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Digestion During Treatment

Liz Puris, MS, RD, LDN
Oncology Dietitian Nutritionist
Dana Farber Cancer Institute

It’s a question you’ll likely hear many times throughout treatment, “are you experiencing diarrhea or constipation?” At first, it can be an uncomfortable question to answer, although it’s important to communicate any concerns so your care team can best support you. Let’s discuss nutritional recommendations for symptoms of diarrhea and constipation during treatment.


There are some medications known to commonly cause constipation, such as anti-nausea medications and pain medications. While taking either of these medications, it’s a good idea to adjust your diet and take any bowel medications as recommended by the team. Focus on insoluble fiber, the type of fiber that can’t be fully broken down, to help to move things through your digestive system. Think of roughage – salads, fruits and vegetables with skins, seeds, nuts, prunes, whole grains, and bran. Make sure that you also drink
enough fluid. The fiber won’t be able to effectively move through your body without enough fluid.

Water is not the only beverage that counts as a fluid. Seltzer, flavored waters, juice, popsicles, pudding, decaf coffee and tea, among others, also count. In particular, try emphasizing warm beverages such as tea, which can be especially helpful for constipation. Most people need about 2 liters (64 oz) of fluid daily, although ask your dietitian for your personalized recommendation. If you are interested in a fiber supplement, such as those that dissolve in water or other fluids, this is also something to ask your dietitian about.


There are some diagnoses and treatment regimens that unfortunately can cause diarrhea. As long as you are having symptoms of diarrhea, it is important to take any bowel medications and take the appropriate dietary adjustments. You’ll want to focus on soluble fiber when having diarrhea. Soluble fiber will help to slow down movement through your digestive tract while also absorbing fluid. Some examples of foods that contain soluble fiber include banana, applesauce, ground flaxseed, barley, and oats. The key is to also try and limit insoluble fiber- roughage, skins, and seeds.

Some people may notice dairy is a trigger, even if it is normally well tolerated. Try dairy-free or plant-based dairy products instead. You may have also heard of the “BRAT” diet (bananas, rice, applesauce, toast). While this is a good starting point, it is important to speak with your dietitian who can guide you through an “expanded BRAT diet,” making sure you have enough choices. It is also important to get an adequate amount of fluid to avoid dehydration. Look for beverages that contain electrolytes (such as sodium and potassium) to replace losses. Some examples include commercial electrolyte beverages, sports drinks, and coconut water.

Tip: Before treatment, your care team will review potential side effects and advise which (if any) bowel medications (for example: senna, colace, miralax, imodium, or lomotil) to pick up and have on hand. Definitely pick up these medications. If you end up needing them, you’ll be glad you don’t have to make another trip to the pharmacy.

If, despite dietary adjustments, something with your digestion is still not quite right, don’t be afraid to speak with your care team. Most people find they need a balance between diet and medications to best treat their symptoms. Untreated bowel side effects can unintentionally cause or worsen other issues such as nausea, dehydration, etc.

Remember everyone is different – ask your healthcare team, including your dietitian, for your own personalized recommendations.

Special Note:

If you have a GI (gastrointestinal) cancer diagnosis or a diagnosis that inadvertently affects the GI system, you might need an even more specific diet to help better manage your symptoms. These diagnoses may include pancreatic cancer, NET, colon cancer, rectal cancer, and gynecological cancers, among others.

What’s the Bottom Line?

If you are experiencing gastrointestinal side effects of treatment, such as diarrhea or constipation, make sure to communicate your concerns. In addition to bowel medications, diet adjustment can help.

Constipation – Make sure to prioritize:

  • Insoluble fiber which cannot be fully broken down, adds bulk, and moves more quickly through your digestive system (roughage such as salads, fruits and vegetables with skins)
  • Fluid including water, and warm beverages such as tea

Diarrhea – Make sure to prioritize:

  • Soluble fiber which helps slow down digestion and absorb fluid (banana, applesauce, and oats)
  • Electrolyte containing fluids