For about 30 percent of the population, coffee stimulates more than just the brain. According to a study in Gut, coffee can trigger activity at the other end, in the colon. Researchers aren’t exactly sure why coffee makes some people need to defecate, but they have some hypotheses — and they’ve ruled out a few reasons, too.
Although caffeine is a stimulant, it’s not the chemical stimulating the colon. Soda doesn’t make people need to go number two, and researchers have found that coffee’s effect occurs with both caffeinated and decaffeinated coffee.
The urge to defecate also doesn’t come from any diuretic effect that coffee has.
First, if coffee were a diuretic, it would have an opposite effect. It would make people need to urinate, which would dehydrate them and could, potentially, lead to constipation. In other words, needing to pee after you drink coffee wouldn’t make you need to poop — it’d do the exact opposite.
Second, while caffeine is recognized as a diuretic, a study in the Journal of Human Nutrition and Dietetics threw coffee’s diuretic properties into question. Similar to how people build up a tolerance to caffeine’s stimulating effects on the brain, study participants built up a tolerance to the diuretic effects of caffeine.
The study in Gut took a close (really close) look into how coffee affected a few participants digestive system, and the researchers found that coffee stimulated the distal colon. They aren’t sure why it cause the colon to become more active, but the increased activity would certainly lead to a need to defecate at times.
Thus, this at least one mechanical reason why coffee can make you poop, but scientists still don’t know why it happens.
Another theory suggests that the acids in coffee could make some people need to go. Chlorogenic acid, specifically, decreases the pH in the stomach (making the stomach more acidic). Chlorogenic acid also prompts the stomach to produce more acid, which further lower’s the stomach’s pH. A big decrease in the stomach’s pH could cause the stomach to empty its contents more quickly than it otherwise would, which would eventually lead to a need to go number two.
Some chemicals in coffee may further add to this theory. There may be chemicals that make the body release hormones which speed up the digestive process. There are over 1,000 compounds in coffee, though, and scientists don’t know which ones, if any, do this.
Finally, for some people, going after drinking coffee could simply be a habit. People’s bodies naturally fall into patterns, and some people may simply be used to going after having a cup of coffee, especially if they drink coffee at the same time every day.
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