The purpose of the dietary changes prescribed in the days before a colonoscopy is to clean the bowel out completely, according to Harvard Medical School. An empty colon allows the doctor to see any anomalous growths in the colon more clearly.
Several days before the colonoscopy, patients should work fiber out of their diet. This means eliminating raw produce, dried fruit, seeds, nuts and whole grains. During the day right before the colonoscopy, patients should avoid all solid foods. Instead, they should stick to clear liquids such as tea, black coffee, bouillon or broth. Clear soft drinks are acceptable, as are Jell-O, popsicles and sports drinks. On the day in which the colonoscopy takes place, that same clear food diet is necessary up until two hours before the procedure, when all eating and drinking should come to a stop, reports Harvard Medical School.
During the afternoon or evening before the day of the colonoscopy, the doctor prescribes a preparation liquid that leads to the sort of diarrhea that clears the colon out completely. The precise steps for taking the treatment vary with the specific preparation that the doctor wants the patient to use. Some methods involve drinking a gallon of the prep fluid the day before, while others incorporate taking half the day before and the other half on the day of treatment, notes Harvard Medical School.