Why we love the f-word: is fermentability the key to understanding fibre?

Author: Christine Stewart

September 2021



Practitioners worldwide have long understood that a diet rich in dietary fibre is healthy and great for promoting regular bowel motions. However, we now know that dietary fibre has many other uses in the body beyond laxation. The way different fibres behave during digestion can be explained by their characteristics of solubility, viscosity and fermentability. Hence the new realisation that not all fibres are the same!

While the Nutrient Reference Values for Australia and New Zealand provide recommendations on the total amount of dietary fibre to be consumed, the time has come for health care practitioners to provide more detailed guidance on types of dietary fibre for clinically meaningful outcomes.

Understanding the chemical structures of various fibres is central in explaining their actions in the gut and can play an important role in clinical decision making. Traditionally, fibre has been classified according to its ability to dissolve in water (soluble vs insoluble) however, this method alone has its limitations. When it comes to clinical practice, considering other fibre characteristics such as fermentability and viscosity are equally important, as these properties can determine the therapeutic effect of fibre consumption1.


Physiochemical characteristics of dietary fibre

Solubility, viscosity and fermentability of dietary fibres are primarily determined by chemical structure but also from interactions with other cell wall compounds, digestive processes and even food processing2.


The solubility of dietary fibre largely depends on its ability to interact with water molecules2,3. Fibre structures that are branched (such as pectins and gums) are more likely to interact with water molecules, making them soluble3. In contrast, linear fibres with ordered structures as seen in cellulose (Fig. 1), have limited solubility3. An easier way to think about this might be to compare how pectin rich jam would behave in a glass of water compared to a piece of cellulose rich lettuce. In truth, classifying fibres in this way is arbitrary as it does not reflect the spectrum of solubility seen in dietary fibre – possibly because solubility can depend on many factors such as temperature or pH3.


Viscosity refers to the degree of resistance to flow (i.e. slowing the movement of digesta through the GI tract) and is typically associated with the solubility of fibres2. Pectins, β-glucans and psyllium are good examples of fibres that are both viscous and soluble2 (See Table 1). One mechanism for determining viscosity is the capacity of the fibre structure to cross link with other structures. The concept of crosslinking is well understood by primary school teachers: if you imagine a playground full of children running around, the ‘flow’ would be fast, but if the teacher asked the children to hold hands (cross link), their movement would be restricted and slowed. The more links made, the harder it would be for the children to run around freely. Fibres with the capacity to cross link such as β-glucans and pectin show high viscosity.


Fermentability of dietary fibre refers to how rapidly or efficiently gut microbes are able to breakdown fibre – a process known as fermentation. The fermentability of fibre depends on the size and structure of the molecule as well as the presence of bacteria with the enzymes required to digest the bonds within the fibre structure4 (Fig. 1). Interactions between the characteristics of fibre can affect fermentability. For instance, solubility typically increases fermentability, while viscosity decreases fermentability by impacting the ability of microbes to access and digest the fibre structure.

Figure 1. Physicochemical structures of various dietary fibres5 (Williams et.al, 2017)

Why fermentability matters!

Most fibres are, in essence, a collection of monosaccharides (glucose, fructose, xylose etc.) arranged in various chemical structures (BioChem flashbacks anyone?). These structures may resist digestion in the small intestine because they are linked with β linkages, which, unlike α linkages can’t be digested by human enzymes. Bacteria are able to ferment dietary fibres because they produce enzymes that can break the β bonds which human digestive enzymes can’t access. An exception to this is resistant starch which is linked by α bonds but has a structure which is too complex to allow complete digestion within the small intestine’s transit time. The fermentation of resistant starch in the large intestine relies on the presence of specialised bacteria which can access these complex structures, such as Ruminococcus_E bromii.

