Quirky Questions: Why does coffee make me poop?

Last updated November 12, 2018.

If you notice that your daily coffee ritual is often accompanied by a timely bowel movement, you’re not alone. For some people this can be an inconvenience, but for others, coffee can be one way of keeping regular. Some coffee drinkers will readily feel this gastrointestinal effect, some less so.

Researchers believe that the bowel-stimulating quality of coffee comes from caffeine and/or other substances contained within the coffee brew. Although there have been no large-scale studies on this subject, what we do know is that drinking coffee can stimulate movement of the colonic muscles, thus promoting peristalsis (the coordinated contraction and relaxation of intestinal muscles that causes bowel movements). One study noted that the magnitude of this peristaltic effect of caffeinated coffee is similar to one induced by eating a meal. It’s also 60 percent stronger than the effect induced by drinking water, and 23 percent stronger than the effect due to drinking decaffeinated coffee.

Aside from promoting bowel movements, coffee can also cause looser stools because increased peristalsis leaves less time for the colon to perform one of its key functions–reabsorbing water from fecal matter to produce well-formed stools. Be aware, however, that other common accompaniments to coffee can be culprits in this matter. Dairy products, excess sugar, even “sugarless” sweeteners like sorbitol (a well-known substance used as a laxative) can cause diarrhea.

So how much coffee is needed to get results? The above-referenced study used a black Colombian coffee containing 150 milligrams (mg) of caffeine, a fairly average caffeine content for an 8-ounce (oz) cup of joe. A 12-ounce “tall” drip coffee from Starbucks contains about 240 mg of caffeine, and single-shot latte drinks approximately 90 mg. In the end, the effect will vary by the individual.

You might be wondering about the dehydrating effects of coffee: Shouldn’t drinking coffee cause constipation instead? While we’ve been told for years that coffee (and caffeine in general) is a potent diuretic, research studies show that coffee only seems to have diuretic effects when consumed in larger amounts — adding up to more than 500 to 600 mg of caffeine a day. The most recent studies on this subject show that there is, in fact, no significant difference between urine output among people who drink caffeinated drinks compared to those who drink water or other comparable non-caffeinated drinks. Therefore, the typical coffee drinker shouldn’t experience significant dehydration from a one- or two-cup habit.

So now that you’ve learned more than you may have cared to about your colonic function, perhaps it’s time to test out your newfound self-awareness with a cup of coffee. However, if you care for the mental stimulant effects of caffeine without the gastrointestinal stimulation, try a cup of tea instead. Studies show that tea, by itself, doesn’t seem share the same bowel movement-inducing quality of coffee.