We know that caffeine is the culprit because regular coffee is a lot better at this laxative effect than decaf, according to William DePaolo, a molecular microbiology and immunology professor at the Keck School of Medicine at USC. There can be anywhere from 80 to 135 milligrams of caffeine in one eight-ounce cup of brewed coffee, and up to 175 milligrams in drip coffee. In contrast, Coca-Cola has only 34 milligrams of caffeine, which explains why soda doesn’t have the same laxative reputation as coffee.The acidic nature of coffee also causes an increase in the production of bile acids in the body, said DePaolo. The liver makes bile and stores it in the gall bladder, and coffee can make the gall bladder release the bile into the intestines, causing diarrhea as well. Beyond the coffee beans themselves, sweeteners, dairy products or non-dairy additives may also be putting additional pressure on coffee drinkers’ guts, according to American Gastroenterological Association expert Dr. Jay Kuemmerle of Virginia Commonwealth University. The artificial sweeteners in coffee mixes can cause bloating, gas and diarrhea. Dairy products like milk, cream and half-and-half contain a sugar called lactose, which can trigger diarrhea and other gastrointestinal complaints among people who are lactose intolerant. Even among those who are not intolerant, the ability to digest lactose tends to decline with age. Taken together, it’s amazing that this witches brew of poop-producing ingredients doesn’t send everyone running to a bathroom after a couple of cups.Source: Why Does Coffee Make You Poop?
"We’ve all been told that coffee -- specifically, the caffeine in coffee -- is a mild diuretic, which means that it makes the body excrete more liquid than normal (read: pee more). The diuretic effects of caffeine can vary based on many factors, including your sex, activity levels and whether or not you’re used to its effects. But caffeine can also have a laxative effect in some particularly sensitive people. The chemical stimulates muscle contractions in the large intestines, which are similar to the contractions that happen after you eat a meal."