What is Dietary Fiber?
Defining Dietary Fiber
“What is dietary fiber?” It’s a simple question, but it doesn’t have a simple answer.
The Codex Alimentarius Commission - a United Nations/WHO body - spent almost two decades developing a definition of dietary fiber that would be recognised globally. The definition adopted in June 2009 is as follows:
Dietary fiber denotes carbohydrate polymers1 with 10 or more monomeric units2, 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
- Includes also lignin and other compounds if quantified by AOAC 991.43.
- Decision on whether to include carbohydrates with a degree of polymerisation from DP 3 to 9 should be left to national authorities.
For consumers, this means that the term ‘dietary fiber’ refers to any carbohydrate that is not broken down by the body’s own enzymes in the small intestine. Instead, it passes into the large intestine (colon) where it is partially or fully fermented. These characteristics of dietary fiber are associated with well documented benefits to human health.
For food scientists, the CODEX announcement in 2009 was particularly significant in that it included resistant starch (RS), a component that was not accurately measured by the dietary fiber methods available at that time. As a result of the Codex definition, the analytical measurement of Dietary Fiber had to change.
Find out more: open access paper ‘Evolution of a Definition for Dietary Fiber and Methodology to Service this Definition’
Types of Dietary Fiber
A. Bulk Properties
Some methods of classifying dietary fiber are based on the physical characteristics of the fiber.
Soluble vs insoluble
This divides types of fiber based on their solubility in water.
Fermentable vs non-fermentable
This separates dietary fibers based on whether they can fermented by the microbiota in the large intestine. This more recent classification is speculated to become increasingly useful in the context of health.
Viscous vs non-viscous
Perhaps the least-used distinction, this classification arises from evidence that certain health benefits of dietary fiber might be related to fiber viscosity.
B. Ingredient Components
Dietary fiber can also be classified by its ingredient components. These are wide ranging (but very well-defined) classes of chemical entities, including the following 11 components:
1. β-Glucan: Polymers of glucose containing various bond types between individual glucose units including β-1,4, β-1,3 and β-1,6.
2. Galactomannan: 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 (see table).
| Source | Mannose:Galactose |
| Fenugreek gum | 1:1 |
| Guar gum | 2:1 |
| Tara gum | 3:1 |
| Locust bean gum | 4:1 |
3. Arabinoxylan: 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.
4. Pectin: 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).
5. Arabinogalactan: 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.
6. Resistant Starch: This is a term given to any form of starch that is resistant to hydrolysis by the enzymes of the small intestine. There are four different types of resistant starch (RS1-4).
| Resistant Starch | Description |
| RS1 | Physical inaccessible starch, such as that found in seeds or legumes and unprocessed whole grains. |
| RS2 | Resistant starch that occurs in its natural granular form, such as uncooked potato, green banana flour and high-amylose corn. |
| RS3 | Resistant starch that is formed when starch-containing foods are cooked and cooled, such as in legumes, bread, corn flakes, pasta salad or sushi rice. This occurs due to retrogradation, i.e. the recrystallisation of amylose and amylopectin on cooling. |
| RS4 | Starches that have been chemically modified to resist digestion. This type of resistant starch can have a wide variety of structures and is not found in nature. |
7. Inulin/Fructooligosaccharides (FOS): 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.
8. Galactooligosaccharides (GOS): 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.
9. Resistant Maltodextrins (RMD): 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.
10. Polydextrose: Polydextrose is a synthetic polymer of glucose that is resistant to digestion. Polydextrose is formed industrially by the forced condensation of glucose, sorbitol and citric acid.
11. Cutin/Suberin: 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.
