Dietary Fibers – Classification and functions

Certain types of carbohydrates are classified as dietary fibers. These fibers are generally indigestible polysaccharides (complex carbs), but other organic compounds are also considered fibers. Usually, plant-based foods, such as grains, legumes, and vegetables, deliver the vast majority of fiber in our daily diet. That is because these indigestible polysaccharides serve the plants in several ways. For example, they are involved in building the cell’s structural framework, and they provide cellular protection (Schusdziarra et al., 2011). 

Principally, dietary fibres can neither be absorbed in the digestive tract, particularly in the small intestine, nor can enzymes reduce them to smaller digestible pieces. Thus, they get undigested into the colon. Hence, fibers don’t play a significant role in energy supply. Nevertheless, on their way through the alimentary canal, fibers can unfold various effects and thus can positively impact the whole body’s metabolism. The following article covers, among other things, how dietary fibers can be categorized and how these undigestible sugars influence several parts of the digestive system (Biesalski et al., 2017). Download a PDF of this article for FREE. You will find the link below the post.


Dietary fibres - spoon full of chia seeds - Picture by ValeriaLu from Pixabay
Dietary fibers – spoon full of chia seeds – Picture by ValeriaLu from Pixabay

Fibers – Classification

Because of their distinct properties, fibers can be categorized into different classes. For this purpose, the solubility of fibers is often considered. However, other properties, such as the structure or the fermentability, could also be considered (Ströhle et al., 2012). When it comes to solubility, fibers can be distinguished into the following classes:

Soluble fibers

As their name suggests, soluble fibers dissolve in water. This means this fiber can absorb much fluid (< 60 ml/g) and swell up. Imagine you eat something that contains some fibers. You take a bite, and after chewing for a while, a piece slides down your esophagus and lands in your stomach. Then, the gastric acid dissolves the bolus, and the soluble fibers within will dissolve in the fluid to form a jelly-like structure. And, because of this jellylike structure, the bolus will move slowly, just little by little, from the stomach into the small intestine. Most fruits and vegetables contain more soluble fibers, especially pectin (Ströhle et al., 2012).

Insoluble fibers

Unlike their soluble counterparts, insoluble fibers can absorb considerably less water (about 3 ml/g), resulting in limited swellability. Roughage is primarily contained in whole-grain products and legumes (Ströhle et al., 2012).

The following table displays soluble and insoluble fibers (Schusdziarra et al., 2011).

Tab. 1: The table shows some members of soluble and insoluble fibers.

Soluble fibres

Insoluble fibres






Some hemicelluloses

Some hemicelluloses


Fibers – Effects

Fibers can influence several organs of the digestive tract. The overview below will look at different stages of the alimentary canal regarding the effects that dietary fibers can have. (The following stages are considered on the understanding that the food was rich in fibers.).


Effects of dietary fibers in the mouth

If you compare the consumption of fiber-rich food, such as eating a whole-grain roll, with white bread, which is relatively low in fiber, you will probably agree that it needs to be chewed more to make it smaller. This implies that fibers necessitate intensive and prolonged chewing. If chewing took longer, ingestion would take longer, too. And as a consequence, there is a chance satiation sets in before the plate is empty. Thus, fibers can contribute to satiation. But besides that, because of prolonged chewing, saliva release is stimulated. Since saliva contains, among other things, antibacterial substances, increased salivation may help neutralize unwanted bacteria and contribute to dental health (Ströhle et al., 2012).


Effects of dietary fibres on the stomach

Dietary fiber effect in the stomach: Certain dietary fibers can swell when combined with water. Extensive chewing causes the moistened food pulp, which is already mixed with liquid, to slide down the esophagus and end up in the stomach. There, the chyme is mixed with the gastric acid by circulation processes. The dietary fibers have had and continue to have plenty of time to swell. The more the dietary fiber swells, the greater the volume and viscosity of the porridge. You may already notice how your stomach is filling up. (Mix 100 – 150 g of oatmeal with a bit of water for a self-experiment. Then eat it as muesli and drink 1 – 2 glasses of water. Now it’s time to wait.). More volume and more viscosity of the chyme lead to slower stomach emptying and increased satiety (Ströhle et al., 2012).


Effects of dietary fibers in the small intestine

The chyme now gradually moves from the stomach into the small intestine. The small intestine is the principal place where nutrients are received. H. something like simple sugars or amino acids z. B. from here to the liver. If the food pulp is not passed on so quickly, but rather gradually, from the stomach to the small intestine, the nutrients also “only” migrate to the liver. That can’t be so bad because, e.g. B., the blood sugar level and thus the insulin level rises rather slowly and can be kept constant, compared to a rapid rise in blood sugar levels after eating quickly digestible foods such as white bread (Ströhle et al., 2012).

Effects of dietary fibres on the colon

The large intestine is populated with essential bacteria for intestinal health. Dietary fibers can significantly limit the proliferation of undesirable microorganisms (Ströhle et al., 2012) by, e.g., ensuring that the “good” bacteria present (Bifidus and Lactobacillus) multiply (Biesalski et al., 2017). The soluble dietary fibers, in particular, can ensure that the microbial cell mass increases. The water-insoluble fiber can increase stool volume (Ströhle et al., 2012). Increased stool volume can promote defecation (Biesalski et al., 2017).

Other effects of dietary fibers

Dietary fiber can help eliminate cholesterol and thus hinder the formation of cholesterol-containing gallstones by promoting bile acid production (Biesalski et al., 2017). The chyme is transported more quickly in the small and large intestines due to the presence of roughage. One speaks of a shortened transit time (transit time) due to increased peristalsis (movement of hollow organs to move the contents). Dietary fiber can help prevent cancer in the colon and rectum (Biesalski et al., 2017).

Dietary fiber-rich food

Tab. 2: The table shows some dietary fiber-rich foods.

FoodDietary fiber per 100 g
Wheat flour (type 405)4.0
Wheat, whole grain13.3
Peas, seed, dried16.6
Lentils, seed, dried17.0


Biesalski, H. K., Pirlich, M., Bischoff, S. C., & Weimann, A. (Eds.). (2017). Ernährungsmedizin: Nach dem Curriculum Ernährungsmedizin der Bundesärztekammer. Georg Thieme Verlag.

Schusdziarra, V., Hausmann, M., Sassen, M., Kellner, M., Mittermeier, J. & Erdmann, J. (2011). Ballaststoffe, Energieaufnahme und Lebensmittelverzehr. Aktuelle Ernährungsmedizin. Vol. 36. Issue 1. pp. 23-30.

Ströhle, A., Wolters, M. & Hahn, A. (2012). Gesundheitliche Aspekte von Ballaststoffen – Ein Update, Teil 1: Von der Struktur zur Funktion. Deutsche Apotheker Zeitung. Nr. 31.



My newsletter comes once a month and shows you a selection of articles that I have recently published or updated. Since I can’t publish with a regular schedule, that’s a great thing, isn’t it?😉

Information on the registration process, service provider, statistical evaluation and revocation can be found in my Data protection

In order to make my newsletter interesting for you, I statistically record which links the users have clicked in the newsletter. By registering, you agree to this statistical recording.

Be the first to comment

Leave a Reply

Your email address will not be published.