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Dietary fiber (DF)

Dietary fiber not only promotes digestion but also strengthens the heart and immune system. Frequent constipation? This could be a sign of a fiber deficiency. Learn in our article which foods are particularly rich in fiber and what benefits they offer.

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Definition
- Classification of Dietary Fiber
- Specific Dietary Fibers
Examples of
- Storage and processing losses
Nutrition - Health
Functions in the body
Dietary
Bibliography

A balanced, plant-based diet with few to no industrially processed foods generally provides sufficient macro- and micronutrients, with the exception of vitamin B₁₂ . However, phytochemicals are particularly relevant for maintaining health and healing, even though they are not considered essential nutrients – apart from vitamins.

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Definition:

Dietary fiber (DF) is edible plant material consisting of indigestible carbohydrates and lignin. It is indigestible because we lack the necessary enzymes. ^Dietary fiber resists digestion in the human small intestine and ferments partially or completely in the large intestine. In the large intestine, it influences, ^{among other things}, the composition and consistency of stool as ^well as the gut bacteria (microbiota).

Accordingly, the European Food Safety Authority ( EFSA ) defines dietary fiber as non-digestible carbohydrates plus lignin. ⁶

The majority of dietary fiber consists of non-starch polysaccharides (NSPs, i.e., complex sugars = polysaccharides that differ from the complex sugar starch). ^1,5 In addition to NSPs, the term dietary fiber also includes lignin (plant substances that cause lignification by being incorporated into the cell wall), which is often associated with fiber. ⁵ Examples of dietary fiber are cellulose, pectins, hydrocolloids, fructo-oligosaccharides, and resistant starch. ⁶

Dietary fiber acts in many parts of the digestive system, including the mouth, stomach, small intestine, and large intestine (colon ⁾ ^. ^Gut bacteria process certain types of dietary fiber into absorbable food components. The positive effects on health depend on their different physicochemical properties: they are water-soluble or water-insoluble, solid or liquid, and fermentable by bacteria or not.

Classification of Dietary Fiber

There is no uniform classification of dietary fiber in the literature. Detailed distinctions are made according to (a) the type of source (plant or animal polysaccharides and polysaccharides from native or synthetic sources), (b) the type of polysaccharide structure (linear or non-linear molecular structure), and (c) solubility. ^Dietary ^fiber can also be distinguished with regard to its solubility in hot water, its water-holding capacity, and its viscosity.

The focus, however, is usually on their solubility (water-soluble or water-insoluble) . ^2,4,7 Soluble fiber is the edible part of the plant that resists digestion but can be partially or completely fermented by bacteria in the large intestine. Insoluble fiber, on the other hand, usually passes through the digestive tract intact. ² Most foods contain both types of fiber, about one-third soluble and two-thirds insoluble. ⁷ Since the main sources of soluble fiber are fruits and vegetables, grains and whole-grain products represent the largest proportion of insoluble fiber . ⁶

In addition to the aforementioned classification, there is also a distinction between "intrinsic" and "functional" dietary fiber: "intrinsic" dietary fiber (dietary fiber = indigestible carbohydrates that occur intact in plants) ⁷, "functional" dietary fiber (extracted and/or synthetically produced dietary fiber that has been shown to have beneficial effects on the body in humans). ^7,8 Functional dietary fibers include: β-glucans, cellulose, chitins and chitosan, fructans, gums (plant gums; sticky plant excretions such as gum arabic – one of the oldest binding agents), lignin, pectin, polydextrose and polyols, psyllium ( psyllium husks ), resistant dextrins, and resistant starch. ²

Specific Dietary Fibers

Here we will discuss some of the dietary fibers mentioned above in more detail. These include lignin, cellulose (crude fiber), hemicellulose (pentosans), pectin (pectin compounds), storage polysaccharides, resistant starch, and oligosaccharides:

lignin

Lignin: Lignin is classified as dietary fiber, although it bears no resemblance to carbohydrates. It consists of phenylpropane units and supports the hardening and lignification of plants. We ingest very little lignin through our diet. It is neither fermentable nor water-soluble. ^1

Non-starch polysaccharides (NSP)

