Dietary fibre and maintaining healthy bowel function

The term "dietary fibre" was first used in scientific literature in 1953. At that time, the author applied it mainly to the indigestible components of plant cell walls, such as cellulose, hemicellulose and lignin. In the decades that followed, fibre ceased to be regarded solely as "bulk" in the diet, and its importance for the functioning of the digestive tract began to be recognised. Currently, in Europe, fibre is discussed primarily as a group of various substances present in plant-based foods that can influence bowel function.

1. Characteristics of dietary fibre
2. Types of fibre
3. The benefits of fibre
4. FAQ – Dietary fibre and maintaining healthy bowel function

Characteristics of dietary fibre

Dietary fibre is the part of food that passes through the upper gastrointestinal tract largely undigested by human digestive enzymes. This means that it does not behave in the same way as starch, sugars, protein or fat, which are broken down into smaller molecules and absorbed mainly in the small intestine. Fibre continues on to the large intestine, where some of its components can retain water, increase the volume of intestinal contents, or be broken down by gut bacteria.

Fibre is not a single substance, but a large group of compounds with varying structures and different behaviours within the digestive tract. Put simply, fibre comprises those edible plant components and certain similar carbohydrates that humans do not digest in the same way as typical energy-providing nutrients. Its presence in the diet stems mainly from the consumption of wholegrain cereal products, vegetables, fruit, pulses, nuts and seeds.

Types of fibre

In practice, fibre is often divided into soluble and insoluble types, but this distinction is a simplification, as viscosity, water-binding capacity and susceptibility to fermentation in the large intestine are also important factors. Fibre includes, amongst other things, cellulose, hemicelluloses, lignin, pectins, gums, plant mucilages, beta-glucans, arabinoxylans, inulin, fructooligosaccharides, galactooligosaccharides and resistant starch.

Cellulose and some hemicelluloses are components of plant cell walls and are generally poorly soluble in water; they are therefore more commonly associated with the volume of intestinal contents.

Lignin is not a carbohydrate, but is traditionally classified as fibre because it is also not digested by human enzymes and occurs alongside the components of plant cell walls.

Pectins (from fruit), gums, mucilages (from herbs) and beta-glucans (from fungi) form viscous solutions or gels more readily, causing them to behave differently in the digestive tract.

Inulin and oligosaccharides are shorter-chain carbohydrates that can be utilised by gut bacteria.

Resistant starch (from potatoes) has the structure of starch, but part of it is not digested in the small intestine; therefore, physiologically, it can behave similarly to dietary fibre.

Common sources of dietary fibre

Nutritional information per 100 g of product (USDA National Nutrient Database)

The benefits of fibre

Fibre helps maintain normal bowel function, mainly through its effect on stool consistency, the water content of the intestinal contents, intestinal transit time and the activity of the gut microbiota. Less soluble fibre fractions, particularly wheat bran fibre, can increase the volume of faecal matter, as they are not digested themselves and also promote water retention in the intestinal contents. A greater stool volume may more strongly stimulate the intestinal wall to perform the natural movements that propel the intestinal contents.

For this reason, health claims regarding the increase in stool mass and the acceleration of intestinal transit have been authorised in the European Union for fibre from wheat bran. Rye fibre has an approved claim regarding the support of normal bowel function, whilst fibre from oats and barley may be claimed to increase stool mass.

“Fibre works best when it is consumed regularly as part of the diet and comes from a variety of plant sources — the gut benefits not from a single ingredient, but from the overall rhythm of daily nutrition.” Łukasz Domeracki – Dietitian

The EFSA (European Food Safety Authority) considers that, for adults, a daily intake of 25 g of fibre is sufficient to ensure normal bowel movements. As well as the amount of fibre consumed, the regularity of its intake and an adequate intake of fluids are also important for bowel function, as fibre requires water to perform its physical function properly.

FAQ – Dietary fibre and maintaining healthy bowel function

How much fibre should you eat every day?

The European Food Safety Authority (EFSA) recognises that an adequate intake of fibre for adults is 25 g per day, which promotes healthy bowel function and regular bowel movements.

Does fibre help with constipation?

An adequate amount of fibre, particularly when combined with proper hydration, can support healthy bowel function. In the European Union, health claims have been approved, amongst other things, for wheat bran fibre, which increases stool bulk and accelerates intestinal transit.

Is it possible to consume too much fibre?

A sudden increase in fibre intake can cause bloating, a feeling of fullness or gastrointestinal discomfort. It is therefore best to increase your fibre intake gradually, whilst ensuring you drink enough fluids.

Why do you need to drink more water when following a high-fibre diet?

Fibre binds water in the digestive tract. Without adequate hydration, its benefits may be reduced, and in some people it may even exacerbate problems with bowel movements.

What foods are the best sources of fibre?

The richest sources include wholegrain cereals, vegetables, fruit, pulses, nuts, seeds, linseed and chia seeds. Regularly eating a variety of plant-based foods makes it easier to achieve the recommended fibre intake.

Does fibre affect the gut microbiota?

Yes. Certain types of fibre, such as inulin and fructooligosaccharides, can be utilised by gut bacteria. Research suggests that dietary composition, including regular fibre intake, plays an important role in shaping the gut microbiota, although the effects of specific types of fibre are still being extensively studied.

Sources:

• HIPSLEY E. H. (1953). Dietary "fibre" and pregnancy toxaemia. British medical journal, 2(4833), 420–422. https://doi.org/10.1136/bmj.2.4833.420
• Stephen, A. M., Champ, M. M., Cloran, S. J., Fleith, M., van Lieshout, L., Mejborn, H., & Burley, V. J. (2017). Dietary fibre in Europe: current state of knowledge on definitions, sources, recommendations, intakes and relationships to health. Nutrition research reviews, 30(2), 149–190. https://doi.org/10.1017/S095442241700004X
• de Vries, J., Miller, P. E., & Verbeke, K. (2015). Effects of cereal fiber on bowel function: A systematic review of intervention trials. World journal of gastroenterology, 21(29), 8952–8963. https://doi.org/10.3748/wjg.v21.i29.8952