It's proven that the caffeine in coffee stimulates the brain, but there's something in a cup of joe that can also jump-start the other end of the body.

That's right: Coffee can make you poop.

And though a number of studies have attempted to explain the effect of a cup of java on the bowels, scientists have yet to determine what it is about the beverage that sends some imbibers running to the restroom.

Stimulating studies

One study, published in 1990 in Gut, a journal of gastroenterology, found that coffee induces a "gastrocolonic response" in some individuals just minutes after they consume the beverage. While the study failed to identify the exact cause of this response, researchers hypothesized that coffee somehow affects the epithelial tissue lining the stomach and the small intestine.

The Gut study also found that coffee promotes the release of gastrin, a hormone produced within the stomach and known to increase motor activity in the colon. As this area of the of the colon is closest to the rectum, researchers concluded that increased activity there could be responsible for coffee's laxative effects.

While certain health professionals believe it's the caffeine in coffee that causes heightened motor activity — or contractions — in the colon, the Gut study found that in certain individuals even decaffeinated coffee stimulates the need to defecate. This led the researchers to conclude that it's not caffeine, but some other substance in coffee that's responsible for the drink's reputation as a purgative.

Coffee conundrum

It might seem contradictory that drinking coffee, which has long been considered a diuretic (or dehydrating) beverage, can result in bowel movements. After all, dehydration is a common cause of constipation. But in recent years, scientific research has demonstrated that coffee does not have the diuretic properties long ascribed to it.

A 2003 study published in the Journal of Human Nutrition and Dietetics found that a strong tolerance for the diuretic properties of coffee often develops in individuals who regularly consume the beverage. In fact, the authors of the study found that the dose of caffeine contained in two to three cups of coffee does not affect the average amount of urine excreted from the body on a given day.

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