Dietary fibre

Definition and occurrence 

As early as 1883, Max Rubner described wheat bran as “roughage for the stomach” (Ballast für den Magen) on the basis of his own experiments, coining the term “roughage” (Ballaststoffe) [12]. In 1953, Hipsley referred to a group of nutritional components, which are found in plant cell walls and cannot be digested, as dietary fibre [13] by which he understood plant “fibres” like cellulose and hemicellulose, including lignin. Trowell expanded the term in 1976 to embrace chemically distinct, indigestible components of edible plants [14]. Despite this definition, discussions continued as to which plant components were supposed to belong to the dietary fibre group. The definition of dietary fibre was therefore revised and adopted at international level by the Codex Alimentarius Commission in June 2009. The Codex [15] describes dietary fibre as chains of carbohydrates (polymers) that comprise ten or more links in the chain (monomers) that can pass through the human small intestine without being broken down by endogenous enzymes and subsequently absorbed. They fall into one of the following categories:

•    carbohydrate polymers that occur naturally in a food in its edible form
•    carbohydrate polymers that are derived from food raw materials through physical, enzymatic or chemical processes and that have physiological health benefits, as demonstrated to the competent authorities by generally accepted scientific evidence
•    synthetic carbohydrate polymers that have physiological health benefits, as demonstrated to the competent authorities by generally accepted scientific evidence

Moreover, the Codex Alimentarius states that lignin and other accompanying substances can be defined as plant fibres if they are closely associated with polysaccharides in the plant cell walls and are detected using official analytical methods. Depending on the regulatory or analytical definition, chitin, which occurs in fungi and insects, also counts as dietary fibre. The present scientific classification exclusively uses the definition in the Codex Alimentarius which does not include chitin as dietary fibre [15].

In a footnote referring to the definition of dietary fibre by the Codex Alimentarius Commission, national authorities were allowed to classify carbohydrate polymers as dietary fibre even if they only consist of three to nine monomers. This means that low molecular weight, soluble dietary fibre like fructooligosaccharides (FOS) can also be included.
This expanded definition was incorporated into the Food Information Regulation [16]). Here, dietary fibre is defined as carbohydrate polymers of three or more monomers that can neither be digested nor absorbed in the human small intestine. In doing so, the European legislators made the footnote provision in the Codex definition mandatory for a degree of polymerisation of ≥ 3 and explicitly included low molecular weight soluble dietary fibre within the scope of nutrition labelling. In 2021, the DGE integrated this expanded definition in its revised reference values for dietary fibre [1].

What are the groups of dietary fibre? 

In dietary fibre, a restricted number of monosaccharides, such as glucose, galactose, mannose, fructose, arabinose, xylose, rhamnose and fucose, are linked into structures of varying complexity (primary structure), resulting in a large number of possible secondary (hydrogen bonds within the chain) and tertiary structures (spatial arrangement of the secondary structure). Examples of different secondary structures include microfibrils, such as those formed by cellulose, or spiral (helical) structures, as in the case of pectin.
As presented in Figure 1, dietary fibre can be ascribed to various categories according to molecular weight and solubility. High molecular fibre (High Molecular Weight Dietary Fibre, HMWDF) with a degree of polymerisation of ≥ 10 can be divided into water-soluble (Soluble Dietary Fibre, Precipitated,SDFP) and water-insoluble (insoluble Dietary Fibre, IDF) high molecular fibre. Resistant starch (RS) is classified in up to four different categories from RS1 to RS4. The Low Molecular Weight Dietary Fibre (LMWDF) category contains carbohydrates with a degree of polymerisation of three to nine which are soluble in 78% ethanol and cannot be broken down by the human digestive system (SDFS). Due to its differing structures, dietary fibre has different characteristics which eventually lead to different physical and physiological effects (cf. section on the physiological effects of [various] dietary fibre).
 

Dietary fibre analysis

The total dietary fibre content is the sum of non-soluble (IDF) and soluble dietary fibre (SDFP and SDFS).  

