What are the types of dietary fibre, and how do they differ?

Dietary fibre is carbohydrate that the small intestine cannot easily digest. The US Food and Drug Administration (FDA) describes it as many sugar molecules linked together in a way that resists digestion. Fibres are grouped in two main ways: by whether they dissolve in water (soluble fibre, such as beta-glucan in oats, pectin in fruit, inulin and psyllium; or insoluble fibre, such as cellulose and lignin), and by how readily bacteria in the large intestine ferment them. Most plant foods contain several kinds at once. Some fibres are also made or extracted for use in food, such as polydextrose made from glucose and sorbitol, psyllium husk, and resistant dextrin made by heating starch. The Thai Reference Daily Intake (Thai RDI) for dietary fibre is 25 g a day, for all kinds together.

In short:

• Definition: carbohydrate that the small intestine cannot easily digest; it can occur naturally in plants or be isolated or made for food use
• Soluble fibre: dissolves in water, can form a gel, and is broken down by bacteria in the large intestine
• Insoluble fibre: does not dissolve in water and passes through the gut relatively intact
• Fermentable and viscous: two further properties that differ between fibres
• Fibres made or extracted for food use: inulin from chicory, polydextrose, psyllium husk, resistant dextrin (from starches such as corn, potato, rice, tapioca or wheat)
• Thai rice: about 90% of the fibre in rice bran is insoluble, in a Kasetsart University study
• Thai RDI: 25 g of dietary fibre a day; increase it slowly and drink enough

This article explains what the different types of fibre are made of and how they are grouped. It does not discuss what they do in the body. For simple ways to add fibre to Thai meals, see Eating few vegetables? How can you add fibre to Thai meals?, and for prebiotic fibres, see What is the difference between probiotics and prebiotics?

What counts as dietary fibre?

A carbohydrate the small intestine cannot easily digest

The FDA describes dietary fibre as a type of carbohydrate made up of many sugar molecules linked together. Unlike other carbohydrates, it is bound together in such a way that it cannot be easily digested in the small intestine. Harvard T.H. Chan School of Public Health puts it simply as a type of carbohydrate that the body can't digest.

Natural fibres and fibres added to food

On US nutrition labels, dietary fibre includes the fibres that occur naturally in plants and certain isolated or synthetic fibres that are added to food and that the FDA has reviewed and accepted. The international food standards body, the Codex Alimentarius Commission, adopted its own definition in 2009. A 2023 review from University College London and the University of Suffolk notes that the 2009 Codex definition made it optional whether to count short chains (oligosaccharides) as fibre, and that inulin counts as fibre by default because it is a non-digestible carbohydrate polymer.

One daily amount for all kinds

The Thai RDI covers all kinds of fibre together. The Thai RDI for dietary fibre is 25 g a day; there is no separate Thai reference amount for soluble or insoluble fibre, or for any single type.

How are types of fibre grouped?

Soluble and insoluble

The FDA describes the two main groups as follows:

Fermentable and viscous

Harvard's Nutrition Source adds two more properties: a fibre can be viscous, with a gel-like quality, or fermentable, meaning bacteria in the gut can break it down. The two properties do not always go together. Harvard lists psyllium as soluble and viscous but non-fermentable, and pectin as soluble and highly fermentable.

Most foods contain a mix

Harvard lists oatmeal, chia seeds, nuts, beans, lentils, apples and blueberries as foods with soluble fibre, and whole wheat products (especially wheat bran), quinoa, brown rice, legumes, leafy greens, almonds, walnuts, seeds and fruits with edible skins as foods with insoluble fibre. Several foods appear on both lists, because plant foods contain more than one kind of fibre.

Which types of fibre are there?

Fibres that occur naturally in plant foods

The table is in alphabetical order, not in order of amount or importance.

Fibres made or extracted for food use

This table is also in alphabetical order. Inulin, listed above, comes from chicory root as well as onions.

Which fibres does the US FDA accept for nutrition labels?

