why is fibre so important

24 August 2020

Although many studies on fibre simply look at the association between total dietary fibre consumption and certain health markers, not all fibre has the exact same function, as such, it is important to consume a variety of fibres rather than just one. 

Firstly -  let me explain what fibre actually is, very basically it’s a carbohydrate that as humans, our digestive system cannot digest. Being a carbohydrate, it’s plant foods - vegetables, fruits, grains, beans, legumes, nuts seeds, and - that  contain fibre, of which there are two main categories:

  • Soluble fibre – This type mixes with water to create a gel-like substance in your gut. It aids in many areas of health, including helping to keep you full longer and reduce blood sugar spikes.

  • Insoluble fibre – Generally the type that passes through as is, being used to bulk up your stool to aid passing a stool and elimination. 

Fibre can also be either fermentable or non-fermentable. The fermentable form provides a “food’ for  bacteria in the colon which produce health giving biologically active by-products called short chain fatty acids (SCFA). Some of these SCFA’S stay close to home in the gut, but others travel far and wide throughout the body, taking part in complex interactions that produce various effects on health and are the subject of active scientific study:- .

  • In the gut they serve as a source of energy for cells inside the colon.

  • SCFAs also have an effect on how energy is metabolised in the body, and thus, a possible protective effect against metabolic disease and obesity

  • Another influence of SCFAs is on the immune system. Research shows that SCFAS (in particular butyrate) have anti-inflammatory effects and seem to play a part in kick-starting the specialisation of immune cells that help ‘keep the peace’, called regulatory T cells.

Some of the more common types of fibre include:

  • Beta-glucans – found in cereal grains and mushrooms

  • Cellulose – found in plant cell walls (remember your Biology O level!)

  • Fructan – found in cereal grains like wheat and rye (a FODMAP)

  • Galacto-oligosaccharides (GOS) – found in onions, garlic, legumes and lentils (a FODMAP)

  • Glucomannan – a water-soluble polysaccharide found in the cell walls of some plants

  • Gums – acacia gum, guar gum, locust bean gum, xanthan gum, carrageen

  • Inulin – a type of fructan; found in onions, chicory root, and Jerusalem artichokes

  • Lignin – found in plant cell walls and algae

  • Pectin – found in beetroot, citrus peels, apples, peaches, pears, and cherries

  • Psyllium – a seed 

Many with IBS and other digestive disorders will look at this list and immediately pick up on some fibres they struggle with, often the FODMAP foods. 

What are FODMAP’S

FODMAPS’s are again carbohydrates, that are rapidly fermented: 

F stands for fermentable – and refers to indigestible carbohydrates producing gas. It’s a more general overview of symptom-causing foods.

Oligosaccharides – includes fructans and galacto-oligosaccharides (GOS) in foods like wheat, onion, garlic and beans as described previously.

Disaccharides –particularly means lactose (mostly in milk, soft cheese and ice cream)

Monosaccharides – pertains to fructose, often found in apples, pears and honey. Fructose in excess of glucose in certain fruits and honey is what causes the problem. 


Polyols – includes sugar alcohols, such as sorbitol and mannitol, which are found in some stone fruits( like peaches, apricots or cherries), veggies (like mushrooms) and gum.

However, studies have demonstrated that fibre, especially soluble fibre, can help with functional digestive conditions as well, so do not completely avoid all fibre.  It is about finding the type and amount that works for you! 

And finally, what is resistant starch?

While resistant starch is not technically fibre, it can act like fibre because it does not get digested.

Resistant starch is considered a prebiotic fibre. Prebiotic by definition is, “a non-digestible food ingredient that beneficially affects the host by selectively stimulating the growth and/or activity of one or a limited number of bacteria in the colon, and thus improves host health.”