Cellulose: A component of plant cell walls, consisting of glucose units. It is found in grains, vegetables, and fruits, is insoluble in water, and cannot be fermented.
Hemicellulose: Like cellulose, it is an important component of plant cell walls. It is found primarily in wheat and rye and can consist of glucose as well as other carbohydrate units. Depending on its composition, hemicellulose (not hemicellulose) has different properties. ¹
Pectins: The majority are found in fruits. They are completely soluble in water and, in this process, cause gel formation or lead to the creation of viscous (thick) solutions. Intestinal bacteria completely ferment pectins and produce acetate (acetic acid) as a byproduct. ¹
Storage polysaccharides such as guar and inulin:
- Guar is found primarily in the seeds of legumes. It is water-soluble and readily fermented by bacteria. ¹
- Inulin consists of approximately 30 fructose units and is found primarily in artichokes, leeks, garlic, wheat, rye, bananas, and Jerusalem artichokes . It is also highly water-soluble, fermentable, and a recognized prebiotic (a food component that promotes the growth of predominantly beneficial bacteria). ¹
Other NSP dietary fibers include plant gums and mucilage . The latter are found in particularly high concentrations in the seed coats of certain plants and possess a high water-binding capacity. An example is psyllium husk, which is also taken for its laxative and cholesterol-lowering effects. ^1

Resistant starch

Resistant starch is defined as the portion of starch that cannot be broken down by human enzymes. A small amount of the starch we ingest passes into our large intestine unchanged, where it is fermented by gut bacteria. ¹ Foods such as potatoes, rice, pasta, cereals, and bread contain little resistant starch (<2.5%, dry matter). Cooked legumes, peas, and cooked and cooled starchy foods have a high resistant starch content (5.0–15%, dry matter). ⁹ This is because the cooling process of heated starchy foods alters their chemical structure, making them difficult for our bodies to break down. In other words, cooled rice contains more fiber than freshly cooked, warm rice and is more beneficial for gut bacteria. This is also a way to increase fiber intake in everyday life if you otherwise consume too little fiber.

Oligosaccharides

Oligosaccharides consist of 3–9 linked sugar units, while polysaccharides consist of more than 10 linked sugar units. Oligosaccharides are highly water-soluble and readily fermented by the microbiome (microbiota). Two well-known examples are fructooligosaccharides (FOS) and galactooligosaccharides (GOS). ^1

Fructooligosaccharides occur naturally in plants such as onions, chicory, garlic, asparagus, bananas, and artichokes. They consist of a network of fructose units and pass undigested into the large intestine, where they serve as a food source for gut bacteria. Because of their sweetness, they are ^often used as sugar substitutes in processed foods and beverages.
Galactooligosaccharides consist of chains of galactose molecules. They are found in lentils, chickpeas, and beans, among other ^foods . Diets enriched with GOS increase the populations of Bifidobacteria and Lactobacillus species and their fermentation products in the ^large intestine.

Examples of

water-soluble dietary fiber include pectin, mucilage, fructans, and some components of resistant starch. ²

They can be found in whole grains (e.g., oats, barley, and wheat ), peas and beans (e.g. , lentils, black beans, kidney beans, etc.), some fruits and vegetables ( apples, oranges, carrots, etc.), as well as in seeds and nuts (e.g. , flaxseeds and psyllium seeds ). ⁷
They bypass digestion in the small intestine and are exposed to fermentation processes in the large intestine by our microflora. Important properties therefore include water solubility, the ability to form viscous solutions, and fermentability. It is ^now known that soluble fiber lowers blood cholesterol levels through various ^mechanisms .

Water-insoluble dietary fiber consists mainly of lignin, cellulose, and hemicellulose.

Whole grain products, wheat bran, brown rice, nuts, and seeds are particularly rich in these fibers. ^2,7 Certain vegetables (e.g., celery, cauliflower, Brussels sprouts ) as well as fruit and vegetable peels often contain a lot of insoluble fiber. ⁷
Insoluble fiber leads to faster gastric emptying and thus shortens intestinal transit time. It therefore has a laxative effect. In addition, it increases stool volume, which promotes regularity of digestion. ^However, its fat-reducing effect is less pronounced compared to soluble fiber ^.