The dietary fibre data in BLS version 3.02 were determined using AOAC methods 985.29 and 991.43 [17, 18]. As described above, starting with BLS version 4.0, there will be a gradual transition to data analyzed using AOAC method 2022.01 [19]. Knowledge of the analytical method used is necessary for interpreting and applying the dietary fibre contents in the BLS. The dietary fibre listed in the current Codex Alimentarius definition is not recorded correctly when using the two older methods, AOAC 991.43 and 985.29. For the resistant starches in categories RS1 to RS3, the measured content is too low, for RS4, it is too high. The two older methods do not take SDFS into account. With regard to IDF and SDFP, there are no differences in the results of the methods used for the BLS. The current AOAC 2022.01 method enable the correct determination of IDF and SDFP, including resistant starches in categories RS1 to RS4 as well as SDFS, and thus records all dietary fibre [20]. Consequently, the AOAC 2022.01 method yields higher dietary fibre content compared to the two older methods. 

The following illustration shows the dietary fibre that is taken into account using the two older methods, AOAC-991.43 and 985.29, in comparison with the newest AOAC method 2022.01: 

Physiological effects of (various) dietary fibre

The health-promoting effects of dietary fibre are not based on a single mechanism but on the interaction of various physiological effects along the gastrointestinal tract as well as systemic metabolic processes. Two particular characteristics of dietary fibre play a crucial role in this: water solubility and fermentability, that is, the extent to which dietary fibre can be broken down by microorganisms in the gut, known as the microbiota, through fermentation. When insoluble dietary fibre, like cellulose, is eaten with sufficient fluid, it swells up and increases the stool volume, stimulating bowel motility and increasing the frequency of bowel movements [21]. If too little fluid is imbibed this dietary fibre can cause gastrointestinal discomfort, such as bloating or constipation. If eaten in very high quantities, it can also mildly impair the absorption of certain minerals; if a balanced diet is maintained, however, this is normally not clinically relevant [22]. It is almost impossible for human gut microbiota to ferment cellulose [23]. It therefore exerts its effects primarily through the physical properties described. 

A key mechanism involves water-soluble dietary fibre such as pectins and β-glucans which increase the viscosity of the chyme in the stomach, thereby delaying gastric emptying [24, 25]. This effect continues in the small intestine and slows down the mixing with digestive enzymes as well as delaying the absorption of glucose and lipids [26]. This, in turn, causes post-prandial blood glucose and insulin spikes to flatten out, which improves long-term insulin sensitivity and is associated with a lower risk of type 2 diabetes mellitus [22, 27]. A delay in emptying the bowel also has a positive impact on feeling full. If the feeling of fullness begins earlier and lasts longer it can reduce energy intake and thus help prevent overweightness and obesity [24, 25, 28]. In parallel, some water-soluble dietary fibre, including pectins and β-glucans, can bind to bile acids in the small intestine and promote their excretion. The resulting increase in de novo synthesis of bile acids in the liver helps lower LDL cholesterol and thus reduces cardiovascular risk factors [29, 30].

In most cases, soluble dietary fibre is easily fermented by the gut microbiota, serving as a food source and, in the form of the compounds produced by fermentation, offering a raft of physiologically beneficial effects. A significant portion of dietary fibre is broken down into the short-chain fatty acids (SCFAs) acetate, propionate, and butyrate which primarily serve as an important source of energy for the lining of the large intestine [31]. Part of this is absorbed into the blood stream where, amongst other things, the SCFAs have anti-inflammatory effects, modulate lipid metabolism and influence glucose homeostasis [32–37]. In this way, the gut microbiota can influence systemic metabolism through these breakdown products.

