The science of fibre

Key points

• Dietary fibre comprises a group of different substances in plant foods which cannot be completely broken down by human digestive enzymes.
• A high fibre intake has been associated with reduced risk of several conditions including constipation, heart disease, type 2 diabetes and colorectal cancer. It is a common feature of healthier dietary patterns.
• Dietary fibres have different properties (solubility, viscosity and fermentability) which vary greatly depending on origin and processing and influence their effects on health.  
• Some dietary fibres are fermented by gut bacteria and dietary fibre consumption may affect the gut microbiota.
• Adults in the UK are recommended to consumer 30g of fibre a day but are currently consuming about 20g a day on average. Children are also eating less fibre than recommended.

Summary

Dietary fibre refers to a complex group of substances in plant foods which cannot be completely broken down by human digestive enzymes and so pass through to the colon.

Sources of less fermentable fibre can act as bulking (laxative) agents and help prevent constipation. For fibre to have the best effect on preventing constipation, an increase in fibre intake should be accompanied by an increase in fluid intake. Some oligosaccharides (a type of fibre with a shorter carbon chain length) can be fermented by gut bacteria and may have a beneficial effect on gut flora.

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.

Based on its review of the evidence, SACN recommended a new daily intake of dietary fibre for adults of 30g with proportionally lower recommended intakes for children. But most people in the UK do not eat enough dietary fibre. A low fibre intake is associated with constipation and gut diseases such as diverticulitis and bowel cancer. Average intakes of dietary fibre in adults are 20g fibre per day.

It is important to eat a variety of fibre-containing foods, which include wholegrain and high-fibre starchy carbohydrates like wholewheat pasta, brown rice, wholegrain breakfast cereals; fruit and vegetables; potatoes with skin; pulses; and nuts and seeds. In the UK, most dietary fibre in the average adult’s diet comes from vegetables, pasta and rice, white bread, fruit and chips and other potato products. In children (aged 4-10 years) and teenagers, the biggest contributors to fibre intake are fruit, vegetables, white bread, pasta and rice and high-fibre breakfast cereals.

What is dietary fibre?

Dietary fibre refers to a complex group of substances in plant foods which cannot be completely broken down by human digestive enzymes. This includes waxes, lignin, beta-glucans and polysaccharides such as cellulose and pectin.

Originally it was thought that dietary fibre was completely indigestible and did not provide any energy. It is now known that some types of fibre can be fermented in the large intestine by gut bacteria (collectively known as the gut microbiota), producing short-chain fatty acids and gases (methane, hydrogen and carbon dioxide). The fatty acids are absorbed into the blood stream and provide a small amount of energy which can be used by the cells lining the colon and are thought to strengthen gut barrier function.  The amount of gas produced depends on the type of fibre eaten and the gut bacteria present. After a substantial increase in the amount of fibre in the diet, some people experience symptoms such as abdominal distension, discomfort and wind. However, the large intestine and gut bacteria gradually adapt to the increased intake and symptoms usually decrease.

While the terms ‘soluble’ and ‘insoluble’ fibre are still used today, they are not considered to be scientifically accurate as some types of fibre vary in their solubility depending on their source and level of processing. Fermentability is considered a more relevant way to categorise fibres, in terms of how the affect health, and fibres may be classified as more or less fermentable. Fibres can also be described in terms of their viscosity. Viscous fibres (mostly soluble fibres) thicken when mixed with fluids and include gums, pectins, psyllium, and beta-glucans. They form a gel-like substance in the gut which can slow the rate of absorption of nutrients such as glucose.

Table 1 below shows some examples of the variability in the characteristics of different types of fibre in terms of the solubility, viscosity and fermentability.

Table 1. The physiochemical characteristics of common dietary fibres (adapted from Gill et al. 2021)

Definition of dietary fibre

For more than 15 years, the international CODEX Alimentarius Commission debated a definition of fibre which was agreed in 2009. The European Commission (EC), in line with discussions at CODEX, agreed a definition for fibre in 2008 which falls under Regulation (EU) No. 1169/2011 on the provision of food information to consumers defines fibre as:

Except for non-digestible edible carbohydrate polymers that occur naturally in foods, the definition states that there should be evidence of a beneficial physiological effect of any other material captured by the definition. Any beneficial physiological effect of other material needs to be supported by generally accepted scientific evidence.

