Why does coffee make you poop?

You know it when you feel it. You've just sat down at your breakfast table, or settled in at your favorite café. You're a few sips into your brew when, out of nowhere, the urge to download a brownload is becoming urgent. Just a few swigs of coffee and it can feel like you've mainlined a laxative — but why?

The short, somewhat boring answer is that coffee stimulates muscle contractions along the last couple of stops on the #2 Metro that is your large intestine (a terminal stretch of bowel that science calls the rectosigmoid colon). Way more interesting are the details behind the study that helped bring this quivering canal to light, not to mention questions that have been raised about the mechanisms linking coffee to its contractions.

For these, we turn to "Effect of coffee on distal colon function" — a research paper with an encouragingly no-nonsense title published in the April 1990 issue of Gut, a scientific journal dedicated to the digestive system. The paper recounts the findings of a study in two parts: a relatively painless questionnaire portion, and a second, somewhat more inquisitorial section involving anal probes.

The Questionnaire

To those of you reading along in confusion, wondering what the hell we're talking about and thinking to yourself "coffee's never made me have to poop — is there something wrong with me?": jealous though you may be of this borderline superpower, you're actually in the majority, here. When polled, roughly two-thirds of the healthy young adults who participated in the study's questionnaire portion claimed to be unmoved by coffee's colonic clarion call. In fact, of the 58 men and 34 women surveyed, just 29% claimed that coffee brought their bowels to rapt attention within 20 minutes of first sup. Writes the study's first author, S.R. Brown1:

These [29%] included 53% of the women and 19% of men. The response to coffee in the responders varied in intensity and frequency. Fifty-two percent claimed the response occurred only in the morning and 58% reported that the effect occurred only if there had been no previous defecation that day.

In other words, it's a little more complex than a cup of joe makes you go. Time of day, diurnal rhythms, and even gender are liable to play a role in coffee sensitivity. The researchers speculate, for example, that the higher proportion of affected females may reflect a natural variation in bowel reactivity between men and women.

Such gender disparities have been well-documented in GI-research. Past studies have shown that the rectum is more sensitive to distension in women than it is in men. Intestinal troubles like irritable bowel syndrome are more common in women, as well. And while fecal incontinence tends to affect women more often than men (thanks alot, childbirth), the same can be said for constipation. Just a few things to keep in mind the next time you're tempted to roll your eyes at a woman who claims to have a stomach ache.

But enough about past research — we both know you're here to read about the anal probes.

The Anal Probe

Even excluding those apparatuses intended for recreational purposes, anal probes come in a variety of shapes and sizes and perform a wide range of functions.

Why does coffee make you poop?

During a colonoscopy, for instance, a physician will commonly equip herself with a flexible endoscope. When measuring the effects of coffee on the rectum and colon (an anatomical coupling known collectively as the "rectosigmoid"), Brown et al. reached instead for what is known as a multilumen manometric probe, the insertion of which was aided by a rigid sigmoidoscope2.

The multilumen manometric probe's prime directive? Measure the effect of black, unsweetened coffee on muscle contractions inside the rectosigmoid. When these muscles constrict, it registers on the probe as an increase in pressure. Changes in pressure are used to calculate a "motility index" that reflects the intensity and duration of the contractions. To put it in simple terms: the probe measures whether the business end of your large intestine gets jittery after a cup of java. Generally speaking, an increase in motility — i.e. an increase in the intensity and duration of rectosigmoid contractions — corresponds to an increased urge to void one's bowels.

Here's how the experiment went down: Brown and his colleagues probed a total of 14 subjects, six of whom claimed coffee inspired in them zero need to poop. All probings commenced at approximately 11:00 am. Beginning half an hour after initial insertion (gotta give the participants time to relax, after all), rectosigmoidal pressure was monitored for thirty minutes to determine baseline motor activity. Subjects then quickly drank 200 ml of 45°C coffee, decaffeinated coffee, or water. Rectosigmoidal pressure was then monitored for an additional 60 minutes.

As expected, the six subjects who claimed to be unaffected by coffee's colonic caress registered no increase in rectosigmoid activity after any of the drinks tested. But the eight subjects who didclaim that coffee induced defecation reacted very differently. These two figures illustrate coffee's physiological effect on those eight subjects (click to enlarge):

Why does coffee make you poop?

Figure 1, above and on the left, shows that coffee triggered an increase in rectosigmoid motor activity, and quickly. Within four minutes of ingesting regular coffee, the intensity and duration of pressure in the rectosigmoid colon increased rapidly, and remained higher than the basal period for at least thirty minutes. Interestingly, a similar increase was seen following consumption of decaffeinated coffee. No increase in colonic activity was seen after the drink of hot water. Figure 2 illustrates these spikes in motor activity by comparing the average motility index for each thirty minute period before and after drinks of coffee, decaf coffee and hot water. The increase in rectosigmoid activity brought on by coffee — caffeinated or otherwise — is obvious, as is water's failure to elicit a response.

The Questions

There are many. Yes, as the researchers write, "drinking coffee can stimulate a motor response of the distal colon in some normal people"; but what, exactly, is doing the stimulating?

Why does coffee make you poop?

Reason and experience would suggest that caffeine is responsible, but the results demonstrate that even decaffeinated coffee can have a significant effect on one's bowels. The results also clearly indicate that the response is triggered by neither temperature nor volume, as hot water registered no effect on rectosigmoid motility. [Photo Credit: zoonabar via flickr cc]

That the response occurs within four minutes of drinking the coffee is particularly interesting. "It seems unlikely," note Brown and his colleagues, "that coffee would reach the colon in this time either via [the intestines] or the blood stream," which suggests an indirection action on the colon. To this end, the researchers note that coffee may induce a "gastrocolonic response" by acting on receptors in the stomach or small intestine:

Coffee has been shown to promote release of Gastrin, which can increase colonic spike and motoractivity. Cholecystokinin is another putative mediator of the "gastrocolonic response," but there are no data on the effect of coffee on [its] release [followup studies have since demonstrated that both regular coffee and decaffeinated coffee stimulate cholecystokinin's release]. Another posibility is that the colonic response to coffee could be mediated by exorphines. Both regular and decaffeinated coffee contain exorphines that can bind to opiate receptors. Opiate receptors, both in the brain and within the gut wall, mediate important effects on colonic motility.

In other words: the gastrocolonic response induced by coffee is probably mediated by a bunch of different hormonal and neural mechanisms, many of which remain to be elucidated.

Studies that have been performed since this one have yielded conflicting results; some indicate that coffee has one effect on sphincter mechanisms at the lower end of the esophagus, others indicate the exact opposite. One study will suggest that coffee slows the rate at which food passes through your intestines, while another will indicate it has no such effect. Through it all, though, the effects of coffee on the furthest reaches of your bowels, so clearly laid out by Brown and his colleagues, have stood the test of time, and have even been found to be "comparable to those of a 1000 kCal meal."

You know, when you think about it, the schizophrenic nature of the results gleaned from studies on coffee's impact on gastrointestinal function are pretty much par for the course when it comes to this beverage. Scientists are perpetually trying to make sense of the various effects that this mysterious brew has on our bodies, often with conflicting results — a fact that has led some to consider coffee both the best and worst beverage ever invented.

1. I shit you not; the first author's name is Brown. The second author's name? Cann. You can't make this stuff up, people.
2. See also: the anoscope, proctoscope, rectoscope, etc. When we said "a variety of shapes and sizes," we damn well meant it.

Top image via Shutterstock; Figures via Brown et al.
This io9 Flashback originally appeared in January 2013