The colonoscopy diet

A colonoscopy reigns as the gold standard screening test to look for colorectal cancer and other bowel problems. It's an outpatient procedure that allows your doctor to peek inside your colon using a thin, flexible tube with an attached camera; inspect the colon lining; and even remove potentially cancerous growths.

But the doctor's expertise isn't all that's needed to make the procedure a success. Your preparation — what you eat, and the way you clean out your gut beforehand — is essential to give your doctor the best view of the colon lining. And the post-procedure diet is important for your comfort and gut health. Here's what you need to know about a colonoscopy diet.

Three days beforehand

The mission to prepare your colon for a colonoscopy starts a few days before the procedure. "During this time, we advise that you eat a low-fiber diet. These foods are easy to digest and move through the colon faster than high-fiber foods. This will make colon prep easier," says Dr. Andrew T. Chan, a gastroenterologist and director of epidemiology at Harvard-affiliated Massachusetts General Hospital Cancer Center.

Eating a low-fiber diet doesn't mean you can ditch all healthy food and reach for chips and soda instead. It means eating foods such as animal protein (eggs, cheese, fish, poultry); certain cooked vegetables (carrots, green beans, potatoes, pumpkin, yams, or zucchini and other squash); low-fiber fruits (bananas, peaches, pears); and (if your doctor says it's okay) low-fiber breads such as white, sourdough, or refined wheat breads.

A low-fiber diet also nixes foods such as legumes (beans, lentils, peas), whole grains (whole-wheat crackers, cereals, breads, popcorn, granola), nuts, seeds, dried fruit, and high-fiber raw fruits or vegetables (especially asparagus, broccoli, brussels sprouts, cabbage, cauliflower, and celery).

The day before the colonoscopy

Unless your doctor has prescribed the type of colonoscopy prep that's done only on the day of the procedure, the day before a colonoscopy is when you should have only clear liquids. Examples include clear broth or bouillon, black coffee, plain tea, clear juice (apple, white grape), clear soft drinks or sports drinks, Jell-O, and popsicles. Then, in the late afternoon, you take laxative pills and solutions and drink lots of fluids to clean out your colon.

Note: Avoid foods or drinks that are red, blue, or purple. Just like they can temporarily stain your tongue, they can also stain the lining of your colon, which might make it hard for your doctor to examine it properly. So stay away from grape, raspberry, or cherry Jello-O or popsicles; go for the lighter-colored flavors instead, such as banana.

The day of the colonoscopy

A colonoscopy requires you to receive a powerful sedative: most people remain conscious, but are very relaxed and less likely to feel any discomfort. You also may have little memory of the procedure. Because of the sedative, you probably won't be allowed to eat or drink anything on the day of your procedure, unless your doctor has advised you to continue taking your usual prescription medications (such as drugs that treat high blood pressure) with a sip of water, or your doctor has directed you to begin the colon prep on this day.

After the procedure, you'll be allowed to resume your normal diet. But should you? A sudden injection of fiber — such as a bowl of black bean soup with whole-wheat crackers on the side, or chickpea pasta tossed with olive oil with grilled asparagus — might cause gas, bloating, and discomfort.

"Some people are able to start eating a full diet right away without discomfort, while others may require a slower introduction of their usual foods. So it might be wisest to restart your normal diet gradually over the next day or so," Dr. Chan says.

Gut health reset?

With your colon cleaned out, you might wonder if you'll now have an opportunity to create a healthier population of gut microbes than you had before the procedure. Is it possible? The jury is still out. "There is no clear evidence that you can more favorably repopulate your gut bacteria over the long term after a colonoscopy with any particular dietary changes," Dr. Chan says.

What we do know is that your gut is home to trillions of mostly beneficial microbes — bacteria, viruses, fungi — that help digest your food and keep you healthy. These bugs fight harmful pathogens; make vitamin K and other important chemicals; affect the way medications work; influence your immune system, heart health, and cancer risk; and possibly play a role in healthy aging and longevity.

We also know that what you eat can help gut microbes thrive. For the best results, feed them a fiber-rich diet that includes fruits, vegetables (especially dark, leafy greens), legumes (beans, peas), and whole grains (quinoa, oats, whole wheat, brown rice). These are the kinds of foods found in a Mediterranean-style diet, which also includes lean proteins (poultry and fish), olive oil, nuts and seeds, dairy foods (milk and cheese), and limited amounts of red wine.

It's a diet that will keep your gut microbes happy and help you maintain many other aspects of good health.

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