Dietary fibre requirements and intakes

When SACN assessed evidence for the links between fibre and health outcomes, it found a significant reduction in risk of cardiovascular disease, type 2 diabetes and colorectal cancer at 30g fibre or more per day. This is consistent with authoritative bodies around the world, who typically recommend dietary fibre intake of 25-30g per day for adults. SACN therefore recommended 30g/day as a revised adult intake for adults in the UK.  Average intake of UK adults is 20g fibre. Socio-economic disparities exist with lower intakes in the lowest income quintile compared to the highest, in all age groups and in both sexes.

The recommendations for dietary fibre intake for both adults and children can be seen in the table below. Children need proportionally less fibre in their diets. These recommendations have been rounded to the nearest 5g and are informed by comparative intakes for dietary fibre in different age groups in the National Diet and Nutrition Survey. For pre-school children, introduction of more fibre to the diet should be done gradually.

Table 1. Recommended dietary fibre intakes according to age in the UK

Most people do not eat enough dietary fibre. Total intakes and the main contributors of fibre in the UK diet for different age groups can be seen below.

Table 2. Total dietary fibre intakes (and main contributors) for UK children and adults (National Diet and Nutrition Survey, years 9 to 11; 2016/2017 to 2018/2019)

The British Nutrition Foundation carried out some simple dietary modelling to develop a 7-day menu plan which illustrates, in practice, what a diet that provides 30g of fibre and meets all other dietary recommendations looks like for adults. It is possible to achieve the recommendation if meals are based on starchy foods (mostly wholegrain and high-fibre options), high-fibre snacks are selected and around eight portions of fruit and vegetables are consumed daily. However, this dietary pattern does not reflect most diets in the UK at present and would require people to make significant changes to the foods and drinks they consume.

The evidence on fibre and health

The 2015 SACN report on Carbohydrates and Health reviewed the wealth of available evidence for the links between fibre and health outcomes. SACN concluded that there is strong evidence from prospective cohort studies that diets rich in fibre, particularly cereal fibre and wholegrains, are associated with a lower risk of cardiovascular disease, coronary events, stroke, type 2 diabetes and colorectal cancer. The randomised controlled trials included in the report did not show an effect of dietary fibre intake on type 2 diabetes or cardiovascular risk factors such as fasting blood lipids, glucose or insulin, but did support the role of fibre in bowel function (decreased intestinal transit times and increased faecal mass). Find out more about the report from SACN's website.

Fibre and digestive health

Sources of less fermentable fibre can act as bulking (laxative) agents and help prevent constipation. For fibre to have the best effect on preventing constipation, an increase in fibre intake should be accompanied by an increase in fluid intake. Some oligosaccharides (a type of fibre with a shorter carbon chain length) can be fermented by gut bacteria and may have a beneficial effect on the gut microbiota.

Eating a diet low in fibre is associated with diverticulitis (where the bowel wall becomes inflamed and ultimately damaged) and colorectal cancer. Evidence suggests a protective effect of eating a diet rich in dietary fibre on diverticulitis (particularly fruit and cereal fibre) and colorectal cancer.

Fibre and heart health

Some fermentable fibres eaten in large amounts can help reduce blood cholesterol levels. Diets rich in fermentable fibre, such as fruits and grains, and in particular oats, have been shown to reduce low density lipoprotein (LDL)-cholesterol.

Fibre and type 2 diabetes

Dietary fibre has been shown to improve glycaemic control and has an important role in managing diabetes.

Studies suggest that a high intake of dietary fibre reduces the risk of developing type 2 diabetes. There is also evidence specifically for higher intakes of cereal fibre and higher wholegrain consumption and lower incidence of type 2 diabetes.

Fibre, energy balance and bodyweight

The SACN carbohydrates report concluded that there was no evidence from randomised trials to show that dietary fibre intake influences bodyweight changes or energy intake. They said that there was limited evidence from trials that a higher intake of wholegrains may decrease total dietary energy intake, but that more evidence is needed to confirm this. A systematic review published in 2020 did not find evidence that wholegrains had an effect on body weight.

Studies looking at the effect of fibres and fibre-rich foods on appetite, energy intake and body weight have had inconsistent results. However, a systematic review and meta-analysis of randomised controlled trials published in 2020 found that intake of viscous fibres such as beta-glucan, psyllium or guar gum can result in modest reductions in body weight, BMI and waist circumference. It was suggested that viscous fibres could affect energy intake as they increase the viscosity of the gut contents, promote gastric distention and slow gastric emptying as well as potential effects on satiety hormones.

