Fibre & Gut Health
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What are dietary fibres?
Dietary fibres are a diverse group of complex carbohydrates found naturally in or isolated from plants. They are unable to be fully digested by human enzymes in the small intestine, allowing them to reach the colon largely intact. Fibres are important for our gut health; some playing a role in supporting normal intestinal transit, whilst others act as ‘prebiotics’, which can be broken down and fermented by gut bacteria when they reach the colon.[1]
Depending on the specific type of fibre and the amount consumed, they may influence gut comfort, appetite regulation, blood glucose and cholesterol concentrations.[1]
Structure of typical dietary fibres:
Foods rich in fibre
Fruit and vegetables
Nuts and seeds
Pulses and legumes
Grains and wholegrains
Fibre-fortified foods, e.g. yogurt with added grains or fruit
Types of dietary fibre
There are two categories of fibre origin: dietary and functional.
Dietary fibres naturally occur within plants e.g. cellulose, beta-glucans, lignins
Functional fibres are carbohydrates extracted from plants by physical, chemical or enzymatic processes, or synthetically made, and added to manufactured foods and beverages that lack fibre e.g. soluble corn fibre, psyllium.
Some fibres may fall into both categories e.g. oligosaccharides, inulin, pectin, which can be both naturally occurring, or extracted/synthesised.
Fibres are often classified by their solubility, viscosity and fermentability. Additionally fibres can also be classified on their chemical structure; interactions with other cell wall compounds, digestion and food processing can also impact their nature. For example, fibre fermentability not only depends on the fibre structure, but the presence of bacteria and enzymes to break them down. Classifications can also impact the characteristics of fibres, for example, solubility typically increases fermentability, whilst viscosity decreases fermentability. This is important when identifying fibres as substrates for probiotics.[2]
Solubility: Ability to dissolve in water. Soluble fibres dissolve in water and can help lower cholesterol, slow digestion and feed gut bacteria, e.g. inulin, pectin and beta glucans found in fruit and oats. Fibres that are mainly insoluble, such as cereal fibres, help regulate the bowel and prevent constipation, e.g. cellulose[3]
Fermentability: ability to be broken down by probiotics. Fermentable fibres are broken down by gut bacteria, promoting a healthy microbiome e.g. resistant starch, pectin, inulin, oligofructose. Fibres with low fermentability can act as bulking agents and help prevent constipation e.g. cellulose[2,4]
Viscosity: ability to thicken when mixed with fluids. Viscous fibres thicken when mixed with fluids to form gels in the gut, slowing the rate of nutrient absorption and stabilising blood sugar levels e.g. pectin and beta-glucans.[2]
Examples of the variability in the characteristics of different types of fibre in terms of solubility, viscosity and fermentability.
Adapted with permission from the British Nutrition Foundation: Fibre - Nutrition Information - British Nutrition Foundation[5]
Due to the complex structure of fibre, each type may behave differently in the body and can trigger unique physiological responses or benefits. Also, many foods contain a mix of different types of fibres, particularly fruits and vegetables, therefore, a varied diet that includes a mix of fibres is important for well-being and digestive health.
Recommended fibre intakes
Fibre intake recommendations are derived from clinical studies that evaluate the connections between fibre consumption and various health conditions, such as cardiovascular disease, type 2 diabetes, normal bowel activity and colorectal cancer. This evidence largely formed the Scientific Advisory Committee (SACN) recommendation of 30g fibre per day in healthy UK adults, which shows that higher fibre intake can improve bowel health and a reduced risk of cardiovascular disease, type 2 diabetes, and colorectal cancer [6]
However, UK adults are not consuming enough fibre and there is a fibre deficit: the latest National Diet and Nutrition Survey (NDNS) Years 9-11 data, reveals that only 9% of UK adults aged 19-65 meet these recommendations.[7]
A Fibre Gap in the UK
Common barriers to fibre consumption include:
Many people lack of knowledge about just how much fibre is needed, which foods contain fibre and the benefits of fibre
Perceived inconvenience of sourcing, preparing and cooking high-fibre foods
Individual tastes and preferences
Cost: fruits, vegetables and grains are considered costly
Concern over potential side effects such as bloating, flatulence and abdominal discomfort, particularly if transitioning from a low fibre diet to a high fibre diet.
