Patients are advised not to eat solid food on the day before a colonoscopy, as the bowels need to be clear for observation of any polyps or lesions, explains Harvard Health Publications. Diet restrictions begin three days before the procedure.
Preparing the bowels for a colonoscopy can take up to 16 hours, according to Harvard Health Publications. Doctors recommend that patients eat a low-fiber diet in the days leading to up the colonoscopy. On the day before the procedure, patients must drink only clear liquids such as water, clear broth, black coffee or tea and clear soft drinks or juices. That evening, patients drink a bowel-clearing liquid such as polyethylene glycol, oral sodium phosphate or magnesium citrate preparations.