If you're anything like me, you can't start your day without a cup of coffee. However, if you're anything like me, about 20 minutes after that first cup has been drained, you can be found sitting on a porcelain throne, taking your first poop of the day.
Coffee makes you poop. That's a fact, right? It must be, since you can buy t-shirts or mugs with that saying, and mugs never lie. But is it actually the coffee that's prompting our poops, or is it something else?
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It was previously thought that caffeine was the poop provoking ingredient in coffee. However, this theory is easily tested. A 1998 study gave 12 volunteers coffee, decaf coffee, water or a 1000 calorie meal and measured their colonic function in response. Specifically, they used a manometer (a device that measures pressure) to measure the colon’s activity, as there is a pressure change when part of the digestive tract contracts to push food towards the anus. They found that coffee stimulated colon activity just as much as a 1000 calorie meal and that while the effect is lessened with decaffeinated coffee, it’s not gone. Decaf coffee increased colonic activity more than water, just less than caffeinated coffee.
These results point to caffeine contributing to coffee’s laxative effect, but not explaining it in full. Indeed, a 1999 review concludes that, “caffeine cannot solely account for these gastrointestinal effects.” Caffeine does certainly play a role, however, as was shown in a 2008 study. Researchers gave 10 subjects either water, or caffeine powder dissolved in water and measured their anorectal function via manometry. The researchers found that ingesting caffeine led to stronger anal sphincter contractions, and an increased desire to defecate.
If caffeine alone cannot explain coffee’s laxative effects, what can? Possibly, coffee’s interaction with stomach acid.
Both caffeinated and decaffeinated coffee stimulate the production of a hormone called gastrin. Gastrin prompts the stomach to release more hydrochloric acid and digestive enzymes, increase stomach contractions, relax the valve between your small and large intestines, and relax the sphincter between your stomach and small intestine, amongst other things. To sum up all these various effects, we can say that gastrin promotes digestion. Therefore, if coffee stimulates gastrin and gastrin stimulates digestion, this pathway may be a mechanism by which coffee makes us defecate.
There could be another effect at play here though: the gastrocolic reflex. This reflex is triggered by the stomach stretching when you eat or drink something and causes the colon to increase its motility. It’s essentially your body trying to push old food out of your intestines to make room for new food. This reflex is controlled by a few different molecules, including gastrin. So, if coffee leads to gastrin production, which leads to increased colon contractions, this is another way coffee may trigger our morning bathroom breaks.
This reflex is especially active in the mornings, which could explain why a 9 a.m. cup of coffee sends you running to the bathroom but not a 3 p.m. one. Studies looking at the relationship between bowel activity and circadian rhythms found that colonic activity is greatly reduced at night and in the evenings, but increased right after waking up. It’s not clear if our circadian rhythms control bowel movements directly, or if they only control our eating habits, which in turn control our bathroom schedules. Either way though, it seems that coffee isn’t the only thing pushing us to poop before noon.
Rest assured that if coffee doesn’t make you poop at all, you’re not the odd one out. A 1990 survey of 99 volunteers found that coffee induced a desire to defecate in only 29% of them. The same study went on to measure the colon activity of 14 volunteers, eight of whom said that coffee made them poop. In those eight, but not the other six, ingesting coffee (regular or decaf) caused increased colon motility.
It seems that some people are just not affected by coffee’s laxative effects, while for others just a whiff of java can have an almost Pavlovian effect on their bowels. Like most physiological effects the interactions between coffee and the colon are complex, numerous and we don’t fully understand them. Personally, I’m going to settle myself with just being happy that my daily cup of joe helps keep be regular, however it manages to do so.