Specific diets: gluten-free and low-carbohydrate diets are often low in fibre
How fibre affects gut health
A rich and diverse community of micro-organisms such as bacteria, fungi and yeast live in the gut. Within this gut microbiota, microorganisms interact with each other and with their host to modulate biological processes essential for health, such as supporting gut comfort, immunity and nutrient absorption. In the human diet, fibre is the primary source of "fuel" for gut bacterial fermentation[8] and prebiotic fibres are essential to support the growth and development of beneficial bacterial populations in the gut. Therefore, a diet rich in different fibres can greatly impact gut microbiome diversity.[9]
When fibre is fermented in the colon, it produces gases and beneficial metabolites, predominantly short-chain fatty acids (SCFAs), including acetate, butyrate and propionate. SCFAs are essential in human functions and play a part in physiological processes such as metabolism, immune support and cell proliferation, as well as being the primary fuel source for intestinal epithelial cells.[10]
A fibre deprived diet can lead to an imbalance in the gut microbiota and functional gut disorders or complaints. It has also been shown that a prolonged low fibre diet may lead to an increase in mucin-feeding bacteria which, in turn can harm the protective colonic mucosal layer, bringing pathogens closer to the epithelial cells and increasing the risk of infections.[11]
Image indicating the possible mechanisms of a fibre rich diet and a fibre free diet on the colonic mucus barrier in a gnotobiotic mouse model. In a fibre deprived diet, the gut bacteria start using the mucus lining of the colon as a food source, which causes the mucus barrier to break down. Mucus-eroding bacteria allows harmful pathogens to reach the colon lining more easily, leading to infections, like colitis.[11]
As already noted, fibre also helps prevent constipation by regulating the bowel (insoluble fibres) and acting as bulking agents (low fermentability fibres). Some types of dietary fibre, particularly soluble fibre are thought to be of interest in the management of Irritable Bowel Syndrome (IBS) symptoms.[12]
Other health benefits of fibre
Evidence suggests that diets high in dietary fibre have a number of other health benefits due to the byproducts of fibre digestion, some of which are known as postbiotics. The evidence for the links between dietary fibre and health was reviewed in 2015 by the Scientific Advisory Committee on Nutrition (SACN), which found that there is strong evidence that diets rich in fibre, particularly cereal fibre and wholegrains, are associated with a lower risk of many health conditions, including cardiovascular disease, coronary events, stroke, type 2 diabetes and colorectal cancer.[5-6]
Fibre and heart health
Diets rich in fermentable fibres – like fruits, grains and most notably oats - have been shown to reduce low density lipoprotein (LDL)-cholesterol[5] and may play a role in cardiovascular disease prevention.[13]
Fibre and type 2 diabetes
Since fibre is linked with controlled blood sugar levels and insulin demands, it can play an essential role in reducing the risk of developing type 2 diabetes and the risk of associated certain complications.[14]
Fibre, satiation & satiety[10]
While there is currently no authorised health claim in Europe linked to fibre’s direct effect on satiation and satiety, there are several suggested ways in which eating fibre may influence feelings of fullness or hunger, such as fibre-rich foods taking longer to chew, delaying gastric emptying and producing SCFAs that stimulate enteroendocrine cells into secreting the satiety related hormones like ghrelin, glucagon- like peptide 1 and polypeptide YY.[15]
Potential issues with low fibre intake
Since high-fibre diets are associated with a lower risk of many health conditions, a low fibre diet may increase the risk of those conditions.