If you look for foods with the highest levels of these substances in our diet, they are psyllium husks (80.8 g/100 g), wheat bran (42.8 g), chia seeds (34.4 g), flax seeds (27.3 g), various beans (20-25 g), nuts (6-12.5 g), and whole grains (11 g). ²⁰

Storage and processing losses

: When grains are milled into refined flour, the outer, fiber-rich layers are removed. This reduces the total dietary fiber content. In particular, the insoluble fiber content decreases, which is the cause of this reduction.

The heat treatment of dietary fiber alters its properties in various ways. Increasing the temperature breaks the molecular bonds of the polysaccharides, affecting the analysis, function, and nutritional significance of the fiber. Reduced association or depolymerization (the breakdown of large molecular chains into smaller units) of dietary fiber increases its solubility. Strong depolymerization results in alcohol-soluble fragments, reducing the fiber content. Moderate depolymerization or reduced association has only a minor effect on the fiber content but can alter the functional and physiological properties of the fiber. ^22

Nutrition - Health:

Dietary fiber has a multitude of positive effects on our health and acts through a number of mechanisms throughout the digestive system (including the mouth, stomach, and small and large intestines). In addition to maintaining a healthy gut, ^it also protects us from certain lifestyle diseases. The ^effects vary depending on the physicochemical properties, which is why it is recommended to consume a wide range of dietary fibers through a varied diet.

Dietary fiber supplements are available in many forms, including capsules, powders, and chewable tablets. They often consist of a single type of fiber, such as inulin, methylcellulose, psyllium, and wheat dextrin. ⁷ They are primarily marketed for the prevention of constipation and are considered a healthy option for increasing fiber intake, but clinical evidence shows that most dietary fiber supplements do not provide any of the health benefits associated with a high-fiber diet. ^8

This is not just for vegans or vegetarians:
Vegans often eat unhealthily. Avoidable nutritional mistakes.

Long-term daily requirement:

Adults should consume between 25 and 30 g of dietary fiber per day. ^1.19

Some countries and organizations provide gender- and age-specific recommendations for daily fiber intake.

Children: 1-3 years (19 g), 4-8 years (25 g), 9-13 years (31 g) ²
Adolescents: Girls 9-18 years (26 g), Boys 14-18 years (38 g) ²
Adult women: 19-50 years (25 g), ^2,4,8 from 50 years (21 g) ⁸
Adult men: 19-50 years (30 g ⁴ /38 g), ^2.8 from 50 years (30 g) ⁸

These recommendations apply to healthy individuals. Certain chronic diseases can affect fiber requirements. ²

Deficiency symptoms:

The low-fiber "Western diet" in industrialized nations is associated with an increased risk of diseases such as obesity, inflammatory bowel disease, type 2 diabetes, colorectal cancer, metabolic syndrome, and various heart diseases. ^4,6,17 Furthermore, a comprehensive study of the global impact of diet showed that an inadequate intake of whole grains, fruits, and sodium is responsible for half of all diet-related deaths and two-thirds of disability-adjusted life years (DALYs). ¹⁸

As Thomas M. Barber et al. aptly summarize, there is ample evidence that dietary fiber intake plays a vital role in overall metabolic health, not just in gut health. ^6 The authors point out that several factors contribute to our modern, low-fiber diet. The diverse and varied health benefits of dietary fiber mean that optimizing dietary habits is an important public health strategy for improving both metabolism and overall health. ^6

How widespread is a lack of dietary fiber?

