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?
Coffee can reduce how much iron get’s absorbed. This doesn’t have anything to do with the caffeine and the resulting stimulation though, but with something in the coffee binding the receptors or something like that. Don’t know if this happens with other nutrients as well.