Dietary Fibre: A Dietitian’s Guide

Fibre sources

What is Fibre?

Fibre is technically a form of carbohydrate. It consists of long strands of glucose, arranged in a way that the body’s digestive enzymes are unable to break down.

Fibres’ resistance to digestion is due to how to it is organised in the grain, or, the way glucose is chemically bonded to itself. Either way, your enzymes are unable to breakdown the long strands into molecules small enough for the body to absorb.

Unlike other nutrients, fibre moves through the gastrointestinal system un-digested and unabsorbed until it reaches the large intestine. Here, gut bacteria ferment fibre into by-products that are healthful and absorbed.

Fibre has a unique role in the maintenance of overall gut health and it also is responsible for other benefits which many are unaware of. Fibre is protective against colon cancer, high cholesterol, coronary heart disease, type two diabetes, obesity and diverticular disease.

Let’s explore fibre’s activities in the body and how it is able to be protective against so many conditions.

But There Seem to Be Conflicting Messages About What Fibre Does

Let’s clear this up, shall we? There are three main types of fibre: soluble fibre, insoluble fibre and resistant starch.

Soluble Fibre – Constipation, Diarrhoea & Cholesterol

Soluble fibre is found hiding away in the inside part of the plant e.g. the flesh of a fruit. The reason it is called soluble fibre is because of its ability to absorb water and form a gel. When this ‘gel’ material comes into contact with the stool in the large bowl it softens it, allowing one to pass the stool easier.

When a person is constipated, passing a hard stool can be very uncomfortable and even painful, so eating plenty of soluble fibre foods should help with this discomfort and also reduce the future risk of constipation.

Nonetheless, if you are experiencing diarrhoea, soluble fibre can also assist with that. Soluble fibre helps thicken the stool and slows transit time through the bowel, allowing it to pass as a firmer stool.

Soluble fibre effects on cholesterol

Another wondrous function of soluble fiber is its ability to bind to fatty acids and cholesterol that are floating around in the gut. Once bound, fatty acids and cholesterol are unable to form a structure called a micelle, the next step of their metabolism.

Due to the fatty acids and cholesterol not being able to form micelles and progress with metabolism, they are sent to the large intestine for either fecal excretion or colonic degradation.

The excretion of bile and cholesterol leads to a decreased cholesterol content of liver cells. This, in turn, promotes the removal of LDL (bad) cholesterol from the blood.

Soluble fibre includes food such as:

  • Bananas
  • Oats and oat bran
  • Barley
  • Chia seeds
  • Legumes and pulses (e.g. kidney beans, chickpeas)
  • Okra, eggplant
  • Psyllium (Metamucil)
  • Benefiber

Insoluble Fibre – Constipation

Insoluble fibre is found in the outer layer of the plant e.g. in the husk or bran of a grain.

From its name, it is obvious that insoluble fibre does not absorb water and hence acts very differently to how soluble fibre acts.

A defining characteristic of insoluble fibre is its ability to increase gut motility and decrease transit time.

Insoluble fibre is quite rough in texture and it acts by pushing along the faecal bulk in the large bowel.

By pushing along or accelerating the faecal bulk, insoluble fibre, in turn, decreases the time that the bulk is spent in the large bowel. The reason this is important is that our waste and toxins are packed into this bulk and the longer they are in contact with the bowel wall, the more damage they can cause.

Although we are after a quicker transit time, we don’t want the other extreme of a super quick passage, as otherwise, things like water and electrolyte absorption and other functions of the large bowel will not be able to be performed. 

Food which are high in insoluble fibre include:

  • Fruit (skin and seeds have the most fibre)
  • Vegetables (skin and seeds)
  • Whole-grain products e.g. bread, pasta, brown rice, quinoa
  • Nuts and seeds
  • Wheat and rice bran

Resistant Starch

Resistant starch is comprised of (mostly) carbohydrate polymers which resist digestion in the small bowel and is transported to the large bowel where it provides a substrate for either partial or complete fermentation.

When resistant starch ferments, it is converted into what are called short-chain fatty acids (SCFA). One of the SCFA’s that are produced is called butyrate.

