Understanding Fibre

Fibre contributes to several aspects of health. Depending on its particular properties, it can:

• support regular bowel function
• influence stool consistency
• help prevent or manage constipation
• provide substrate for the gut microbiota
• contribute to the production of short-chain fatty acids (SCFAs)
• support colonic health
• influence post-meal blood glucose responses
• contribute to lowering LDL cholesterol
• influence satiety
• contribute to long-term cardiovascular and metabolic health
• form part of a dietary pattern associated with a lower risk of colorectal cancer.^1,4,5

Fibre is often described as soluble or insoluble, but this is only one way of classifying it. Its physiological effects are also influenced by whether the fibre is:

The last of these describes fibre that is selectively used by microorganisms in a way that confers a health benefit.^3,6 These characteristics overlap. For example, one soluble fibre may be readily fermented, whereas another soluble fibre may form a viscous gel and be largely resistant to fermentation.

This helps explain why two foods containing a similar number of grams of fibre can have very different effects on the gastrointestinal tract.

Soluble fibres dissolve or disperse in water. Some form viscous gels, whereas others do not.³

Sources can include

Foods generally contain mixtures of different fibres rather than one single type.

What can soluble fibre do?

Depending on its physical properties, soluble fibre may:

• retain water
• influence stool consistency
• form a gel within the gastrointestinal tract
• provide substrate for gut microorganisms.³

Some viscous, gel-forming soluble fibres can also slow the digestion and absorption of nutrients, moderate post-meal glucose responses and contribute to lowering LDL cholesterol.^3,4 Some soluble fibres can therefore be helpful in constipation and bowel regulation.

But soluble does not automatically mean gentle.

Some soluble fibres are readily fermented and can produce gas. Others, particularly psyllium, are largely resistant to fermentation and may therefore produce less fermentation-related gas.^3,7

Insoluble fibre does not dissolve in water and generally remains more structurally intact as it travels through the gastrointestinal tract.

Sources include

Soluble fibre sources

Insoluble fibre sources

Some poorly fermented insoluble fibres can increase faecal bulk and stool output. Their effects depend partly on their structure, particle size and water-holding properties.³

When can it be uncomfortable?

In some people with a sensitive gut, particularly those with IBS, large quantities of coarse insoluble fibre such as bran can aggravate symptoms.

NICE therefore recommends reviewing fibre intake in people with IBS and discourages insoluble fibre such as bran when it is contributing to symptoms. Where an increase in fibre is appropriate, soluble fibre such as ispaghula/psyllium or foods such as oats may be better tolerated.²

Some fibre and other non-digested carbohydrates reaching the large intestine can be fermented by the gut microbiota. Fermentation is a normal physiological process.

Gut microorganisms metabolise these substrates and can produce compounds including the short-chain fatty acids acetate, propionate and butyrate.⁶ SCFAs have a range of physiological functions within the colon and elsewhere in the body.⁶

Fermentation can also produce gases. The amount and rate of fermentation vary considerably between different carbohydrates and between individuals. This means that a nutritionally valuable food can still produce uncomfortable gastrointestinal symptoms in a susceptible person.

Highly fermentable carbohydrates may contribute to:

This does not necessarily mean that the food has “irritated” or damaged the bowel.

A simplified pathway

People with IBS may have altered gastrointestinal sensitivity and/or motility, meaning that intestinal distension can produce more noticeable symptoms.⁸

This distinction is important. Fibre and FODMAPs overlap, but they are not interchangeable terms.

In susceptible people, particularly those with IBS, these changes can contribute to:

Sources: 8, 9

Fructans

Sources can include wheat- and rye-based foods, onion, garlic, some other grains and some vegetables.^8,9 The FODMAP content of a food depends on the particular food, how it has been processed and the amount eaten.

Galacto-oligosaccharides (GOS)

Sources include beans, chickpeas, lentils and other pulses.^8,9 These foods can be highly nutritious and provide fibre, protein, micronutrients and other plant compounds. However, larger quantities may cause symptoms in somebody who is sensitive to particular FODMAPs.

A reaction to a wheat-containing food is not automatically a reaction to gluten.

In people without coeliac disease or wheat allergy, fructans or other components of the food may sometimes be contributing to symptoms.^8,9

The answer is therefore not necessarily to remove fibre, wheat or whole food groups. It may be more appropriate to investigate the type, amount, portion size, frequency and distribution of the foods being eaten.

