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Nutrition and daily life

Fibre: the gut’s quiet helper

Fibre is the part of plant food that the body cannot digest. It provides no calories, but it keeps the bowels regular, feeds helpful bacteria and helps prevent constipation. During treatment, however, the gut can be sensitive. This topic explains what fibre is, and when to eat more of it and when less.

Last updated: ~4 min read
On a dark navy background, a bowl of oat porridge, a bowl of cooked green lentils, a slice of wholegrain bread, cooked carrot and broccoli, a pear and a few dried apricots.
Figure 1.Fibre-rich foods: oats, lentils, wholegrain bread, vegetables and fruit.

01Short answer

Fibre is found in whole grains, legumes, vegetables, fruit, nuts and seeds. Soluble fibre turns into a gel in water; insoluble fibre adds bulk to the stool. Both matter for gut health. During treatment, some medicines (such as painkillers and anti-sickness drugs) can cause constipation, while others cause diarrhea. With constipation, slowly increasing fibre and drinking enough fluid helps; with diarrhea or colitis, fibre may be reduced for a while. Vegetables and fruit must be washed carefully, and during neutropenia they are often eaten cooked.

02Simple explanation

Fibre is made of the substances that form the walls of plant cells. The human gut cannot digest them.

Soluble fibre (oats, legumes, apples) forms a soft gel in water, softens the stool and slows the entry of sugar into the blood.

Insoluble fibre (bran, whole grains, vegetable skins) adds bulk to the stool and helps it move along faster.

The helpful bacteria of the large bowel feed on fibre and make substances that protect the bowel wall.

During treatment the gut can be sensitive, so the amount and form of fibre (cooked, puréed) is adjusted to how you are.

03Scientific explanation

General advice: the 2023 WHO guideline recommends at least 25 g of naturally occurring fibre a day for adults; the NHS suggests about 30 g. These figures are for healthy people.

Constipation: according to NCI, causes of constipation during cancer treatment include opioid painkillers, some anti-sickness and chemotherapy drugs, little movement and low fluid intake. Fibre, fluids and movement help, and doctors prescribe laxatives when needed.

Diarrhea: NCI and Cancer.Net advise switching for a short time to soft, low-fibre food during diarrhea and replacing lost fluid. Diarrhea can be caused by infection, so diarrhea that lasts must be reported to a doctor.

Food safety: the FDA recommends washing raw fruit and vegetables well under running water. For people with weak immunity, many centres advise eating vegetables cooked or peeled during neutropenia.

04What happens in the body?

Where is fibre found?

Oats, buckwheat, wholegrain bread, lentils, chickpeas, beans, vegetables, fruit, dried fruit, nuts and seeds. Animal foods (meat, fish, eggs, milk) contain no fibre.

What happens in the gut?

Fibre passes through the stomach and small intestine unchanged. In the large bowel, bacteria ferment soluble fibre, while insoluble fibre adds bulk to the stool.

On a dark navy background, a translucent section of the large bowel with soft teal gel clouds and fine golden strands moving inside, small bacteria glowing around them.
Figure 2.Soluble and insoluble fibre do different jobs in the gut.

With constipation

Increase fibre slowly: oat porridge, stewed prunes, cooked vegetables. Drink enough through the day and walk as much as you are allowed. If it does not help within a few days, tell your team.

With diarrhea

For a while, soft low-fibre food: white rice, mashed potato, banana, white bread, soup. Cut back for a time on raw vegetables, bran, legumes and fruit with skins. Replacing fluid and salts is important.

On a dark navy background, a plate of smooth white rice porridge, a sliced peeled banana, a cup of clear chicken broth and a slice of white toast.
Figure 3.On hard days, soft low-fibre food is gentle on the gut.

How to soften fibre

Cooking vegetables until soft, puréeing them, peeling fruit, and choosing oats instead of bran bread make fibre gentler and reduce wind and bloating.

Safety

Fibre-rich vegetables and fruit are in contact with soil. Wash them well under running water and during neutropenia often eat them cooked or peeled. Avoid raw sprouts.

05Fibre: four steps

What it is, what it does, when to soften it, always with water.