Butyrate or, butyric acid provides an energy source for colonic cells. It also acts as an anti-inflammatory agent keeping the colonic cells healthy.

Another mechanism of butyrate is cell cycle arrest (stopping the transformation of cells) and apoptosis (cell death) of transformed colonocytes.

The way it performs this is via inhibiting the action of histone deacetylase which is an enzyme that affects gene transcription. However, the jury is still out on whether this is, in fact, protective against colon cancer, so stay tuned!

The presence of these short-chain fatty acids in the colon causes a decrease in the pH of the colon’s environment. A decreased pH means a more acidic environment.

Whilst this might sound bad, an acidic environment causes bile acids to be less soluble and hence less able to undergo metabolism. As a result of this, fat is not fully metabolised or absorbed.

As well as this, a more acidic environment allows calcium to become more available to bind with bile and fatty acids. Once bound, these fatty acids and bile are excreted. This process may also be protective against colon cancer.

What Else Can Resistant Starch Do?

Another trick resistant starch has up its sleeve is its ability to act as a prebiotic.

A prebiotic is a non-digestible food ingredient that stimulates the growth and activity of beneficial strains of bacteria in the colon.

The growth of beneficial bacteria improves the health of the host… The host, in this case, is the human in question.

Food high in resistance starch include:

  • Cooked and cooled potato, rice, quinoa, and pasta
  • Unripe banana and green banana flour
  • Legumes
  • Oats

Is Fibre Ever Bad for Me?

Fibre is never ‘bad’ per se, however, each type has his strengths and weaknesses.

Excessively high fibre intake, or quick increases in fibre intake, can lead to gastrointestinal symptoms like bloating and gas. 

Therefore if you are increasing fibre intake, it makes sense to do so in a slow and controlled fashion.


One circumstance in which a certain type of fibre is advised against is during a diverticular disease flare-up or, diverticulitis.

Diverticular disease is when people to develop little pockets in their large bowel. These pockets or sacs can become inflamed during a flare or diverticulitis.

A healthy diet high in fibre and water is generally the dietary recommendation for this condition.

During aflare-up however, limiting insoluble fibre foods is recommended as they can further irritate the bowel. Once symptoms subside, those insoluble fibre containing foods should be reintroduced.

Diverticulitis infographic

Irritable bowel syndrome or IBS is a condition of the gut experienced by many people. The cause is unknown and unfortunately, there is no known cure. Although there are certain dietary strategies that can potentially help

It presents differently in each case though the most common symptoms are bloating and diarrhoea or constipation.

Having a sound knowledge of the types of fibre and how to manipulate them in your diet could give you an edge over your individual symptom control.

We know that insoluble fibre can speed up gut motility, then we wouldn’t want to be adding too much of that into our diet if diarrhoea is an issue.

Similarly, if one is constipated and finding it difficult to pass a stool, then including more soluble fibre and water into your diet could help to soften the stool making it easier to pass.

Food Sources of the Different Types of Fibre

Soluble: Oats, psyllium, legumes, fruit flesh, vegetable flesh

Insoluble: Bran, wholegrain bread and wholegrain crackers skins of fruit and vegetables

Resistant starch: The content of resistant starch in each food is different as is determined not only by the type of food but also by things such as whether the food has been cooked or not, how much the food has been cooked, whether it has been cooled after cooking as well as by physical manipulation of the food i.e. chewing.

For example, cooked and cooled rice, pasta and potatoes: the cooking and cooling process causes starch molecules to become inaccessible to enzymatic hydrolysis (breakdown), so they pass into the large colon undigested and exert the above beneficial effects.

Other resistant starch-containing foods include raw green bananas (as bananas ripen starch breaks down), wholegrain foods as well as Hi-Maize (corn starch) containing products e.g. Wonder White bread, Sanitarium Up and Go’s, and Uncle Toby’s fruit bars.

How Much Fibre Should I Be Having Daily?

In general, adults should be having 25g (women) or 30g (men) of fibre/day.

This is a general recommendation though and there are certainly benefits to going higher or lower based on your personal needs and preferences. 

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