Fibre and the gut microbiome

The human gut microbiota (the community of microorganisms in the gut) is estimated to number approximately 100 trillion organisms – mainly bacteria but also viruses, fungi and other micro-organisms. The gut microbiota has important roles in immunity, digesting food, gut hormone and neurological signalling, the metabolism of drugs and elimination of toxins. There has been growing interest in the potential impact of the bacteria in our gut on health and disease, and how our diet could modulate this.

The gut microbiota develops from birth and early childhood and then tends to remain relatively stable from later childhood to adulthood. The balance of different bacterial species in the gut varies between individuals and can be affected by factors including mode of birth (vaginal or caesarean section), infant feeding, lifestyle, medications, and genetics.  There is currently no agreed definition of exactly what makes a ‘healthy’ gut microbiota but having a greater diversity of species is associated with positive effects and certain bacterial strains produce compounds that are associated with health benefits.

A plant-rich diet can provide fermentable fibres such as inulins and galacto-oligosaccharides which are ‘pre-biotic’, that is, they provide food for beneficial bacteria species such as Bifidobacterium and Lactobacillus, selectively increasing their numbers in the gut. When such species ferment fibres, they produce the short-chain fatty acids acetate, butyrate and proprionate. These compounds provide an energy source for the cells in the gut as well as having potential roles in in modulating hormones such as peptide YY (PYY) and glucagon-like peptide 1 (GLP-1), that are involved in appetite control and glucose metabolism. Fermentation of prebiotic fibres also promotes the protective layer of mucus on the gut wall, providing a strong barrier function that is important for maintaining a healthy gut. Conversely, diets high in animal fats and protein and low in fibre may promote ‘dysbiosis’, an unfavourable imbalance of bacteria in the gut, resulting in lower production of short-chain fatty acids, a leaky gut mucosa and intestinal and systemic inflammation.

Studies in animals and in humans have shown differences in gut microbiota between healthy individuals and those affected by a range of conditions including obesity, type 2 diabetes, eczema, inflammatory bowel disease and arterial stiffness. A lack of diversity, lower levels of beneficial bacteria and higher levels of harmful bacteria seem to be common features of the gut microbiota in disease states. The relationship between the gut microbiota and obesity has generated particular interest as studies have found that faecal transplants from obese or lean humans into germ-free mice can cause or prevent obesity respectively. However, much of the work in this area has been in mouse models, and we need more research in humans to better understand how the gut microbiota may be linked with bodyweight.

The relationship between the gut microbiota, diet and health is a complex and emerging field. What does seem clear is that a healthy dietary pattern that is rich in a variety of plant foods is likely to support a more favourable balance of bacteria in the gut, alongside the other beneficial effects that we know are associated with this type of diet. A ‘healthy’ gut microbiota may have several health benefits in terms of maintaining a healthy gut and reducing risk of disease.

Sources of fibre

Because the components of dietary fibre are found in different proportions in fibre-containing foods and have different properties, it is important to eat a variety of fibre-containing foods. Some examples of different components of fibre and their food sources are listed below.

Fibre on the label

Similarly to carbohydrate, fats and proteins, fibre is a source of energy providing on average 2kcal per gram. Nutrition information on food labels must comply with a format set out within the EU Food Information for Consumers Regulations. Whilst it is mandatory to state the energy, fat, saturates, carbohydrates, sugars, protein and salt content of a food, the provision of fibre content is voluntary.

Manufacturers may voluntarily claim foods as a ‘source of fibre’ if it contains at least 3g of fibre per 100g or at least 1.5g of fibre per 100kcal, or ‘high in fibre’ if it contains at least 6g of fibre per 100g or at least 3g of fibre per 100kcal. Where these claims are used, the fibre content must be provided in the nutrition information on the back-of-pack.

No health claims have been approved for ‘dietary fibre’ as a claim must be based on robust evidence for a specific cause and effect relationship, which is not possible for the diverse category of compounds included in the fibre definition. A range of health claims relating to specific fibre types have been authorised for use in Great Britain (England, Wales and Scotland), which describe the relationship between consumption of the fibre type and health. There are conditions of use associated with each claim that must be met for a product to carry a claim, including in some cases specific information that must be provided to the consumer where a claim is made. Some examples of authorised claims are given below. A full list of authorised claims can be found in the Great Britain nutrition and health claims (NHC) register.

*Foods must contain at least 6g of fibre per 100g or at least 3g of fibre per 100kcal to qualify as ‘high in’ fibre

Last reviewed June 2021. Next review due June 2024.