Many people consume too little fiber, even though it offers numerous health benefits. As the authors of a scientific review published in 2020 summarize, while fiber intake is high in many countries, it is generally below recommended levels in high-income countries. ^4 It is a fact that modern dietary habits have led to a significant decline in fiber consumption. ^{17}

A clear example of dietary fiber deficiencies is provided by the authors of a scientific paper: they mention that, according to estimates, in 2016 only 5% of the US population consumed the recommended amount of dietary fiber. Adults consume an average of about 15 g of fiber per day, with those following a low-carbohydrate diet consuming less than 10 g per day. ^{8,2} Another review focusing on dietary fiber consumption in the United Kingdom shows that while the recommended daily intake has been increased to 30 g, only 13% of men and 4% of women meet this recommendation. According to the analyzed data, the average adult intake in 2019/2020 was 21 g for men and 17 g for women. ^4

These examples demonstrate the need to increase dietary fiber intake. Peter Cronin 's team writes in a publication that the staple foods of the Western diet have transformed from formerly fiber-rich to energy-rich and high-glycemic (high in readily available carbohydrates) foods. ^{17} In addition to lower fiber intake, the high protein and fat content of the Western diet also contributes to the significant differences in the composition of gut bacteria between individuals from industrialized and developing countries. The authors also note that a comparison of the microbiome (the entirety of microorganisms) of individuals from various industrialized countries (USA, Denmark, Spain, France, Italy, and Japan) showed hardly any differences in structure and composition. ¹⁷ In contrast, vegans and vegetarians from these societies have gut microbiomes that more closely resemble those of non-industrialized populations with a high habitual intake of fiber and a low incidence of metabolic diseases. ¹⁷

Overconsumption:

Excessive fiber intake can lead to abdominal discomfort, bloating, or difficulty with bowel movements. In people with irritable bowel syndrome, it can worsen constipation symptoms. In diabetics with severe gastroparesis (a condition in which people digest food more slowly than usual), blood sugar levels may also change with increased fiber intake. ⁷

In some people, fiber supplements can cause additional side effects such as allergic reactions, asthma, gastrointestinal problems, and interactions with medications and nutrients. Fiber ^supplements can also affect the properties of some medications when taken concurrently. For example, they can reduce the absorption of certain drugs and thus affect plasma levels (the concentration of a substance in the blood plasma), which can impact their effectiveness. Therefore, medications should generally be taken at least one hour before or two hours after taking a fiber supplement. ^This is usually not a concern when consuming natural fiber in normal amounts.

Functions in the body:

Dietary fiber contributes to health in many ways, and the type of dietary fiber is crucial.

Key health benefits

Delayed gastric emptying reduces feelings of hunger by lowering ghrelin levels, an appetite-stimulating hormone. ^1 This supports overweight or obese adults on a calorie-reduced diet in losing weight and helps them stick to the diet. ^{15}
Binding of bile acids, which leads the body to use more cholesterol to increase bile secretion and balance the bound bile acids. This also inhibits the formation of cholesterol-containing gallstones. ^1,4,7
Delaying nutrient absorption, particularly of easily digestible carbohydrates and starchy foods, reduces the blood glucose load . ^{1,4} There is evidence that total dietary fiber reduces the risk of type 2 diabetes by 6% for every 7 g of fiber consumed. ^4
Increased gastric peristalsis (muscle activity in the digestive tract that contributes to the further processing of food components) and increased food transit through the digestive tract, leading to faster elimination of food. ¹
Particularly swellable dietary fiber increases stool volume and helps prevent constipation because it has a high volume and, due to its indigestibility, positively influences stool consistency. ^1,8
Dietary ^fiber contributes to the healthy development of the gut flora, as it often serves as a nutrient for gut bacteria and thus acts as a prebiotic. A healthy gut flora protects us by depriving pathogenic germs, which can cause disease, of their habitat.
They help reduce the risk of colorectal cancer ( ^one of the most common cancers of the colon or rectum) as well as other cancers. ^4,2
Dietary fiber reduces the risk of cardiovascular disease, including heart disease and stroke, and may contribute to lower blood pressure . ^4.2
Dietary fiber, metabolized by gut bacteria into SCFAs (short-chain fatty acids ), promotes immunity and suppresses inflammatory responses in the gut and other organs. ¹
According to the book Nutritional Medicine by Biesalski et al., dietary fiber increases the absorption of certain micronutrients such as calcium, magnesium, zinc, and iron —fructooligosaccharides and inulin, in particular, play an important role in this process. ^1 However, there are also studies that suggest that certain micronutrients are less bioavailable due to the presence of dietary fiber. ^{16}

