Coffee gets us moving in more ways than one. Not only does it wake up our brains, it also - ahem - wakes up our bowels. About a third of all coffee drinkers have experienced the consequences, and yet no one really knows why.

Researchers at the University of Texas are determined to crack the mystery, and the first hypothesis scratched off their list is one of the most commonly cited. Whether caffeinated or not, they argue, a morning cup of joe will still get your bowels moving by stimulating the muscles in your gut.

"When rats were treated with coffee for three days, the ability of the muscles in the small intestine to contract appeared to increase," says lead author of the study, gastroenterologist Xuan-Zheng Shi.

"Interestingly, these effects are caffeine-independent, because caffeine-free coffee had similar effects as regular coffee."

It's important to note that the preliminary results, presented this week, are based on animal models. However, they do build on other studies which suggest decaf coffee also has a laxative effect in humans.

Not only did the team observe these results in live rats, they were also clear in the lab when muscle tissue from the gut was directly exposed to coffee. In both cases, the black concoction caused a burst in the small intestines and colon, pushing the contents of the gut along faster.

This last point isn't exactly novel, either. In the 1990s, several studies found that coffee could induce movement in the distal colon for some coffee drinkers.

Unlike this past research, however, the new paper follows the process even further. In the poop of the rats that weren't given any coffee, the researchers found more bacteria, and the same thing was found when replicated in a petri dish. When the rat poo was treated with coffee, both caffeinated and decaf, the growth of microbes was suppressed.

"That's really interesting, because that means coffee could be an antibacterial agent, and we could see this again with decaffeinated coffee," Shi told Gizmodo.

"But that we need to study more - why coffee could have this suppressing effect on the microbiome."

Plus, the authors also want to find out whether all of these changes are ultimately positive or negative for gut health - given what we know about the delicate balance of intestinal microbiome, at this stage the effects of coffee are anyone's guess.

The preliminary results were presented at Digestive Disease Week.