Mind & Body

Why Does Coffee Make You Poop?

For many people, it's a routine as regular as their morning coffee: a half-hour or so after that first cup of joe, nature calls. What is it about coffee that jump-starts the plumbing? Scientists are still trying to figure this out, but they know one thing: The caffeine is almost certainly not to blame.

Brew 'n' Poo

If all it took to make your bowels bellow was caffeine, we'd all be heading to the restroom after a few cups of tea, cans of cola, or bars of chocolate. Yes, caffeine does have an effect on your digestive tract, but it's not as much as you'd think.

Research has found that regular and decaf coffee have a remarkably similar effect on the colon, for instance. One study found that regular coffee was only 23 percent more effective than decaf at moving things along, whereas it was 60 percent more effective than water and about as effective as a full meal. And in a 2008 study where 10 participants got a solution of pure caffeine to drink, the researchers found that while it did make the participants more sensitive to the sensation of needing to poop, it also increased how hard they could squeeze to hold it in.

Full Steam Ahead

So what exactly does coffee do to speed things up downstairs? Its biggest effects seem to be on involuntary muscle contractions in the intestines. Back in 1998, researchers found that regular and decaf coffee triggered contractions in the colon, especially in the higher regions closer to the small intestine. Coffee also seems to accelerate "gastric emptying," the technical term for food leaving the stomach and heading for the small intestine.

The most recent data on this question comes from researchers at the University of Texas. In a study presented last month at Digestive Disease Week 2019, a team led by Xuan-Zheng Shi, Ph.D. studied the effects of coffee both in vitro and in vivo — that is, in a petri dish and in living animals (rats).

They exposed fecal matter to a few different concentrations of coffee — both regular and decaf — over three days, then measured the growth of the gut bacteria within the poop. Both the regular and decaf coffee significantly reduced microbial growth overall. The same was true after feeding rats coffee for the same period, though the evidence suggested that coffee might reduce so-called "bad" bacteria slightly more than "good" bacteria. That might help with digestion overall, although the team warns that more research is needed on that front.

The researchers also found that feeding mice coffee boosted contractions in the muscles of the lower intestine and colon regardless of caffeine content, and it did so even more at higher concentrations. The same was true when they exposed muscle cells to coffee directly in the lab.

There's still research to be done on exactly what compounds in coffee cause it to jump-start colon contractions, but one thing's clear: It's not just you. Coffee clearly helps to hasten nature's call. Let's just take a moment to be thankful for coffee-shop restrooms.

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If you'd like to learn about coffee without all the bodily functions, check out "Uncommon Grounds: The History of Coffee and How It Transformed Our World" by Mark Pendergrast. We handpick reading recommendations we think you may like. If you choose to make a purchase, Curiosity will get a share of the sale.

Written by Ashley Hamer June 17, 2019

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