Modes of action according to fiber type

Soluble fiber: According to a 2019 review article, long-term randomized controlled trials provide strong evidence that soluble fiber intake is associated with an improved lipid profile, better inflammatory markers, and better overall health. ^2 It also contributes to lower blood pressure, improved blood glucose control, weight loss, and improved immune function. ^7
Insoluble fiber binds, for example, carcinogens (cancer-causing substances), mutagens (mutation-inducing substances), and toxins. This blocks or prevents their absorption and promotes their elimination. In ^particular, insoluble fibers such as cellulose, hemicellulose, and psyllium shorten transit time (the time required for stool to pass through the intestines) and increase stool volume. ^Therefore, a diet high in legumes and whole grains is especially effective in preventing constipation.

Physical effects: cholesterol, blood sugar and digestion

The physical effects of dietary fiber in the small intestine depend, among other things, on the viscosity of the gel-forming fibers. ^8 For example, β-glucans and psyllium increase viscosity and reduce the reabsorption of bile acids. ^4,8 Since our body needs cholesterol to produce bile acids, β-glucans thus lower cholesterol levels. ^2 Pectin, psyllium, and guar gum have a similar, though less pronounced, cholesterol-lowering effect. ⁴ The extent of cholesterol reduction depends on the type of fiber, the amount consumed, and the type of diet. Medium- to high-viscosity gel-forming fibers are particularly effective, and dose-response models show that 3 g of β-glucan per day from oats or barley is sufficient to lower cholesterol levels. ^7

Soluble fiber binds sugars in the small intestine and, due to its viscosity, increases the barrier effect against our enzymes. This leads to slower glucose absorption and a less rapid rise in insulin levels. Furthermore, the fiber barrier inhibits the enzyme amylase, which we need to break down starch. This results in less starch digestion and improves insulin sensitivity. ^4

This leads to the aforementioned gel-forming and viscous dietary fibers effectively lowering cholesterol levels. They also improve blood sugar control in patients with type 2 diabetes and metabolic syndrome (a combination of obesity, impaired glucose and lipid metabolism, and high blood pressure). ⁸ However, these benefits are limited to viscous soluble dietary fibers. This effect could not be demonstrated for less viscous soluble dietary fibers such as inulin or insoluble dietary fibers (e.g., wheat bran). ⁸

The latter can, however, have positive effects in other ways, as studies using inulin as an example show. A meta-analysis published in 2021 showed that increased inulin intake (averaging between 10 and 30 g per day in the observed studies) significantly lowered blood glucose, total cholesterol, and triacylglycerol levels in patients with prediabetes and diabetes. The researchers proposed several mechanisms to explain the observed health benefits in patients with prediabetes and diabetes. However, these mechanisms remain largely unclear. ^{21}

In the large intestine, dietary fiber can have a laxative effect if it (a) resists fermentation and thus remains intact throughout the colon, and (b) increases the percentage of water content to soften or bulk up the stool (e.g., wheat bran and psyllium husks). ^8 This is also one reason why consuming dietary fiber with adequate fluid intake helps to prevent or relieve constipation . ¹

Non-physical effects: prebiotics, gut flora, metabolism and immune system

Certain dietary fibers act as "prebiotics" in the large intestine. This class of functional fibers supports the activity or growth of beneficial bacteria, primarily lactobacilli and bifidobacteria. Examples of prebiotics include galacto-oligosaccharides and oligofructose (fructooligosaccharide, FOS), a complex sugar from the fructan group. ^2

Certain gut bacteria can synthesize and excrete short-chain fatty acids (SCFAs) from certain dietary fibers, which have a variety of effects. On the one hand, they promote the production of appetite-suppressing hormones such as peptide YY (peptide tyrosine-tyrosine or PYY) and glucagon-like peptide-1 (GLP-1) in the large intestine. On the other hand, SCFAs can cross the blood-brain barrier and act as signaling molecules in the brain to directly influence both appetite and food-related decision-making. ^Dietary fiber sources containing high amounts of β-glucan, resistant starch, and α-galactosides appear to provide higher levels of SCFAs. In addition to the parameters mentioned, the metabolic pathways influenced by SCFAs also include the production and breakdown of fatty acids and cholesterol synthesis. ^Furthermore, SCFAs boost immunity and suppress inflammatory responses in the gut and other ^organs .