The end products of fibre fermentation include the creation of short-chain fatty acids (SCFAs) and gasses such as hydrogen and carbon dioxide which lower luminal and faecal pH6. The benefits of lowering pH in the colon include inhibition of pathogen growth, reducing protein degradation and lowering activity of less desired bacterial enzymes6. SCFA’s support integrity of the gut epithelium as well as playing anti-inflammatory roles in the body2. Ensuring your patient is consuming sufficient amounts of fermentable fibre is an important consideration for clinicians aiming to improve patient health through modulation and restoration of the gut microbiome.


Understanding fibre degrading potential

Luckily for us, metagenomic sequencing has opened the door for practitioners to learn the capacity of their patients’ microbiome to ferment fibre. The fibre degradation potential within the Insight™ report provides information on the overall capacity of the microbiome to break down fibre. In the Insight™ report, an average-to-high potential to break down fibre is considered beneficial, while a low potential indicates that the microbiome may not contain sufficient enzymes to efficiently ferment dietary fibre.

If your patient’s report is showing a low or average potential to degrade fibre, it is suggested you address your patient’s fibre intake. Utilising the FFQ and the Insight Prebiotic Reference Guide and can assist you in selecting the right options for your patients.


Patient showing low or average potential to degrade fibre?
Download our Prebiotic Reference Guide.



A recent paper7 investigated two microbial targeted diets, one high in fibre and the other high in fermented foods. After 10 weeks of dietary intervention, the high-fibre diet increased the microbiome’s capacity to ferment fibre despite microbial diversity remaining stable. This is a great example as to how diet can favourably influence and modulate the functional potential of the gut microbiome.


Microba’s food frequency questionnaire (FFQ) can provide practitioners with an estimation of their patient’s daily fibre intake. While helping your patient increase their total fibre intake may improve gut health, understanding the characteristics of different dietary fibres will help identify the ideal dietary sources of fibre for your patient.

Find out more about our Food Frequency Questionnaire


Considering fermentability in practice

Rapidly fermented fibres such as fructooligosaccharides (FOS) and galactooligosaccharides (GOS) are an easy fuel source for gut bacteria – they love it! Increased fermentability bestows the benefits of altering abundances of specific bacteria and boosting SCFA production4 – which we love! However, fibres that are readily broken down can also lead to accelerated gas formation, increased flatulence and digestive discomfort in sensitive individuals9 (who probably don’t love that!).

Fibres with medium fermentability are not broken down as swiftly, slowing the formation of gas – some happy news for patients experiencing painful bloating. Additionally, once a fibre has been fully fermented, it’s gone,9 meaning gut bacteria in the distal colon rely on fibres with slower fermentation rates which are more likely to reach them before being fully fermented (See Fig. 2). Lastly, poorly fermented fibres will typically pass through the gut untouched and act more like bulking agent in stools2.

Considering the fermentability can reveal the ideal fibre/s for your patient depending on their gut symptoms, bowel habits, medical conditions, tolerance, and microbiome composition.

The Insight™ microbiome test reveals your patient’s microbiome capacity to degrade dietary fibre. A low potential indicates that the microbiome may not contain sufficient enzymes to efficiently ferment dietary fibre and create beneficial SCFAs. While increasing total dietary fibre intake has been shown to increase the microbiome’s capacity to utilise fibre, providing more individualised advice on fibre types is required to get the optimal benefit without adverse gut symptoms. From a bacteria’s perspective, rapidly fermented fibres act as an easy fuel source while more slowly fermented fibre can feed the bacteria in the distal colon, slowing gas production. By understanding the roles various fibres play, practitioners can begin to personalise fibre-based recommendations according to the desired therapeutic application.

Table 1. Physiochemical characteristics of common fibres2,8

About the author


Christine Stewart

Christine Stewart is a nutritionist and registered nurse with experience across the medical (hospital), nutrition and public health industries. She brings more than 15 years of experience of working with patients to her role as a Clinical Application Specialist with Insight™. Christine has a Master’s Degree in Human Nutrition from Deakin University and a Bachelor of Nursing from Griffith University. She has a passion for educating and supporting microbiome restoration through the adoption of positive eating behaviours and works closely with healthcare professionals to implement gut microbiome analysis in practice within Australia.




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