples from the frequent coffee drinkers—even though “faecal bacterial load did not differ between the two groups,” the researchers wrote. The operating theory, according to Bucci and her coauthors, is that this bacterial biodiversity is evidence of the “mixing and thus greater interactions of foods and microbes” you might expect inside a colon churned by these increased coffee-induced contractions. Coffee-inspired medical interventions The team, which also included scientists in Belgium and Singapore, cautioned that more studies would be needed to sort out how much factors like the number of colon contractions, bacterial richness, and pH might contribute to what you’re free to call “the coffee poop effect.” “It is important to note that this study was originally not intended to investigate the effects of coffee consumption,” Bucci and her coauthors underscored. But, that said, study coauthor Henrik Munch Roager, the head of nutrition, microbiome, and metabolomics research at the University of Copenhagen, said he hopes that the suggestive digestive biochemistry identified here might soon inspire real solutions for real people in real gastrointestinal distress. “A great many people struggle with 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 that knowledge to stimulate gut motility in a natural way.”