Viscous and gel-forming fibre

Some soluble fibres become viscous or form gels when hydrated. Examples include beta-glucan in oats and barley, psyllium/ispaghula, and some naturally occurring fruit fibres.³ Viscosity can influence the interaction of nutrients with the gastrointestinal tract and affect glucose and cholesterol absorption.³

Beta-glucan

Oats and barley are particularly useful sources of beta-glucan. Adequate amounts of oat or barley beta-glucan can contribute to lowering LDL cholesterol, moderate post-meal glucose responses and contribute to total fibre intake.^3,4 Beta-glucan can also undergo fermentation within the colon.

Oats can therefore be a useful way of diversifying cereal fibre intake. For people who are modifying FODMAP intake, however, individual grain choice, processing and portion size matter. Some barley- and rye-containing foods can provide significant amounts of fructans.^8,9

Oats & barley

Psyllium / ispaghula

Psyllium is a particularly useful fibre because it is predominantly soluble, viscous, gel-forming, highly water-holding and largely resistant to fermentation.^3,7 Its gel-forming and water-holding properties help explain why it can regulate stool consistency.

Psyllium can be useful in constipation, improving stool form, some loose-stool presentations and bowel regulation in IBS.^2,3 Because it is largely resistant to fermentation, it may produce less fermentation-related gas than rapidly fermented fibres. Adequate fluid intake is important when taking psyllium.

Linseeds / flaxseed

Linseeds contain a mixture of soluble and insoluble fibre as well as mucilage and the plant omega-3 fatty acid alpha-linolenic acid (ALA). When hydrated, their mucilage forms a gel. Ground linseeds can contribute to fibre intake and may help bowel regularity.

NICE advises that people with wind and bloating may find oats and linseeds helpful, with linseeds used at up to one tablespoon per day in its IBS guidance.² As with other fibre sources, they are generally best introduced gradually alongside adequate fluid.

Resistant starch

Resistant starch is starch that escapes digestion in the small intestine and reaches the large intestine. Foods containing it can include beans and lentils, some whole grains, oats, slightly green bananas, and some cooked-and-cooled potatoes, rice and pasta.

Cooking and then cooling some starchy foods can increase their resistant starch content through a process known as retrogradation, although the amount produced varies according to the food and the way it is prepared. Resistant starch can be fermented by colonic microorganisms and contribute to SCFA production, including butyrate. It is fermentable, although fermentation may occur more slowly than with some rapidly fermentable carbohydrates such as inulin, FOS and GOS. Tolerance varies between individuals.⁹

NICE also advises people with IBS that reducing resistant starch may sometimes improve symptoms, particularly resistant starch found in processed or re-cooked foods.² These points are not necessarily contradictory: a dietary component can have potentially beneficial physiological effects while still producing gastrointestinal symptoms in some individuals.

Prebiotics

A prebiotic is “a substrate that is selectively utilized by host microorganisms conferring a health benefit.”⁶ Well-established examples include certain fructans, inulin, fructo-oligosaccharides (FOS) and galacto-oligosaccharides (GOS).⁶

Prebiotics are selectively utilised by microorganisms and, by definition, there must be evidence that this interaction confers a health benefit.⁶ Simply changing the number or type of microorganisms in the gut does not, by itself, make something a prebiotic.

“Prebiotic” does not mean “symptom-free”.

Some prebiotics, including inulin, FOS and GOS, are readily fermented and can increase gas and gastrointestinal symptoms in susceptible individuals. Something can therefore have potentially beneficial effects on the gut microbiome while being poorly tolerated by a particular person or at a particular dose.

There are several possible reasons.

Usually, “irritation” is not the most accurate description. In functional gastrointestinal disorders such as IBS, symptoms following fibre do not necessarily indicate tissue inflammation or damage. Symptoms may instead arise from a combination of:

Sources: 2, 8

Experiencing pain or bloating after eating a food does not automatically mean that the food is damaging the gastrointestinal tract. A food can be nutritionally valuable and physiologically safe while still producing uncomfortable symptoms in a particular person.

Rather than asking only “Am I getting 30 g of fibre?” it can be more useful to consider:

The aim is usually not to remove fibre indiscriminately. Depending on the individual’s symptoms and clinical circumstances, strategies might include:

• reviewing total fibre intake
• increasing fibre gradually when an increase is appropriate
• spreading fibre-containing foods across the day
• maintaining adequate fluid intake
• avoiding very large fibre loads at one meal
• varying cereal and grain sources
• assessing whether wheat fructans may be contributing to symptoms rather than automatically assuming gluten is responsible
• adjusting portions of beans and pulses if GOS is contributing to symptoms
• considering oats, psyllium or ground linseeds where appropriate
• considering portion size, preparation and frequency as well as the food itself
• avoiding unnecessary long-term dietary restriction.^2,8,9

The goal is to find a nutritionally adequate and varied fibre intake that supports long-term health while remaining as well tolerated as possible.