Does Coffee Make Us Poop? Scientists Who Studied Rats Find Drink Affects Gut Bacteria and Muscles

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A cup of coffee is displayed. Scientists at the University of Texas Medical Branch, Galveston, carried out research by feeding rats coffee for three days. Getty Images

It's a question more than a handful of people have likely sheepishly typed into Google after their morning cup: Why does coffee make you poop? And after studying rats, scientists think it's because the drink affects the digestive muscles and gut bacteria.

The research questions the myth that it's the caffeine in coffee which kickstarts the bowels.

Scientists at the University of Texas Medical Branch, Galveston, carried out their research by feeding rats coffee for three days. The team took a number of approaches to see how the drink affected the rats: including by analyzing their feces after the three-day period; dosing gut bacteria from the rodents' fecal matter with coffee in a petri dish; and looking for any changes to the animals' intestines and colon.

Coffee seemed to help the lower intestines and colons of the rats to contract, they found. A similar result was found when the team treated small intestine and colon tissue with coffee.

The scientists also showed that when they mixed fecal matter with coffee, fewer microbes grew. And the effect was more pronounced when the dose was stronger. These changes also happened when decaffeinated coffee was used.

After three days, the rats also had less bacteria in their feces. Future studies should look at whether firmicutes, commonly known as a "good bacteria," are affected by coffee, or enterobacteria, the family of bacteria which can cause diseases such as gastroenteritis and meningitis.

The findings were presented on Sunday at Digestive Disease Week, a meeting of doctors, researchers and academics in fields including gastroenterology. The research has therefore not been peer-reviewed. And while studying rodents provides a useful insight into how substances like coffee can affect the body, the results might not translate in humans.

The study's lead author, Xuan-Zheng Shi, a PhD associate professor in internal medicine at the University of Texas Medical Branch, Galveston, told Newsweek: "When rats were treated with coffee for three days, the small intestine contractility was increased to compare with untreated controls.

"It [the study] indicates that coffee's impacts on digestion are largely not dependent on caffeine. It is also reasonable to believe that coffee and decaffeinated coffee may have similar anti-bacteria effects on human microbiota."

He explained there are many different ingredients in coffee and it's not clear which component or components made the rats need to relieve themselves, but the team hopes to find out in future studies.

"This is important, as if we know of the responsible component(s), we can use the information for medical or therapeutic benefits," Shi said. "While we know now that coffee may have general antibacterial effects on gut bacteria, it would be great to study more on the effects of coffee on individual bacteria species and on the diversity of the bacteria population in the gut."

The research mirrors the findings of a 1990 study published in the journal Gut which found that in a small group of 92 participants, coffee made 29 percent want to use the restroom. They concluded that coffee can trigger a response in the colon.

The scientists found coffee stimulated the colon in the participants, who were aged between 17 to 27, regardless of whether it was regular or decaf.

This article has been updated with comment from Xuan-Zheng Shi​.