What is fibre?

Fibre is a complex group of carbohydrates, which are found in plant-based foods, or derived from them, which our digestive system cannot fully break down, or absorb, and so they pass through the small bowel unchanged into the large bowel where our gut microbiota live. 

Carbohydrates are classified by how large they are, sugars are 1-2 units, oligosaccharides are 3-9 units and polysaccharides are 10 or more units (Hijová et al, 2019). Polysaccharides are often not, or partially digestible due to their length and bonds, some oligosaccharides are also non-digestible, these are resistant oligosaccharides (ROS). As a rough definition, it is these non-digestible carbohydrates that we define as fibre (Hijová et al, 2019).

Fibres are also prebiotics, meaning a food component that arrives in the large bowel unchanged to the benefit of our gut bugs, the microbiome, and by extension, us too (Gibson and Roberfroid, 1995). In short, when we consume fibre, or ROS, it’s a buffet for our belly bugs - partially ones that we think are beneficial, such as bifidobacteria maintaining an overall healthy microbiome (de Vos et al., 2022). They can also break down bonds that we can’t, feeding themselves, but also producing some beneficial products (de Vos et al., 2022)

The products of this are short chain fatty acids and gases such as methane, carbon dioxide and hydrogen. It’s thought that these products are good for our health (Barber et al., 2020; de Vos et al., 2022). However, this gas can actually be a problem if fibre is increased too much, too quickly, as it might lead to bloating, wind and some abdominal cramping. The rule then is to increase gradually to our target. 

What is the target? A target of 14g per 1000 calories consumed can be used, but there are also specific daily targets too, which vary depending upon the source, age and sex of the individual. To make things simpler though, anyone over the age of 16 years old can set a fibre goal of 30g/day or more. (McKeown et al., 2022). Though, it should be remembered that this is simply a goal, there are likely benefits above this amount too, but this should be balanced with your unique needs, diet and response to dietary fibre. If you currently consume far less fibre than this, which is most people, then it is a good idea to slowly, over weeks, increase your daily intake - adaptation does occur, but this is a process that takes weeks to months (McKeown et al., 2022).

Dietary fibres

Cellulose and hemicellulose
Found within the cell wall of plants, such as glucomannan, these can be found in grains, vegetables, fruit, nuts and cereal bran. Latter in legumes too.

Lignin 
Found within woody plants and seeds cell wall, this non-carbohydrate component of cells walls can be found in celery or outer layers of grain cereal.

Pectins 
A non-starch soluble fibre found in fruits, vegetables, legumes, nuts and 
potatoes.

Beta-glucans
Glucose polymers with a branched structure, mainly found in oats and barley

Fructans 
Soluble fibres such as Inulin, found in onions.  

Resistant starches
These include carbohydrates that have been cooked, cooled then reheated again such as rice and pasta. 
They are also naturally present in grains, pulses, unripe bananas and potatoes.

Functional fibres (isolated / synthetic) 

Polyols 
Sugar free food substitutes 

Gums and mucilages
Non-starch fibres, that form thick gels, which hold cell walls together. 
Seeds, seaweed, psyllium seeds 
Often used as thickeners, stabilisers and emulsifying agents

When reading nutrition labels remember: ‘Source of fibre’ refers to any food that contains at least 3g per 100g, or 1.5g per 100 kcal, and high fibre is 6g per 100g, or 3g per 100 kcal. Interestingly fibre has around 2kcal per gram, despite being largely indigestible, however, it is not mandatory for food labels in the EU to include the calories of fibre. 

Why is fibre important?

Adequate fibre intake may reduce the risk of developing cardiovascular disease, type 2 diabetes mellitus (T2DM), constipation, cancer and a whole boat-load more (Barber et al., 2020; Threapleton et al., 2013). Soluble (dissolves in water) fibres pull water into the gut when ingested forming a thick gel, and some of these, such as psyllium, b-glucans (oats) are not fermented so they pass into the stool. These gels can trap cholesterol from the diet (and cholesterol we produce) meaning the liver must capture some of the LDL-cholesterol (‘bad’ cholesterol) in the blood and use it, lowering our cholesterol levels. In addition, the other nutrients within the food are trapped too. This means it takes longer for food to digest, so emptying of the stomach and bowel is slowed, hormones like GLP-1 are released, keeping us fuller, for longer. This trapping of molecules, some of which may be toxic to our bowel wall cells (colonocytes), may also explain, in part, why increased dietary fibre is associated with lower rates of bowel cancer. In addition, the gel by pulling in water bulks up the stool and presses on the bowel, directly stimulating movement.

Other soluble fibres are fermented, such as pectins, inulin, or resistant starches. They act as fuel for the bacteria, or prebiotics as mentioned above. This produces those fatty acids, like butyrate and acetate and the aforementioned gases, providing energy to cells of the bowel wall. Again, this may be providing protection against bowel cancer and there is likely a host of benefits for the rest of the body (de Vos et al., 2022). 

Finally we have the insoluble (does not dissolve in water) fibres, often called ruffage, these fibres are not fully fermented and do not form a gel. As a result they ‘sweep’ through the gut, directly irritating the bowel wall, stimulating mucus and water secretion. This promotes stool formation and motility of the gut. In other words, we poop. However, given these fibres can irritate the bowel, some people may be very sensitive to these types of fibres. 

References

1. Barber, T.M., Kabisch, S., Pfeiffer, A.F.H. and Weickert, M.O. (2020). The health benefits of dietary fibre. Nutrients, 12(10), p.3209. doi:https://doi.org/10.3390/nu12103209.

2. de Vos, W.M., Tilg, H., Van Hul, M. and Cani, P.D. (2022). Gut microbiome and health: mechanistic insights. Gut, [online] 71(5), pp.1020–1032. doi:https://doi.org/10.1136/gutjnl-2021-326789.

3. Ghosh, T.S. and Valdes, A.M. (2023). Evidence for clinical interventions targeting the gut microbiome in cardiometabolic disease. BMJ, [online] 383, p.e075180. doi:https://doi.org/10.1136/bmj-2023-075180.

4. Gibson, G.R. and Roberfroid, M.B. (1995). Dietary Modulation of the Human Colonic Microbiota: Introducing the Concept of Prebiotics. The Journal of Nutrition, 125(6), pp.1401–1412. doi:https://doi.org/10.1093/jn/125.6.1401.

5. Hijová, E., Bertková, I. and Štofilová, J. (2019). Dietary fibre as prebiotics in nutrition. Central European Journal of Public Health, 27(3), pp.251–255. doi:https://doi.org/10.21101/cejph.a5313.

6. McKeown, N.M., Fahey, G.C., Slavin, J. and van der Kamp, J.-W. (2022). Fibre Intake for Optimal health: How Can Healthcare Professionals Support People to Reach Dietary recommendations? BMJ, [online] 378, p.e054370. doi:https://doi.org/10.1136/bmj-2020-054370.

7. Threapleton, D.E., Greenwood, D.C., Evans, C.E.L., Cleghorn, C.L., Nykjaer, C., Woodhead, C., Cade, J.E., Gale, C.P. and Burley, V.J. (2013). Dietary fibre intake and risk of cardiovascular disease: systematic review and meta-analysis. BMJ, [online] 347(dec19 2), pp.f6879–f6879. doi:https://doi.org/10.1136/bmj.f6879.

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