Overview of types of fiber

Fiber is a group of food ingredients that are resistant to digestive enzymes and is found mainly in whole grains, fruits, and vegetables. It is a nutrient with a widely recognized role in human health.

Previous studies have demonstrated that dietary fiber is important in preventing chronic non-communicable diseases such as diabetes, cardiovascular disease, gastrointestinal cancer, dyslipidemia, etc. Furthermore, mechanistic studies show that the physiological functions of different types of fibers are highly dependent on their physicochemical properties, one of which is solubility, viscosity and fermentation ability.

1. Fiber classification

a. Based on structure

+ Fiber can be classified into non-starch polysaccharides (NSPs), resistant starches (RS) and resistant oligosaccharides.

+ Non-starch polysaccharides (NSPs): are long-chain polymers whose monomers are sugar molecules linked together by glycosidic bonds but not starch. Resistant starch (RS) is starch, including its degradation products, that escapes digestion in the small intestine of healthy individuals. Resistant starch occurs naturally in foods, but it can also be added as part of dry raw foods or used as an additive in processed foods.

+ Resistant oligosaccharides, also known as functional oligosaccharides, refer to oligosaccharides with a degree of polymerization (DP) of 3 to 9 simple sugars and also have a prebiotic effect. Preparation of resistant oligosaccharides can be achieved by enzymatic methods, such as hydrolysis and isomerization, and chemical methods.

b. Based on synthetic origin

It can be divided into dietary fiber and functional fiber:

Dietary fiber

A non-starch polysaccharide consisting of carbohydrates and lignin that is not digestible and of vegetable origin.

DF is divided into 2 types according to its ability to disperse in water, which will be discussed in detail under sections 2 and 3.

+ Soluble dietary fiber (SDF): is a type that can dissolve in water, including gums, polyfructose, pectin, mucus, some hemicellulose.

+ Insoluble fiber is the one that does not dissolve in water, including: cellulose, some hemicellulose, lignin.

Functional fiber

It is carbohydrates isolated and industrially extracted from indigestible plant (analysed starch, pectin, gums) animal (chitin and chitosan) origin which is added to foods and beverages to increase the fiber content of the body and has many good physiological effects on health.

c. Based on physical and chemical properties

Fiber is considered to be a carbohydrate polymer with three or more monomer units linked together by glycosidic bonds, resistant to endogenous digestive enzymes and therefore neither hydrolysed nor absorbed in the small intestine.

2. Insoluble fiber

The sugar chains in insoluble fiber are linked together by dense hydrogen bonds, forming hydrophobic and crystalline structures, which can resist hydrolysis by exogenous glucosidase. Thus, it passes through the intestinal tract and out in the stool.

Cellulose: is the most widely distributed and abundant insoluble fiber in nature. Cellulose is a high molecular weight polysaccharide, the monomer is β-glucose linked together by β-(1,4)-glycosidic bonds. Cellulose molecules have the form of a straight chain and long fibers, cellulose are arranged in parallel and attached to each other by hydrogen bonds to form tough, strong, durable and flexible microfibers. It is a major structural component of plant cell walls, often in combination with hemicellulose, pectin, and lignin.

– Hemicellulose: is made up of both hexose and pentose sugars linked together by β-(1,4)-glycosidic bonds forming the backbone and side chains including galactose, arabinose and glucuronic acid linked via β (1–2) and β (1–3). Due to the higher degree of branching and the lower degree of polymerization (DP) of the molecule, the chain length is shorter than that of cellulose, hemicellulose exists in many different forms and is more hydrophilic, more sensitive to acidity, alkalinity and temperature, and therefore more susceptible to hydrolysis than cellulose.

– Lignin is a large molecule with a complex amorphous structure, made up of highly branched phenolic polymers with strong intramolecular bonds. The lignin molecule is not homogenous, forming bonds with hemicellulose and surrounding cellulose via ester bonds in plants to provide rigidity and protection of the cell wall.

3. Soluble diatery fiber

SDF consists of substances with different structures, mainly composed of resistant oligosaccharides and viscousity of high molecular weight. The solubility of SDF can be quite variable depending not only on the structure of the fiber but also on external factors such as temperature and pH value.

– Fructo-oligosaccharides (FOS) and galacto-oligosaccharides (GOS) are the most typical and studied resistant oligosaccharides. They are easily fermented in the intestine because of their low molecular weight and high solubility.

+ FOS: composed of sucrose combined with 1 to 3 fructose linked together by β-glycosidic bonds.

+ GOS: Galactose or glucose combined with 1 to 7 galactose linked by β-glycosidic bond.

– Inulin: D-fructose linked by β-glycosidic bond to glucose at the end with polymerization (DP) from 2 to 60. Inulins with DP less than 10 belong to resistant oligosaccharide, while others belong to viscous fiber.

– Due to their large molecular weight and strong hydrophilicity, viscousity, including β-glucan, pectin and gum, can dissolve in water as well as form gel structures at the required critical concentrations with the ability to water retention, which can inhibit intestinal glucose and lipid absorption.

+ β-glucan: linear macromolecular structure of glucose monomers linked by β (1-4) and β (1-3) and has high viscosity at low concentrations. It is a structural component of plant cell walls and can also be synthesized by enzyme technology.

+ Pectin: Heterozygous polysaccharide with complex structure mainly consisting of D-galacturonic acids linked by α-1,4-glycosidic bonds and containing small amounts of rhamnose, arabinose and galactose.

+ Gum: Gum is a polymer made up of monomers that are sugars or sugar derivatives, including: glucose, galactose, mannose, fucose, glucuronic acid, rhamnose, arabinose, xylose, etc.
Gum is a high molecular weight polysaccharide that at low concentrations can combine with water to increase the viscosity of water or solution, forming a gel state.

References:

[1] G. A. Soliman, “Dietary Fiber, Atherosclerosis, and Cardiovascular Disease”, Nutrients, vol 11, no. 5, p. 1155, May 2019, doi: 10.3390/nu11051155.

[2] A. M. Stephen et c.s., “Dietary fibre in Europe: current state of knowledge on definitions, sources, recommendations, intakes and relationships to health”, Nutr Res Rev, vol 30, p.h 2, pp. 149-190, December 2017, doi: 10.1017/S095442241700004X.

[3] Z.-W. Guan, E.-Z. Yu, and Q. Feng, “Soluble Dietary Fiber, One of the Most Important Nutrients for the Gut Microbiota”, Molecules, vol 26, p.h 22, p 6802, November 2021, doi: 10.3390/molecules26226802.

[4] “Hemicellulose – overview | ScienceDirect topic”. https://www.sciencedirect.com/topics/materials-science/hemicellulose (accessed 23 October 2022).

Article source: Nutrition Research and Development Institute (https://inrd.vn/)

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