Fiber During Cancer Treatment: What to Ask

Fiber is the one nutrient where the healthy-eating advice you have heard your whole life may be exactly wrong for the next few months. Outside cancer, more fiber is nearly always the answer. During treatment, the right amount depends entirely on which end of the bowel problem you are on, and that can flip inside a single chemotherapy cycle.

Fiber is the part of plant food your body cannot digest. It comes in two forms, and knowing which is which explains most of the confusion.

Soluble fiber attracts water and turns to a gel as it moves through you. MedlinePlus notes this slows digestion down. It is in oats, oat bran, barley, beans, nuts, seeds, and the flesh of many fruits.

Insoluble fiber does not dissolve. It adds bulk to stool and appears to speed food through the stomach and intestines. It is in wheat bran, whole grains, and the skins and stalks of vegetables.

For adults and older children, MedlinePlus puts the general target at 21 to 38 grams a day. That number is for general health. It is not a treatment target, and during some phases of cancer care your team may want you nowhere near it.

Cancer care pushes the bowel in both directions at once, sometimes in the same week.

Constipation is common. The National Cancer Institute points to opioid pain medicines, some anti-nausea drugs, less movement, less food, and less fluid. Certain cancers add to it, including colon, rectal, and ovarian cancers and brain tumors.

Diarrhea is just as common, and NCI lists a long set of causes: chemotherapy, immunotherapy, radiation, surgery, targeted therapy, stem cell transplant, infection, and medicines. Radiation aimed at the pelvis is a frequent culprit.

Then there is anatomy. Bowel surgery, a stricture (a narrowed section of bowel), or an ostomy all change what the gut can handle. MedlinePlus lists strictures and obstruction among the reasons someone may need a low-fiber diet long term, not just for a flare.

If diarrhea is the problem, NCI recommends low-fiber foods. Its list is deliberately dull: white bread, white rice, noodles, peeled potatoes, refined cereals, eggs, fish, skinless chicken, cooked carrots, cooked green beans, canned peaches, applesauce, saltines, graham crackers, and plain yogurt.

MedlinePlus describes a typical prescription as no more than 10 to 15 grams of fiber a day, though your dietitian sets your own number. On that plan you drop beans, whole grains, nuts, seeds, raw fruit and vegetables, and all skins and pips.

NCI also lists things to avoid while diarrhea is active: alcohol, milk and dairy, spicy food, caffeine, dried beans, high-fat food, fruit juice, and sugar-free gum or candy. That last one surprises people. The sugar alcohols used in sugar-free products pull water into the bowel.

For constipation, more fiber is the obvious move, but NCI is careful here, and so should you be. Its guidance says to talk to your doctor about dietary fiber, because high-fiber foods and fiber supplements can make constipation worse for some people.

That is not a throwaway line. Adding bulk to a bowel that is slow, dehydrated, or narrowed can make things harder rather than easier. Fiber needs water to work. MedlinePlus advises adding fiber gradually over a few weeks and drinking about eight glasses of water or other non-calorie fluid a day. NCI gives the same fluid figure for constipation and notes that coffee and prune juice can have a laxative effect.

NCI also warns against reaching for suppositories or enemas during cancer treatment. In people with low blood counts these can cause bleeding, infection, or other harm. Ask for a medicine plan instead of improvising.

It is worth saying plainly: the fiber advice given during cancer treatment is less evidence-based than it sounds.

The low-fiber diet for treatment-related diarrhea is standard practice and it is what NCI publishes, but it rests more on physiology and clinical habit than on large trials in people having chemotherapy. The same is true of soluble fiber supplements for loose stools. They are widely used and often helpful. They are not proven to shorten chemotherapy-induced diarrhea.

So expect your team to give you a starting point and then adjust based on what your bowel actually does. Anyone offering you a precise gram target for your cancer is guessing.

• Call 911 or go to an emergency department if you have severe belly pain with a swollen abdomen and vomiting and no bowel movement, which can mean a blockage. Go too for black stool or a lot of blood in the toilet, or if you are too dizzy to stand after a day of diarrhea. Go straight away as well if your temperature reaches 100.4°F (38°C) or higher, the figure CDC gives for people having chemotherapy — fever with low white cells is an oncologic emergency, and antibiotics are meant to start within about an hour, so this is never a message left with the office. Seven or more extra bowel movements a day belongs here too; NCI grades that level as life-threatening.
• Call your care team the same day if you cannot keep fluids down, your diarrhea has jumped by four to six extra bowel movements a day, or you are passing much less urine than normal.
• Call your care team within a day or two if you have had fewer than three bowel movements in a week, or fiber changes you made are making the problem worse rather than better.

• Am I meant to be increasing or decreasing fiber right now?
• If low-fiber, how many grams, and for how long?
• Which of my medicines is causing this, and can it be changed?
• Should I use a soluble fiber supplement such as psyllium, or a laxative, or both?
• I have had bowel surgery. Are there foods I should avoid permanently?
• How much fluid should I drink alongside any fiber you add?
• What symptom means I stop and call you rather than adjust my food?

Try Managing Constipation During Cancer Treatment, Managing Diarrhea During Cancer Treatment, What to Eat After Diarrhea During Treatment, and Hydration Plan During Cancer Treatment.

• https://www.cancer.gov/about-cancer/treatment/side-effects/constipation
• https://www.cancer.gov/about-cancer/treatment/side-effects/diarrhea
• https://www.cancer.gov/about-cancer/treatment/side-effects/nutrition/low-fiber-foods
• https://www.cancer.gov/about-cancer/treatment/side-effects/infection
• https://www.cdc.gov/cancer-preventing-infections/patients/fever.html
• https://www.ncbi.nlm.nih.gov/books/NBK541102/
• https://medlineplus.gov/ency/article/002470.htm
• https://medlineplus.gov/ency/patientinstructions/000200.htm
• https://www.cancer.gov/publications/patient-education/eatinghints.pdf