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What is Dietary Fiber

What is dietary fiberDietary fiber can generally be described as that portion of food that is not digested in the human small intestine. It passes into the large intestine where it is partially or fully fermented. These characteristics of dietary fiber are associated with its numerous well documented health benefits.

CODEX Alimentarius
was founded in 1963 as part of the World Health Organisation Food and Agriculture Organisation (WHO/FAO) and is the international body that sets guidelines for national regulatory authorities. In 2009, they finally reached consensus upon a definition of dietary fiber after almost 20 years of deliberation:

CODEX Alimentarius definition of dietary fiber:  

Dietary fibre denotes carbohydrate polymers with 10 or more monomeric units, which are not hydrolysed by the endogenous enzymes in the small intestine of humans and belong to the following categories: Edible carbohydrate polymers naturally occurring in the food consumed. Carbohydrate polymers, which have been obtained from food raw material by physical, enzymatic or chemical means and which have been shown to have a physiological benefit to health, as demonstrated by generally accepted scientific evidence to competent authorities. Synthetic carbohydrate polymers that have been shown to have a physiological benefit to health, as demonstrated by generally accepted scientific evidence to competent authorities. NOTES: (1) Includes also lignin and other compounds if quantified by AOAC 991.43. (2) Decision on whether to include carbohydrates with a degree of polymerization from DP 3 to 9 should be left to national authorities.     

This definition of Dietary Fiber required an accompanying analytical method that was capable of measuring dietary fiber in a simple, robust, reproducible manner.  Megazyme fulfilled this requirement with the development of the McCleary Method Integrated Dietary Fiber Assay Kit (AOAC 2009.01/AOAC 2011.25).

For further information on this aspect of dietary fiber, see measurement of dietary fiber

Types of Dietary Fiber

Dietary fiber can be classified in a number of different ways.

Bulk properties can in certain cases provide useful information:

Soluble versus insoluble is a commonly used distinction. This divides types of fiber based on their water solubility.  Fermentable versus non-fermentable is a more recent classification but could be more useful in the years to come when discussing many of the health benefits associated with dietary fiber. This separates dietary fiber based on whether or not it can be fermented by the microbiota in the large intestine.  Viscous versus non-viscous is perhaps the least used distinction and arises from the fact that there is some evidence support that certain health benefits of dietary fiber could be related to its viscosity.

Dietary fiber can also be classified by its ingredient components. These are wide ranging but very well defined classes of chemical entities.


Polymers of glucose containing various bond types between individual glucose units including β-1,4, β-1,3 and β-1,6. 

beta glucan analysis

β-Glucan obtained from barley, oats and yeast is available from Megazyme. High, medium and low viscosity grades are available and these are indicative of the molecular weight range in each product.


Polymers of β-1,4 linked mannose units with varying degrees of α-1,6 galactosyl substitution. The ratio of galactose to mannose determines the polysaccharide identity.

Galactomannan structure galactomannan

Galactomannan obtained from carob, guar and konjac is available from Megazyme. High, medium and low viscosity grades are available and these are indicative of the molecular weight range in each product. 


Polymers consisting of a β-1,4 linked xylose backbone with the xylose monomers bearing a mixture of either 2, or 3, or 2 and 3 substituted α-L-arabinose residues.

Arabinoxylan structure

Arabinoxylan obtained from rye and wheat is available from Megazyme. High, medium and low viscosity grades are available and these are indicative of the molecular weight range in each product. 


Pectin describes a range of diverse structural polysaccharides that are found in many fruits and can be subdivided into two broad classes, namely galacturonans (consisting of homo-galacturonans, unsubstituted and substituted galacturonans) and rhamnogalacturonans (consisting of rhamnogalacturonan-I and rhamnogalacturonan-II).

Homogalacturonan is a linear chain of 1,4-linked-α-D-galacturonic acid residues in which some of the carboxylic acid groups are present as methyl esters. Substituted galacturonans include xylo-galacturonan and apio-galacturonan which exhibit a degree of branching of the respective monosaccharides on the polygalacturonic acid backbone.

Rhamnogalacturonan-I (RG-I) contains a repeating disaccharide subunit of galacturonic acid/methyl ester and rhamnose [→2)-rhamnose-α-(1→4)-galacturonic acid-α-(1→]. The 4- position of the rhamnose residues bear long side chains consisting of β-1,4 linked galactose and/or α-1,5 linked arabinose and/or arabinogalactan. Rhamnogalacturonan-II (RG-II) is a complex, structurally diverse heteropolymer made up of 11 different monosaccharides with extensive branching on a linear backbone of α-1,4 linked galacturonic acid.                                                                                                                                                   Pectin

Pectic polysaccharides including polygalacturonic acid, rhamnogalacturonan I (obtained from potato) and rhamnogalacturonan (obtained from soy bean) are available from Megazyme.


A polysaccharide based on a β-1,3 linked linear galactosyl backbone. The 4- or 6- positions can act as branch points for short chains of galactose or arabinose residues. The overall ratio of galactose to arabinose is in the region of 6:1.
                                                                                                                                                                     arabinogalactan structure

Arabinogalactan obtained from larch wood is available from Megazyme.

Resistant Starch

This is a term given to any starch that is resistant to hydrolysis by the enzymes of the small intestine. There are four different types of resistant starch (RS1-4).                                                                     

                  Physically inaccessible starch, such as that found in seeds or legumes and unprocessed whole grains Resistant starch that occurs in its natural granular form, such as uncooked potato, green banana flour and high amylose corn. Resistant starch that is formed when starch-containing foods are cooked and cooled such as in legumes, bread, cornflakes and cooked-and-chilled potatoes, pasta salad or sushi rice. This occurs due to retrogradation – the recrystallization of amylose and amylopectin on cooling.



Inulin is a polysaccharide found in nature containing long chain fructose monomers linked β-1,2 and a terminal glucose residue. Fructo-oligosaccharides (FOS) are the β-1,2 linked fructose oligomers lacking the terminal glucose residue. They are usually formed through the degradation of inulin although they can also be manufactured commercially using enzymatic transglycosylation.

Inulin will soon be available from Megazyme.


Oligomers formed by transgalactosylation reactions with DP usually in the range of 2-8 and possessing either β-1,3, β-1,4 or β-1,6 linkages.

Galacto-oligosaccarides structure

A mixture of β-1,3 and β-1,4 linked galactobiose (DP2) is available from Megazyme.

Resistant Maltodextrins

In terms of industrial production, these are formed from starch using heat/acid/enzymatic treatment to rearrange the standard α-1,4 bonds between glucose monomers to a random mixture of either α- or β- 1-2, 1-3 or 1-4 bonds. The resulting oligomers/polymers are resistant to digestion. 

resistant maltodextrins


Polydextrose is formed industrially by the forced condensation of glucose, sorbitol and citric acid, polydextrose is a synthetic polymer of glucose that is resistant to digestion. 

cutin structure


A small proportion of dietary fiber is made up of the waxes of plant source ingredients. These waxes are generally polymers of long chain aliphatic acid monomers like 18-hydroxy-oleic acid, or aromatic acid monomers like the hydroxy-cinnamic acids.