A low fibre diet has been linked to obesity, constipation, bloating and gas. It is also associated with diverticulitis and colorectal cancer.[10]
Diets high in animal fats and protein and low in fibre may promote ‘dysbiosis’, an imbalance of bacteria in the gut, resulting in lower production of short-chain fatty acids, a leaky gut mucosa and intestinal and systemic inflammation.[10]
References
[1] Slavin J. Fiber and prebiotics: mechanisms and health benefits. Nutrients. 2013; Apr 22;5(4):1417-35.
[3] McRorie JW, McKeown NM, 2017. Understanding the Physics of Functional Fibers in the Gastrointestinal Tract: An Evidence-Based Approach to Resolving Enduring Misconceptions about Insoluble and Soluble Fiber. Journal Of The Academy Of Nutrition And Dietetics, volume 117, n2°.
[5] British Nutrition Foundation. Fibre. Available here [Accessed April 2025].
[6] SACN. Carbohydrates and Health. 2015. ISBN: 978 0 11 708284 7. Available here. Accessed December 2024.
[7] Action on Fibre | The Food & Drink Federation Accessed January 2025.
[8] Thomson C, Garcia AL, Edwards CA. Conference on ‘Gut microbiome and health’ Symposium 2: Gut microbiome, nutrition and health: cause and effect interactions between dietary fibre and gut microbiota. Proceedings of the Nutrition Society, 2021; volume 80. p. 398-408
[9] Sonnenburg ED, Smits SA, Tikhonov M, et al. Diet-induced extinctions in the gut microbiota compound over generations. Nature, 2016; volume 529, n7585. p.212-215
[10] Koh A, De Vadder F, Kovatcheva-Datchary P, et al. From Dietary Fiber to Host Physiology: Short-Chain Fatty Acids as Key Bacterial Metabolites. Cell, 2016; Jun 2;165(6):1332-1345.
[11] Desai MS, Seekatz AM, Koropatkin NM, et al. A Dietary Fibre-Deprived Gut Microbiota Degrades the Colonic Mucus Barrier and Enhances Pathogen Susceptibility. Cell, 2016; volume 167, n5. p.1339-1353.
[12] Moayyedi P, Quigley EM, Lacy BE, et al. The effect of fiber supplementation on irritable bowel syndrome: A systematic review and meta-analysis. Am J Gastroenterol, 2014; volume 109. p. 1374-1367.
[13] Threapleton DE, Greenwood DC, Evans CEL, et al. Dietary fibre intake and risk of cardiovascular disease: Systematic review and meta-analysis. BMJ, 2013; volume 347.
[14] The InterAct Consortium. Dietary fibre and incidence of type 2 diabetes in eight European countries: The EPICInterAct Study and a meta-analysis of prospective studies. Diabetologia, 2015; volume 58, n7°. p. 1408–1394.
[15] Wanders AJ, Jonathan MC, Van den Borne JJGC, et al. The effects of bulking, viscous and gel-forming dietary fibres on satiation. British Journal of Nutrition, 2012; volume 109, n7°.
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*A responder was defined as a subject having an improvement in their gastrointestinal well-being, i.e. answering ‘improved’ on a 3-point Likert scale on at least 2 weeks out of the 4 week double-blind period of product consumption
** GI wellbeing was assessed by using a a 3-point Likert scale (improved, no change, worsened) on a weekly basis. Each participant was classified as responder or as a non-responder to assess the magnitude of the effect. A responder was defined as a subject having an improvement in their gastrointestinal well-being, i.e. answering ‘improved’ on the three-point Likert scale, on at least 2 weeks over the 4-week double-blind period of product consumption.
Frequency of digestive symptoms was measured using a 5-point Likert scale that ranged from 0 (never) to 4 (every day of the week). The composite score for these 4 symptoms ranged from 0 (none of the symptoms) to 16 (all symptoms, every day).
Bowel function: Participants were also asked to report daily bowel movements according to the Bristol stool scale.
Health-related quality of life: The Health-related quality of life (HRQoL) was assessed by self-administration of two questionnaires: the Food and Benefits Assessment (FBA) and the Psychological General Well-Being Index (PGWBI) at three time points: baseline, after 4 weeks, and after 8weeks
