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A Dietitian’s Guide: The functional effects of fibre

By Sascha McMeekin, Accredited Practising Dietitian, Owner and Principal Dietitian at Prosper Dietetics

What is fibre?

Dietary fibre is the carbohydrate component of plant foods that is neither digested nor absorbed in the small intestine. The main sources of dietary fibre are unrefined whole foods such as wholegrains, legumes, vegetables and fruits. Fibre supplements are many and varied. Some of the common ones are wheat bran, psyllium, guar gum, and partially hydrolysed guar gum.

Dietary fibre is well known provide benefits for gastrointestinal health as well as preventing a range of chronic diseases including heart disease, certain cancers and diabetes. Up until recently, the effects of fibre on the way your intestines function has been attributed to whether the fibres are soluble or insoluble. It has now been proposed that solubility of fibre is just one of three characteristics that influence the way fibres influence the functioning of your gut. Fibre viscosity and fibre fermentability also play a role. The role of dietary fibres in gut conditions needs more research – let me share with you what we know so far.

Fibre solubility

Description: The dissolvability of fibre in water. Insoluble fibres stay as intact particles in water whereas soluble fibres are more dissolvable.

How it affects your gut function:

Soluble fibre affects the behavior of the first part of your digestive system, your stomach and small intestine. More specifically it affects how quickly your stomach and small intestine empty and the way certain fats and sugars are absorbed by your small intestine. Insoluble fibre affects the second part of your digestive system, your large intestine or colon. It influences stool consistency and how quickly stools pass through the large intestine.

Dietary sources: Foods often contain a mixture of both soluble and insoluble fibre. Here are some examples of foods that contain mostly one or the other.

  • Predominantly soluble fibre: oats, flesh of bananas and potatoes.

  • Predominantly insoluble fibre: wheat bran, most fruit and vegetable skins, brown rice.

Fibre viscosity

Description: the degree of resistance to flow. It is easy to understand when you think about the consistency of water compared with honey. Water has a low viscosity, honey has a high viscosity. This attribute is usually associated with soluble fibres, and describes the fibres’ ability to thicken when mixed with water.

How it affects your gut function: More viscous fibres slow down the passage of food in the first part of the digestive system (stomach and small intestine). In doing so, they slow down the rate that you digest food. This not only keeps you fuller for longer but also keeps your blood sugar levels more stable and appears to be the reason why certain fibres can lower cholesterol levels.

Dietary sources:

  • Contain medium to high viscosity fibres: guar gum, psyllium

  • Contain non-viscous to low viscosity fibres: lettuce, kale

Fibre fermentability

Description: fermentability refers to the ability of gut flora to breakdown the fibre to use as fuel. The more your gut bugs are being fed fibre, the greater the beneficial byproducts such as short chain fatty acids (SCFAs). All natural plant fibres have some degree of fermentability, with only synthetic fibres being non-fermentable.

How it affects your gut function: Fermentability can indirectly affect how your gut functions through the action of SCFAs. Certain SCFAs can, among other benefits, increase the movement of your intestines (i.e. keep your regular), and regulate your appetite. A byproduct of fermentation is gases. The higher the fermentability and the more rapidly a fibre ferments the more gas that will be released at once. Gases in the intestines are not harmful, however, individuals with Irritable Bowel Syndrome (IBS) may experience abdominal pain and bloating in association with intestinal gas (learn more here).

Dietary sources:

  • Contain highly fermentable fibres: legumes (e.g. lentils, chickpeas)

  • Contain poorly fermentable fibres: leafy greens and seaweed


What does this mean for you and your guts?

Research is still in the early stages when it comes to the exact dose, type and source of fibre required to see a change in your gut function.

The National Health and Medical Research Council recommend that healthy adult women include at least 25g per day of fibre and men include at least 30g per day from a range of dietary sources.

If you have a gastrointestinal condition, it is important to seek advice regarding a diet that meets your nutritional needs while considering your medical history and gastrointestinal symptoms. If you have IBS, discuss the following fibre modifications with your Dietitian.

Constipation-prone IBS:

  • Take soluble fibre in supplement form

  • Adjust total fiber intake according to symptoms including adding naturally occurring sources

Diarrhoea-prone IBS:

  • Reduce intake of insoluble fibre

  • Include soluble fibre in supplement form


Sascha has developed her specialisation in managing IBS and gut health over the past 9 years. She is an Accredited Practising Dietitian and a Monash Certified FODMAP Dietitian.

To learn more about how Sascha can help you apply to work with her here.

For more of the latest nutrition advice for IBS visit Sascha on Instagram @ibs.gut.dietitian


Gill, S. K., Rossi, M., Bajka, B., & Whelan, K. (2020). Dietary fibre in gastrointestinal health and disease. Nature Reviews Gastroenterology & Hepatology, 1-16.


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