Medical review by Elizabeth Gomez, MSN, FNP-BC
Nobody enjoys colonoscopy prep, but the process, which includes several days of unusual dietary restrictions, can be especially tricky for people with diabetes.
When you’re preparing for your colonoscopy, you’ll receive eating guidelines that conflict with everything you’ve ever learned about eating healthy with diabetes. Depending on your gastroenterologist’s specifications, you may spend one day each on low-fiber and clear liquid diets. You’ll also be eating many fewer calories than normal. All of these changes have impacts on your blood sugar and may require proactive adjustment of your diabetes medication.
Do you have a colonoscopy coming up? Read on to learn about best practices for people with diabetes.
Colonoscopy — The Basics
A colonoscopy is a medical procedure in which a doctor examines the lining of the colon (large intestine) for abnormalities, such as polyps or tumors. These polyps can become cancerous and removing them promptly is the best way to prevent colorectal cancer, a condition that is more common in people with diabetes.
The current recommendations, set forth by the U.S. Preventive Services Task Force, call for regular colorectal cancer screening between the ages of 45 and 75.
The actual colonoscopy — during which a gastroenterologist sticks a scope up through the anus, beyond the rectum, and into the colon — is performed under anesthesia. Many people find the multiple days of preparation worse than the actual procedure. It takes days of careful eating and powerful laxatives to flush fecal matter out of the body so that your doctor has a clear view of the intestinal lining.
Blood Sugar Monitoring During Colonoscopy Prep
“You really need to be paying closer attention to your glucose readings,” says Thomas Grace, MD, the medical director of Ohio’s Blanchard Valley Diabetes Center. Dr. Grace, who has type 1 diabetes himself, advises patients on colonoscopy prep frequently.
Your several days of colonoscopy prep will mean big changes to both what and how much you eat, which can easily cause blood sugar unpredictability.
Grace told Diabetes Daily that the risks are generally not extreme for colonoscopy patients with diabetes, but that close glucose monitoring — ideally with a continuous glucose monitor (CGM) — is key. If you’re aware of your glucose levels, it’s easier to make adjustments. And the steadier your blood sugar, the less likely the medical team will be required to manage your insulin or sugar intake during the colonoscopy procedure.
The biggest fear, says Grace, is low blood sugar. “We don’t want you going low. Anything we can do to help prevent that hypoglycemia, we’re going to do that.”
It’s especially important for people with diabetes — especially those that use any drugs associated with hypoglycemia, particularly insulin and sulfonylureas — to always keep a source of sugar at the ready.
So as not to disrupt the colonoscopy prep, that sugar source should be a clear beverage: “I want my patients to have a clear juice, soda or Gatorade around in case their blood sugars are running low.” Fixing a hypo with anything else threatens to cancel the colonoscopy, an expensive hassle for everyone involved.
The Low-Fiber Diet
The first stage of colonoscopy prep usually requires you to adopt a low-fiber diet.
If you’re already eating a healthy diabetes diet — an eating pattern that avoids added sugars, refined grains, and other highly processed foods — you’ll be recommended to eat some of the very foods you normally don’t allow yourself.
According to the Mayo Clinic, a low-fiber diet asks people to eat the type of nutritionally meager foods we’re usually supposed to stay away from, including white rice and pasta, low-fiber bread and biscuits, and processed fruit products like juice, applesauce, and canned peaches. Meanwhile, colonoscopy preppers should not eat healthy whole ingredients like nuts, seeds, brown rice, quinoa, whole grain bread, beans, lentils, and most fruits and vegetables.
If you’re watching your carb and sugar intake, what are you supposed to do? For one thing, a low-fiber diet also allows for the consumption of eggs, lean meats, and dairy. You can also have some soft, well-cooked vegetables, such as carrots or green beans. The United Kingdom’s National Health Service recommends that people with diabetes aim to have the same amount of carbohydrates that they customarily do.
Some doctors will also advise patients with diabetes to tolerate a higher blood sugar for a day or two. A few days of suboptimal eating will not cause any long-term harm.
