You wake up in the morning. You pour some of that sweet delicious hot nectar to get you going through the day but it’s not long before you feel something in your belly. For three in ten people, coffee brings much more than just a morning buzz – it brings a bowel movement. But why?

The morning coffee seems to be often followed by this view. Photo by Elya.

Before we start drawing any conclusions, it has to be said that the science isn’t settled on this. I’ve found no conclusive study about why coffee makes you go to the toilet, but there are a few smaller studies and other hints. So we’re a bit short on the why but we’re good on the how.

It’s probably not caffeine

At first, you’d probably be tempted to blame it on caffeine. After all, it’s the main active ingredient in coffee and it’s what makes coffee… coffee. But that’s almost certainly not the case. But if you think about it, other things have caffeine as well. Some sodas have it, as do some energy drinks and they don’t send you to the loo.

Photo by Blake Richard Verdoorn / Unsplash.

Furthermore, a study has found that decaf has a similar effect. Basically, they found that a cup of hot, caffeinated coffee stimulates the bowel on average by as much as a 1,000-calorie meal. The effect was 60% stronger than hot water and 23% more than decaffeinated coffee. So decaf had an impact, but not as strong.

The most referenced study on coffee and bowel movements recruited 99 healthy volunteers and found that 28 of them reported a coffee-induced urge to defecate. Now, this isn’t the largest sample size but it’s quite significant. The study also notes the time after which the urge is felt: 4 minutes. This indicates an indirect action because four minutes is not enough for the coffee to reach the colon through the intestines or through the bloodstream.

So then… what is it?

The study promotes two mechanisms. The first one proposes that acidity is the culprit. Coffee contains a substance called gastrin, a peptide hormone that stimulates secretion of gastric acid. Gastric acid sends a signal to the receptors on the epithelial receptors in the stomach, which then dumps its contents faster.

This inconspicuous does it all – and we still don’t know exactly how. Photo by Natalie Collins / Unsplash.

Alternatively, the cause could be exorphines, a group of opioid peptides. Both regular and decaffeinated coffee contain exorphines that can bind to opiate receptors and again, stimulate the bowel, but we don’t know which of the many such substances could be responsible.

Another study investigating coffee’s connection to ulcers found that strong coffee and hot water both have a significant effect on bowel movement, though that of coffee is much stronger. However, in time, coffee drinkers can become immune to this. A 2003 study published in the Journal of Human Nutrition and Dietetics found that individuals who consume coffee regularly often develop a tolerance for these effects.

How the magic happens

The process through which it does this is called peristalsis. Peristalsis is is the formal scientific term for ‘muscle contractions which go like a wave in a tube.’ So when you’re drinking coffee, if you’re among the lucky 30%, your distal colon (the last part of the colon) is stimulated and muscle contractions go down it like a wave, forcing you to poop.

A time-space diagram of a peristaltic wave traveling down the esophagus during a water swallow. Data recorded at Department of Gastro-Enterology, University Hospital of Zurich, Switzerland.

Peristaltic waves are vital for our digestive function. In the human gastrointestinal tract, smooth muscle tissue contracts in sequence to produce a peristaltic wave, which propels a ball of food along the tract. But sometimes, your body can be tricked and can create a peristaltic wave without having a ball of food to send forth.

Coffee’s effect is also likely exacerbated by the gastrocolic reflex. The gastrocolic reflex means that when your gastrointestinal tract is waking up after being inactive all night, it sends the colon a message to make some room.

Conclusions

So coffee doesn’t make everyone poop – just around 30% of all people (based on one study). If you’re among the lucky ones, then coffee sends wave-like muscle contractions down your colon, either due to an increase in acidity, a secretion of exorphines, both, or something else we’re missing completely. Some people develop a tolerance to these effects.

I would really love a bigger study would cover this. After all, coffee is one of the most valuable commodities in the world, second only to oil and millions enjoy it every morning. It would be nice to know exactly why it makes us poop, doesn’t it?