Getting ready for a colonoscopy takes longer than the procedure itself, as Harvard Health Publishing points out — and that's largely because of all the dietary dos and don'ts you have to follow. Some of the instructions are focused around what you should eat — but even more of them are about what you can't.
After all, the whole goal of colonoscopy preparation is to empty your intestines so the doctor will be able to see any abnormalities when he or she goes in with the colonoscope. "Emptying increases visibility," says Cordialis Msora-Kasago, MA, RDN, a spokesperson for the Academy of Nutrition and Dietetics. "It allows your doctor to see the lining of your colon more clearly — and it also decreases complications." If the doctor can't see the lining of the intestine clearly, he or she could make a mistake, such as inadvertently nicking it.
Different facilities have different guidelines when it comes to emptying the colon pre-procedure. "Colonoscopy prep and diet vary," notes Msora-Kasago. "It's very important for individuals to follow the guidelines of their physician at the particular site they're going to."
That said, here's what you're likely to read in the instructions when it comes to the foods to avoid before a colonoscopy.
Read more: How to Recover After a Colonoscopy
Many colonoscopy centers will ask you to cut out high-fiber items about three days before the procedure. "That means focusing on low-fiber and low-roughage foods to prepare your gut, to start cleaning it out so [your colon lining is] easier to see," says Msora-Kasago.
This may seem counterintuitive. After all, high-fiber foods — especially plant-based ones — are supposed to help prevent colon cancer, according to the American Institute for Cancer Research. (Fiber basically scrubs out the colon, clearing it of potentially unhealthy buildup, per the University of California San Francisco). The problem is, high-fiber foods fill up your colon and take several days to clear. That's exactly what you don't want before a colonoscopy procedure.
Fibrous foods to stay away from prior to a colonoscopy include nuts, seeds, legumes such as chickpeas, beans and lentils, meat with gristle, whole grains, fresh fruits and raw vegetables (cooked is OK), says Msora-Kasago. Raisins, popcorn, corn and potato chips are also on the list.
But these are the guidelines... for now. A 2018 article published in the journal Gastrointestinal Endoscopy reported that restricting dietary fiber the day before a colonoscopy may not actually be all that helpful. "There's a debate about whether restricting diet is necessary," says Msora-Kasago. "The evidence is still evolving."
Foods Containing Dye
Although no official guidelines specifically recommend avoiding foods with red dye, most colonoscopy centers will tell you to stay away from them before the procedure. The concern is that red dye can stain the colon and make it difficult to see abnormalities, says John H. Ashcraft, DO, chief of colon and rectal surgery at the University of Kansas Health System in Kansas City. So don't be surprised if you're asked to forego fluids, Jell-O, ice pops and any other foods or drinks that are red, purple or orange.
Even more painful than restricting fiber and avoiding food containing certain dyes is stopping all solid foods during colonoscopy prep. Even something as seemingly innocent as mashed potatoes must be eschewed the day before a procedure.
So what does that leave? In short: clear fluids, gelatin and ice pops. Specific items that can be consumed include bouillon, tea, clear fruit juice without pulp (apple, white grape, white cranberry, lemonade), clear sports drinks and yellow or green Jell-O and ice pops. Black coffee is fine as well, even if it contains sugar — but not coffee with creamer or milk.
An exception to the clear-liquids rule? Alcohol. "Even though alcohol is clear, it tends to be dehydrating," says Msora-Kasago. It's also not a good idea to have alcohol in your system when you're going to be sedated.
Read more: What to Eat After a Colonoscopy
According to a 2019 study published in Diseases of the Colon & Rectum, participants who followed a low-fiber diet the day before their colonoscopy saw better prep results than did those who only consumed clear liquids. For now, though, the main thing to remember is to closely follow whatever instructions your own doctor gives you.
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- Harvard Health Publishing: “Preparing for a Colonoscopy”
- American Institute for Cancer Research: “Get the Facts on Fiber”
- Kaiser Permanente: “Low-Fiber Diet for Colonoscopy Preparation”
- University of Wisconsin Health: “Getting Ready for Your Colonoscopy”
- Gastrointestinal Endoscopy: “The Association Among Diet, Dietary Fiber, and Bowel Preparation at Colonoscopy”
- Diseases of the Colon & Rectum: “Randomized Clinical Trial: A Normocaloric Low-Fiber Diet the Day Before Colonoscopy Is the Most Effective Approach to Bowel Preparation in Colorectal Cancer Screening Colonoscopy”
- University of California San Francisco: "Why Fiber Is So Good for You"