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Fibre’s Fantastic Functions: Fermentability, Fermentation, and Fostering Health

By Dr Brad Leech

In the realm of nutrition, the significance of a fibre-rich diet for maintaining digestive health has long been acknowledged by health practitioners worldwide. However, recent scientific insights have illuminated a broader spectrum of roles that dietary fibre plays within the body beyond its traditional association with bowel regularity. This newfound understanding underscores the necessity for a more nuanced approach to dietary fibre recommendations, one that accounts for the diverse characteristics of different fibre types.

Traditionally, dietary fibre has been classified based on its solubility in water, distinguishing between soluble and insoluble fibres. However, this classification alone fails to capture the full complexity of fibre behaviour in the body. To truly appreciate the diverse functions of dietary fibre, it is crucial to consider additional factors such as viscosity and fermentability.

The Physiochemical Characteristics of Dietary Fibre

The behaviour of dietary fibre within the digestive system is governed by its physiochemical properties, which are influenced by both its chemical composition and interactions with other compounds. These properties include solubility, viscosity, and fermentability.


Solubility refers to the ability of dietary fibre to dissolve in water, a characteristic largely determined by its chemical structure. For instance, fibres with branched structures, like pectin and gums, tend to be more soluble, whereas linear fibres such as cellulose exhibit limited solubility. However, it’s essential to recognise that solubility is not a binary trait but exists along a spectrum influenced by various factors such as temperature and pH.


Viscosity measures the degree of resistance to flow and is closely linked to solubility. Fibres that form viscous solutions, such as pectin and β-glucans, have the capacity to slow down the movement through the gastrointestinal tract. The more extensively fibres can cross-link with other structures, the higher their viscosity.


Fermentability refers to the rate and efficiency at which gut microbes break down dietary fibre through fermentation. Factors influencing fermentability include fibre size, structure, and the presence of specific bacterial enzymes. Fermentation of fibre yields short-chain fatty acids (SCFAs) and gases, which have various physiological effects within the gut, including maintaining gut barrier integrity and exerting anti-inflammatory properties.

Understanding Fermentability in Practice

In clinical practice, considering the fermentability of dietary fibre is crucial for tailoring dietary recommendations to individual needs. Rapidly fermentable fibres, like fructooligosaccharides (FOS) and galactooligosaccharides (GOS), can promote beneficial changes in gut microbiota composition and increase SCFA production. However, for individuals prone to digestive discomfort, fibres that ferment more slowly may be preferable to minimise gas formation.

Furthermore, the extent of fermentation determines whether fibres reach the distal colon intact or are fully metabolised earlier in the digestive process. Fibres that remain partially undigested can serve as bulking agents in stools, aiding in regular bowel movements.

What about Prebiotics?

Prebiotics are types of dietary fibre that feed the friendly bacteria in your gut. Unlike probiotics which are the bacteria themselves, prebiotics act as a fertiliser to help the good bacteria grow and thrive.

So what exactly are prebiotics? They are carbohydrates that resist digestion and absorption in the upper gastrointestinal tract. This allows them to pass through to the colon or large intestine where they selectively nourish and promote the growth of beneficial bacteria.

The International Scientific Association for Probiotics and Prebiotics recommends getting at least 5 grams of prebiotics per day for health benefits. Some good sources of prebiotics include:

  • Inulin and fructooligosaccharides (FOS) found in onions, garlic, bananas, artichokes, and chicory root
  • Galactooligosaccharides (GOS) found in beans, root vegetables, and some grains
  • Resistant starch found in cooled cooked starches like potatoes, pasta and rice
  • Pectin found in fruits like apples, oranges, and berries
  • Arabinoxylan found in cereal grains like wheat and rice
  • Beta-glucans found in oats, barley, and mushrooms

The gut-boosting benefits of prebiotics include:

  • Producing beneficial short-chain fatty acids like butyrate which feed colon cells
  • Reducing inflammation and oxidative stress
  • Enhancing mineral absorption
  • Improving gut barrier function
  • Promoting regular bowel movements
  • Supporting immune function

For those looking to increase their prebiotic intake, supplements are also available including inulin, FOS, GOS, resistant starch, and others. However, getting prebiotics from whole food sources is preferable when possible.


Dietary fibre encompasses a diverse array of compounds with varied physiological effects within the body. By considering factors such as solubility, viscosity, and fermentability, health practitioners can provide more tailored dietary recommendations to optimise gut health and overall well-being. Incorporating prebiotics into one’s diet further supports a thriving gut microbiome, highlighting the importance of dietary fibre in promoting optimal health.

More about the author

Dr Brad Leech

Dr Brad Leech is a PhD-qualified Clinical Nutritionist and Herbalist specialising in chronic autoimmune conditions and complex gastrointestinal disorders. He provides complete and personalised care to his patients using functional nutrition, integrative medicine and holistic wellness. 

After entering the profession in 2008, Brad has taught and developed subjects at leading universities and conducted research on intestinal permeability, autoimmune disease management and food-based probiotics.

Brad is the Lead Clinical Educator at Co-Biome by Microba where his expertise in gastrointestinal healthcare enables him to translate the latest science on the gut microbiome into practical clinical applications. 

In addition to being an Adjunct Fellow at the National Centre for Naturopathic Medicine, Brad offers practitioner support through his multiple mentoring programs.