CK Yao, a Monash University researcher talks about her recent research, “I showed in our laboratory experiments using human faecal microbiota that resistant starch and fructans actively and effectively suppressed hydrogen sulfide gas production by 50-70%. Hydrogen sulfide which is usually a by-product of really high sulfur-protein (or animal protein) intake (e.g. if you are body building and consuming a high protein diet + supplementing with whey protein). Both carbohydrates (RS and fructans) are rapidly broken down and so we think they do so by shifting fermentation away from proteins.”  

Bacteria tend to prefer carbohydrates as their primary fuel, but if it is not present in the colon then will ferment protein. Unfortunately, the by-products of protein fermentation tend to have negative consequences in the gut.

Types of resistant starch:

  1. RSI, type I resistant starch (example: whole or partially milled grains, seeds, and legumes, muesli) Thick outer cell walls of legume seeds and protein matrix of whole grain cereals prohibit water penetration into the starch. 

  2.  RSII, type II resistant starch (example: raw potato, raw banana starch, green bananas)
 Cooking alters the starch content, however, and it becomes highly digestible, partly due to starch gelatinisation…. So the next “type” is more practical 

  3. RSIII, type III resistant starch (example: Cooked and cooled starchy foods–think cooked and cooled rice salad or cooked and cooled potato salad).

So, this shows, that  one man’s medicine  is indeed another man’s poison. Following a highly restrictive low FODMAP diet model for longer than 6 weeks is going to have negative  impacts on the hosts’ microbiome diversity as it  reduces intake of key prebiotic fibres, fructans (found in onion, garlic, and wheat) and galacto-oligosaccharides (found in beans, cashews and pistachio nuts), the “O” group in FODMAPs or oligosaccharides. Always work with a practitioner where you are guided and re-introduction of different FODMAP’s encouraged. 

Dr. Rob Knight of  The American Gut project  has listed eleven factors that optimise the gut microbiome -  a KEY finding was that the more vegetables consumed (30 different each week is BEST), the more diverse the microbiome, and that is thought to be associated with health and improved immune status since MANY chronic diseases (see the below Table) have changes in microbiome diversity AND composition [Cantinean et al 2018].






Inflammatory bowel disease

Less bacterial diversity ⬇️  the number of Bacteroides and Firmicutes

Decreasing the concentration of Butyrate

Lucas López et Al (2017)

Irritable bowel syndrome - diarrhea

⬆️  Enterobacteriaceae ⬇️ Faecalibacterium prausnitziiq

Not Known

Dupont (2014)


⬆️ Firmicutes (Lachnospiraceae and Ruminococcaeceae) ⬇️  Bacteroidetes (Prevotella)

Increasing the production of butyrate

Zhu et al (2014)


Changes in the ratio of Bacteroidetes / Firmicutes ⬇️ the abundance Akkermansia muciniphila ⬆️ the abundance Campylobacter, Shigella, Prevotella

Decreasing the production of butyrate

Festi et al (2014), Tremaroli & Bäckhed (2012)

Diabetes T2

⬇️  Bifidobacterium spp significant association of Parabacteroides with diabetic patients

not known

Wu et al. (2010)

⬇️  Firmicutes ⬆️ Bacteroidetes, Proteobacteria It is possible to determine endotoxemia ➡️ oxidative stress ➡️  IL1, IL6, TNF Marlene (2013)

Diabetes T1

⬇️  Lactobacillus, Bifidobacterim, Blautia coccoides-Eubacterium rectale, Prevotella

decreasing the production of butyrate decreasing the synthesis of mucin increasing the intestinal permeability

Murri et al. (2013)

⬇️  Clostidium clusters IV and XIV (species that produce butyrate) decreasing the production of butyrate De Goffau et al. (2014)


⬇️ Lactobacillus

Decreasing Enzymatic

Kumar et al (2012), Ramakrishna (2012)

The Mighty Microbiome Diet Challenge, as part of my Gut to Know online programme ticks off 30 different veg each week! And has been designed to boost prebiotic fibres. In a low-ish FODMAP manner - as always I’m following the science to best help my clients. 

In health,

Tanya x