Basically, as I understand it, when you eat food it goes through your stomach and then it travels through your bowels where the nutrients and water get gradually absorbed along the way. Coffee, as I understand it, stimulates the muscles in the bowels and causes the contents to move through the intestines more quickly. So if drinking coffee means that food will spend less time in the intestines, does that mean that less nutrients will be absorbed from the food than if no coffee was consumed?
You don’t poop out the contents of your stomach, small intestines or large intestines, where the absorption is taking place. You poop out what is in your colon, which is quite short in comparison.
Coffee will not make you poop your food before you have adequately processed it.
Also, up to about half of the dry weight of your poop is microbial waste.