The dietary fibre’s chemical structures produce diverse microbial metabolic pathways. A varied diet involving different plant foods is usually associated with the consumption of various types of dietary fibre as well as high ecological diversity in the gut microbiota [38]. An unbalanced diet involving few different types of dietary fibre, on the other hand, leads to less diversity in the microbiota [39]. If fermentable dietary fibre is missing from the diet altogether, significantly less SCFA is produced. Instead, the microbiota utilises to a greater extent other sources of nutrition like proteins. The fermentation of proteins produces branched-chain fatty acids (BCFAs) as well as compounds containing sulfur and nitrogen, such as hydrogen sulfide and ammonia, which, amongst other things, can have a toxic effect on intestinal tissue [31]. Regarding the prevention of cancers like colon cancer, butyrate and other SCFAs generally have a positive effect whereby other mechanisms also play a role. A shortish transit time, combined with a sufficiently high frequency of bowel movements, reduces exposure to carcinogenic substances. The latter include hydrogen sulfide, which is produced during protein fermentation. On the other hand, butyrate has been shown to have a protective effect against colorectal cancer and inflammatory reactions [8, 31, 40].

The fermentability of dietary fibre partly depends on the length of the molecule chain. Low molecular dietary fibre like FOS is characterised by particularly fast, intensive SCFA production. However, when produced in excess, the gases generated as byproducts of fermentation (primarily hydrogen and carbon dioxide) can no longer be effectively removed via the bloodstream or reused by the microbiota. This is the main reason for bloating and flatulence which are often seen as negative side-effects of a high-fibre diet [24, 39-41]. But the gut microbiota can adapt to an increased intake of fermentable dietary fibre, as a result of which the gut becomes habituated and the undesirable effects gradually decrease [40, 42]. 

Resistant starches occupy a special place. In the large intestine, they are preferentially fermented into butyrate, which serves as the most important energy source for the cells of the large intestine mucosa and plays a crucial role in maintaining the integrity of the intestinal barrier. An intact intestinal barrier prevents inflammatory substances from entering the circulatory system and thus reduces systemic inflammatory processes [43, 44]. Consuming too much resistant starch can also lead to gastrointestinal discomfort which, however, is usually less painful than that caused by rapidly fermentable oligosaccharides.

Apart from the characteristics of dietary fibre itself, the composition and matrix of high-fibre foods is very important. High-fibre foods are often also rich in secondary phytochemicals such as carotenoids and polyphenols [26]. Moreover, health-promoting effects normally occur in the combination of the individual physiological effects of dietary fibre, secondary phytochemicals and other nutritional components [45].

A varied intake of different plant-based food is therefore explicitly recommendable, as also stated in national nutrition recommendations [1, 5]. The reference value for dietary fibre intake of at least 30g/day [1,41] is primarily based on the positive effects that have been observed in the prevention of chronic disease. Physiologically, no strict distinction is made between low and high molecular weight dietary fibre. Rather, what counts is the total intake and the resulting effects on the body. As even low molecular weight dietary fibre and resistant starches present relevant physiological effects, in principle, they can count towards the reference value. The evidence for the current intake recommendation of 30 g/day derives from a state of research in which the content of dietary fibre was analysed using the older AOAC methods.
 

Conclusion

Dietary fibre is a significant part of a healthy diet. According to the representative consumption data available, however, in Germany less than the recommended amount is consumed on average. The use of AOAC method 2022.01 captures additional fractions of dietary fibre, resulting in higher dietary fibre contents being reported for numerous foods in BLS Version 4.0, even though their actual composition has not changed. As food groups that are relevant with regard to dietary fibre, such as bread, cereals and baked goods, have already been thoroughly updated, these changes have a considerable impact on the dietary fibre data in the BLS.

It is important to conduct a comprehensive analytical assessment of dietary fibre, as it unleashes a wide range of effects in the human body that depend heavily on its structural properties, such as solubility and fermentability. Both high molecular weight and low molecular weight dietary fibre, including resistant starches, contribute to different health effects in various ways. It is, therefore, not only the entire intake of dietary fibre that is decisive for the health-promoting effect but also the diversity of high-fibre foods. Against this backdrop, the MRI concurs with the valid recommendations for varied, plant-based diet. Currently, there is a lack of data on whether meeting the recommended intake more easily by consuming low molecular weight, rapidly fermentable dietary fibre – as measured by AOAC method 2022.01 – is actually associated with the same long-term health benefits, or whether the recommendations need to be adjusted to ensure optimum physiological effects.