The list below applies to US nutrition labels only. The FDA lists the isolated or synthetic fibres that can be counted as dietary fibre on US labels: acacia (gum arabic), alginate, arabinoxylan, beta-glucan soluble fibre, cellulose, cross-linked phosphorylated RS4, galactooligosaccharide, glucomannan, guar gum, high amylose starch (resistant starch 2), hydroxypropylmethylcellulose, inulin and inulin-type fructans, locust bean gum, mixed plant cell wall fibres (such as sugar cane fibre and apple fibre), pectin, polydextrose, psyllium husk, and resistant maltodextrin/dextrin.

Eight of these are covered by a June 2018 FDA guidance, which says the FDA does not intend to object to declaring them as dietary fibre on labels while it completes its rulemaking: mixed plant cell wall fibres, arabinoxylan, alginate, inulin and inulin-type fructans, high amylose starch (resistant starch 2), galactooligosaccharide, polydextrose, and resistant maltodextrin/dextrin.

How are polydextrose, psyllium and resistant dextrin made?

Polydextrose

Harvard describes polydextrose as a soluble fibre made of glucose and sorbitol. The 2018 FDA guidance allows a caloric value of 1 kcal per gram for polydextrose on US labels.

Psyllium

Harvard describes psyllium as a fibre extracted from psyllium (Plantago ovata) seeds. Psyllium husk powder is also used in Thai food research: a 2021 Prince of Songkla University study added it at 0% to 4% to surimi (fish paste) from threadfin bream. At 1%, the breaking force of the gel rose by 50% compared with the gel without psyllium; at 2% to 4% it was lower, and the gels became less white as more was added.

Resistant dextrin

US regulations (21 CFR 184.1277) describe dextrin as an incompletely hydrolysed starch, made by dry heating starch from corn, potato, wheat, rice, tapioca and other plants, with or without acids or alkalis. Researchers at Chulalongkorn University made resistant dextrins from cassava starch with 0.04% to 0.10% hydrochloric acid at 100 °C to 120 °C for 60 to 180 minutes; stronger conditions gave darker, more soluble products with more total dietary fibre. A 2020 Soochow University study of dextrins made by dry heating waxy maize starch reports that new chemical bonds formed during heating made them harder to digest in a laboratory test.

How much soluble and insoluble fibre is in Thai rice?

A 2023 study from the Rice Science Center at Kasetsart University, with Thailand's National Center for Genetic Engineering and Biotechnology, measured soluble and insoluble fibre in 31 rice varieties and breeding lines, most of them Thai (including Khao Dawk Mali 105, RD6 and Riceberry), and in their bran.

About 90% of the fibre in rice bran was insoluble. The authors suggest that the insoluble fibre is concentrated in the outer layer of the grain, while soluble fibre is spread through the starchy endosperm. They also found about half as much fibre in milled (white) rice as in whole grain rice. Pigmented rice had significantly more soluble and insoluble fibre than non-pigmented rice; Riceberry had 1.37% soluble fibre.

What the soluble fibre in rice is made of

The same study analysed the soluble fibre in whole grain rice: on average, soluble arabinoxylan made up 7%, beta-glucan 11% and pectin 28% of it, about 46% in total. The authors suggest that the rest may be resistant starch, arabinogalactan and fructan. The rice came from one research centre, so the figures describe these varieties rather than all rice sold in Thailand. For fibre in Thai fruits, see What is the difference between probiotics and prebiotics?

How much fibre a day, and how fast should you add it?

The Thai RDI

The Thai RDI for dietary fibre is 25 g a day, for the general population aged 3 and over. It covers all types of fibre from all foods. For the other Thai reference amounts, see What are the Thai daily reference amounts for each nutrient?

Adding fibre slowly

MedlinePlus, from the US National Library of Medicine, advises adding fibre to the diet slowly, because increasing it too quickly can lead to gas, bloating and cramps. Harvard's Nutrition Source likewise suggests increasing fibre gradually, and notes that drinking more fluids can help lessen these effects.

Who should talk to a doctor or pharmacist?

If a doctor is following a health condition of yours, ask before making large changes to what you eat.

If you are pregnant or breastfeeding, or take medication regularly, talk to a doctor or pharmacist before taking dietary supplements.

Dietary supplements are not intended to prevent, treat or cure any disease.