Soluble and insoluble dietary fiber have numerous non-nutritive health benefits. The abundance of fiber in whole grains, fruits, and vegetables makes them attractive tools for disease prevention and reducing the risk of atherosclerosis and cardiovascular disease. ²

Conclusion:

We all have choices regarding our diet – although it is regrettable that a healthy diet generally costs more than an unhealthy diet based on highly processed foods. However, the widespread availability, convenience, and relatively low cost of highly processed foods should not force us to make these relatively unhealthy choices.

Regular and sufficient consumption of various dietary fibers is crucial for maintaining a healthy body. Depending on their physicochemical properties, different types of fiber influence far more than just our digestive processes in a variety of ways. Among other things, they promote a healthy gut and reduce the risk of type 2 diabetes, cardiovascular disease, and certain types of cancer. While water-soluble fibers (such as the majority of beta-glucans from grains like oats) regulate blood sugar and fat levels and reduce inflammation, water-insoluble fibers (such as psyllium from psyllium husks) are beneficial for increasing stool volume, preventing constipation, and facilitating the faster elimination of undigested food components.

Despite its many benefits, excessive fiber intake can lead to abdominal discomfort, bloating, or bowel movement difficulties, delay the absorption of certain medications, or have adverse effects on specific health conditions. However, such overconsumption is generally avoided when fiber is consumed naturally through a varied diet. Isolated fiber intake in the form of supplements is not comparable to the intake of natural fiber from food (along with its associated micronutrients and phytochemicals ).

Therefore, we recommend consuming enough fiber-rich foods, primarily whole grains, fruits and vegetables, nuts, seeds, and legumes. Most dietary guidelines recommend 25g of fiber per day for women and 38g for men. It's worth investing in your future by prioritizing fiber-rich foods. In a surprisingly large number of countries, the vast majority of people fall short of these recommended intakes, thus jeopardizing their long-term health.

Absorption and Metabolism

As mentioned at the beginning, dietary fiber consists of substances that our bodies cannot digest due to a lack of enzymes. Nevertheless, it can influence our energy levels in two opposing ways. On the one hand, our gut microbiota in the large intestine is able to ferment dietary fiber to varying degrees, depending on its properties. This process primarily produces short-chain fatty acids (SCFAs), which we can absorb and use as an energy source. It is estimated that humans produce 300 mmol of SCFAs daily, which corresponds to 20 g of SCFAs. ^5

However, dietary fiber can also reduce the energy value of foods by inhibiting the digestion and absorption of other energy-providing macronutrients in the food. Some dietary fibers are able to prolong processing time, increase gastric emptying time, and possibly induce metabolic processes in the intestine via SCFA production that may contribute to satiety. ^5 A scientific paper that examined the energy content of dietary fiber concluded that, based on the arguments above, the energy contribution can vary considerably. The net energy value is usually negative and generally does not exceed 8 kJ/g. ^5

Storage - Consumption - Losses:

Dietary fiber from food passes through the small intestine into the large intestine, where intestinal bacteria partially ferment it. The body excretes the unfermented fiber along with other indigestible food residues in the stool. ^23.5

Dietary

fiber is structurally classified into various categories. These include non-starch polysaccharides, resistant starches, resistant oligosaccharides, and lignin, a non-carbohydrate polymer found in plant cell walls. Depending on the source, dietary fiber can be broadly divided into three subgroups: (i) natural carbohydrate polymers from plants such as vegetables, fruits, and grains; (ii) edible carbohydrate polymers obtained through food processing that offer physiological benefits (such as resistant oligosaccharides and inulin); and (iii) synthetic carbohydrate polymers with proven benefits (such as methylcellulose). ²³

Bibliography - 23 Sources

Many researchers do not believe that Wikipedia is an authoritative source. One reason for this is that the information about literature cited and authors is often missing or unreliable. Our pictograms for nutritional values provide also information on calories (kcal).

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