A colonoscopy is a medical procedure used to examine the lining of the large intestine (colon) for abnormalities such as polyps or signs of disease. For the examination to be successful, the colon must be completely empty of residual waste. Proper preparation, involving a multi-stage dietary shift, is mandatory because remaining solid matter can obscure the physician’s view or necessitate repeating the procedure. Understanding when to stop eating is fundamental to ensuring accurate results.
The Initial Dietary Shift
The first step in preparing the digestive tract is transitioning to a low-residue diet, a phase that typically begins three to five days before the colonoscopy. This is not a complete fast but a targeted modification of solid food intake designed to reduce the amount of indigestible fiber that forms stool bulk. The term “residue” refers to the parts of food, primarily fiber, that pass through the digestive tract largely undigested. By lowering this residue, the process starts to clean out the colon gently.
During this period, one must eliminate high-fiber items like whole grains, nuts, seeds, popcorn, and most raw fruits and vegetables. These foods require significant time to move through the colon and can leave behind fragments that interfere with visibility. Conversely, the diet permits refined foods that are easily digested and absorbed, such as white bread, white rice, plain pasta, and lean, tender meats without skin. Cooked vegetables without skins, like peeled potatoes, and pulp-free fruit juices are also generally acceptable during this initial phase.
The Transition to Clear Liquids
The absolute cutoff for solid food intake begins the day before the procedure, usually around the 24-hour mark, and signals the start of the clear liquid diet. The goal shifts from reducing residue to eliminating all solid contents from the digestive system. A clear liquid is defined as any liquid you can see through, which ensures no solid material remains to block the view during the colonoscopy. This diet must be strictly followed throughout the day leading up to the procedure.
Acceptable items include water, clear broth (chicken or beef), clear fruit juices without pulp like apple or white grape, and clear sodas such as ginger ale or lemon-lime. Black coffee and tea are allowed, but no milk, cream, or non-dairy creamers can be added. It is also permissible to consume gelatin and popsicles, provided they do not contain any fruit pieces or cream. A specific restriction is placed on all liquids and foods containing red or purple dyes, as these colors can linger on the colon wall and be mistaken for blood during the examination.
This clear liquid phase is when the prescribed bowel preparation solution is consumed, often in a split-dose regimen. Patients typically drink the first portion the evening before and the second portion several hours before the appointment. The high volume of clear liquids consumed alongside the prep solution is necessary to flush the colon completely clean.
Timing the Final Fast
The final step is an absolute fast, known medically as NPO (nil per os, or “nothing by mouth”), which dictates when liquids must also be stopped. This strict cutoff is a safety measure for the procedure, which involves sedation or anesthesia. Having any contents in the stomach, even clear liquids, increases the risk of pulmonary aspiration—where stomach contents are accidentally inhaled into the lungs—while the patient is under sedation.
For safety, all intake, including the final dose of the prep solution and even small sips of water, must stop a specific number of hours before the scheduled procedure time. This final fast for clear liquids is typically set at two to four hours before the appointment, though guidelines vary. Patients must finish the second dose of the bowel preparation solution within the allowed liquid window, ensuring a few hours pass between completion of the prep and the start of the procedure.