It turns out that the standard explanation leaves out a few things. Three major ones, actually. First, if it takes ten hours for food just to reach the colon and a day or two for all of it to be gone, then your intestines are always working on several meals at any one time. Any symptoms you feel may have nothing to do with what you just ate. Second, lactose has another role to play that isn t always mentioned.
All foods contain water.
Fruits and vegetables are mostly water, and so is anything soft, including most dairy products. In normal digestion this water is absorbed by the intestines, leaving just a little bit left so that your stool doesn t get hard and dry. Undigested lactose, however, reverses this process so that water is actually drawn into the intestines. Third, your intestines work by pushing food along in a process called peristalsis. Food entering the stomach acts as a trigger to start the pushing, all the way through the intestines.