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?
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… 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.
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… 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… 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.