The plant’s skeleton — The part we cannot digest.1The plant’s skeletonThe part we cannot digest.
Feeds bacteria — Protects the bowel wall.2Feeds bacteriaProtects the bowel wall.
Soften when needed — With diarrhea and colitis.3Soften when neededWith diarrhea and colitis.
With water — It does not work without fluid.4With waterIt does not work without fluid.
Regular bowelsLess constipation.
Healthy gut floraHelpful bacteria are fed.
ComfortLess bloating and pain.

06Fibre: choices by situation (simplified)

SituationBetter choicesReduce for a while
ConstipationOats, cooked vegetables, stewed prunes—
DiarrheaWhite rice, banana, mashed potatoBran, raw vegetables, legumes
Bloating, windCooked and puréed vegetablesCabbage, beans, onion
Mouth soresSoft porridge, puréesCoarse bread, seeds
NeutropeniaWell-washed, cooked vegetablesRaw sprouts, unwashed fruit
RecoveryGradually whole grains and legumes—

* The table is general information. With bowel problems, your team decides on your individual diet.

07Key terms

Fibre
The indigestible part of plant food.
Soluble fibre
Fibre that forms a gel in water.
Insoluble fibre
Fibre that adds bulk to the stool.
Gut flora
Helpful microbes living in the gut.
Constipation
Infrequent, difficult or painful bowel movements.
Diarrhea
Frequent, watery stools.
Colitis
Inflammation of the large bowel.
Low-fibre diet
Temporarily reducing fibre to rest the bowel.

08Key points

  • Fibre keeps the bowels regular.
  • Increase fibre gradually.
  • Drink enough fluid with fibre.
  • With diarrhea, fibre is reduced for a while.
  • Cooking and puréeing make fibre gentler.
  • Wash fruit and vegetables thoroughly.
  • Avoid raw sprouts.
  • Tell your team about lasting constipation or diarrhea.

09Frequently asked questions

Will bran help constipation?

It can, but only with enough fluid and introduced slowly. A lot of bran at once causes bloating and pain. Constipation from painkillers often also needs a laxative prescribed by your doctor.

Can I eat fruit with diarrhea?

Banana and peeled baked apple (purée) are usually well tolerated. Cut back for a while on fruit with skins and seeds and on dried fruit.

Can I take fibre powder?

Sometimes a doctor recommends it. It can affect how some medicines are absorbed, so ask your team first.

Can I eat raw salad?

During neutropenia cooked vegetables are often advised. With normal counts, well-washed fresh vegetables are usually fine. Follow your centre’s rules.

Legumes give me wind. Should I stop eating them?

There is no need to stop. Start with small portions, soak and cook them well and purée them. If bloating is strong, cut back for a while.

Is fibre important for children too?

Yes, but the amount depends on the child’s age and condition. The paediatric oncologist and dietitian will advise.

10Questions for your doctor

Take these questions to your appointment. The ones you save are collected on the “My questions” page and stay on this device only.

  1. Will bran help constipation?

  2. Can I eat fruit with diarrhea?

  3. Can I take fibre powder?

  4. Can I eat raw salad?

  5. Legumes give me wind. Should I stop eating them?

  6. Is fibre important for children too?

My questions

12Sources

  1. MedlinePlus (U.S. National Library of Medicine). Dietary Fiber
  2. NHS. How to get more fibre into your diet
  3. World Health Organization (WHO). Carbohydrate intake for adults and children: WHO guideline (2023)
  4. Harvard T.H. Chan School of Public Health — The Nutrition Source. Fiber
  5. British Dietetic Association. Fibre: Food Fact Sheet
  6. National Cancer Institute (NCI). Diarrhea and Cancer Treatment
  7. National Cancer Institute (NCI). Constipation and Cancer Treatment
  8. National Cancer Institute (NCI). Eating Hints: Before, During, and After Cancer Treatment
  9. American Cancer Society. Eating Problems
  10. Cancer Research UK. Diet problems and cancer
  11. Macmillan Cancer Support. Eating problems
  12. U.S. Food and Drug Administration (FDA). Selecting and Serving Produce Safely
Medical disclaimerThis information is for educational purposes only and does not replace medical advice.
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