“One day of glucose readings that are a little bit higher — permissive hyperglycemia — is way better than having a severe hypoglycemic event,” says Grace. “It’s just safer.”
It’s probably best to discuss the details of your low-fiber eating plan with your gastroenterologist. There is also plenty of advice on the Diabetes Daily forums, where regular people have been sharing details of their colonoscopy prep experiences for years.
The Clear Liquid Diet
The day before your procedure, you’ll probably switch to a diet of clear liquids. This will allow you to ingest some calories and stay hydrated without depositing any new fecal matter in your colon.
Aside from water and zero-calorie beverages such as coffee or tea, the Mayo Clinic recommends some of the following foods: clear gelatin (with or without sugar), bone broth and clear soup, pulp-free juice, soda, sports drinks, and ice pops.
It’s practically impossible to eat your fill on a clear liquid diet. You’ll eat virtually no fat, and it’s not easy to find protein sources. Refined and added sugars represent two of the only calorie sources that you’re allowed to have.
Even if you consume more sugar than usual, you can expect to be hungry. If you’re not excited about a day of crash dieting, ask your doctor about high-protein gelatin and clear protein shakes — they may be able to recommend a product that works for you.
It might be smart to have both sugary and sugar-free options at the ready, so you can respond to the changes in your blood glucose levels.
At the same time, you may be required to drink vast amounts of laxative beverages. These solutions, such as GoLYTELY, have zero calories and include plenty of electrolytes. There are many different colonoscopy prep kits and laxatives; your gastroenterologist will have their favorite and may not initially offer you a choice. But if you’re curious, Everyday Health has explored the options: Choosing a Colonoscopy Prep.
The collision of your diabetes medication, colonoscopy prep, and the procedure itself increases the risk of a variety of bad health outcomes, including hypoglycemia, lactic acidosis, and kidney injury. Working carefully with your doctor, you will have to reduce or stop taking some of your medications ahead of your colonoscopy.
A 2022 review offers some standard recommendations:
Metformin: Stop taking when the clear liquid diet begins, and resume once you’re back to eating normal meals after the procedure.
GLP-1 receptor agonists (such as Ozempic and Trulicity): Stop taking when the clear liquid diet begins, and resume once you’re back to eating normal meals after the procedure. If your regular weekly injection is scheduled during your colonoscopy prep period, hold off until you’re back to eating normally.
DPP-4 inhibitors (such as Januvia): Stop the morning of the procedure and resume that evening.
SGLT-2 inhibitors (such as Jardiance and Invokana): Stop taking three days before the procedure, and resume once you’re back to eating normal meals and are adequately hydrated.
Sulfonylureas: Stop taking the day before the procedure and resume once you’re back to eating normal meals.
Insulin, the diabetes drug with the highest risk of hypoglycemia, and an absolute necessity for people with type 1, requires special attention. Please speak to your doctor, because the following are only starting points:
Rapid insulin: People that use an insulin-to-carb ratio to determine how much rapid insulin to use for meals can continue to follow their normal dosing strategy. Those that use fixed doses for each meal will have to cut their usual dose substantially during the clear liquid phase of prep.
Basal insulin: Beginning the day before the procedure, patients will be asked to cut their basal insulin dose by 20 to 50 percent, depending on the insulin they use and the type of diabetes they experience.
Grace reports that his patients using automated insulin delivery (closed-loop systems that use CGM readings to automatically adjust insulin doses administered by an insulin pump) don’t need to do anything; their system will naturally adjust to their changing insulin requirements.
Other Special Considerations for People With Diabetes
It is important that your gastroenterologist knows that you have diabetes. Consider it your responsibility to make sure that they are aware.
According to a 2022 review, diabetes “is a predictive factor for inadequate bowel cleansing,” meaning that people with diabetes are more likely to need a second colonoscopy because their colon wasn’t clean enough for the doctor to perform an evaluation. Diabetic neuropathy, for example, may slow the digestive system, causing a standard prep protocol to fail. If your gastroenterologist knows that you have diabetes, they may recommend modifications, such as a special type of laxative or a customized diet plan.