Health Benefits of Prebiotic Fibers

Diederick Meyer1    Sensus BV, Roosendaal, The Netherlands
1 Corresponding author: email address: diederick.meyer@sensus.nl

1 Prebiotic Fibers

1.1 Definitions and properties of dietary fiber

The definition of dietary fiber has been the subject of an almost endless debate, mainly because dietary fiber is not one single chemical entity like starch or cellulose. Some based the definition on physiological features, whereas other used its chemical composition. The original description by Hipsley (1953) described dietary fiber as nondigestible constituents of plant cells walls. Later, Trowell and others expanded the description to “consisting of plant polysaccharides and lignin which are resistant to hydrolysis by digestive enzymes of man.” More importantly, these authors also came up with the dietary fiber hypothesis related to health observations (Trowell, 1972, 1976). The definition of dietary fiber was based on (one of) its physiological features, namely, its nondigestibility. Based on this feature, fiber determinations were developed mimicking the human digestion in a glass tube, to determine the fiber content of food, e.g., to assist food industry and enforcing authorities with nutritional labeling. In 1985, these efforts led to an approved AOAC method (AOAC 985.29) and in many countries it was used as a de facto definition of dietary fiber: material determined by this method is dietary fiber (AOAC 985.29, 2012). Soon it turned out that many nondigestible carbohydrates with physiological functions as dietary fiber were not assessed by this method or by other methods based on AOAC 985.29. This has led to the development of a battery of assays aimed at determining specific dietary fibers, such as AOAC 997.08 and 999.03 for fructans (AOAC 997.08, 2012; AOAC 999.03, 2012), or AOAC 2000.11 for polydextrose (AOAC 2000.11, 2012). A more detailed description of the development of dietary fiber definitions and assessments can be found in Prosky (2001) and Tungland and Meyer (2002). Figure 1 (left-hand picture) shows the situation before the latest developments that will be described below.

The whole discussion on the definition has now led to two definitions for dietary fiber for labeling; they both are based on the nondigestibility. In the EU, dietary fiber (dietary fiber) means carbohydrate polymers (either naturally occurring or obtained by physical, enzymatic or chemical means, or synthetic polymers) that are not hydrolyzed by the digestive enzymes in the small intestine of humans. The carbohydrate polymers must have a degree of polymerization (DP) of three or more monomeric units (European Commission, 2008/100/EC) and for isolated fibers and synthetic polymers a beneficial physiological effect has to be proven based on generally accepted scientific evidence. The definition adopted by Codex Alimentarius Commission (2009; Alinorm 09/32/26) is based on the same type of carbohydrate polymers, but on those having a DP of 10 and above. However, a footnote which forms an integral part of this definition, states that the decision on whether or not to include carbohydrates from three to nine monomeric units should be left to national authorities (see also Harris & Pijls, 2009). Also this definition requires evidence for a beneficial physiological effect for carbohydrates obtained by physical, enzymatic or chemical means, and synthetic polymers. It should be stressed that from a physiological point of view, there is no reason to exclude oligomers with DP < 10 (Howlett et al., 2010).

With this definition in mind, a universal method determining all dietary fibers in food was developed (McCleary, 2007; McCleary et al., 2010). These methods are now available as validated methods (AOAC 2009.01 and AOAC 2011.25) and they can be used to assess the total dietary fiber content of food irrespective of the type of fiber present (see Fig. 1) (AOAC 2009.01, 2012; AOAC 2011.25, 2012). Not surprisingly, also these methods have their disadvantages; apart from the laborious procedure it now emerges that some fibers still partially escape detection (e.g., Zielinski, DeVries, Craig, & Bridges, 2013). In connection with the issue about the DP required to classify as dietary fiber as in the Codex definition, it should be stressed that these or any other approved analytical method cannot discriminate dietary fibers with DP < 10 from those with DP ≥ 10 (Betteridge, Caers, Lupton, Slavin, & Devries, 2012).

1.1.1 Physiological properties of dietary fibers

Dietary fiber is acknowledged worldwide for its positive effects on health and well-being. The benefits include positive effects on bowel habit, a favorable effect on fermentation in the colon, a reduction of blood (LDL-) cholesterol levels and an improvement of blood glucose and insulin levels. Moreover there are associations from epidemiological evidence mainly between a lowered risk for colon cancer and for obesity with appropriate fiber consumption (e.g., EFSA, 2010b; Health Council of the Netherlands, 2006). An overview of the effects of different kind of fibers is presented in Table 1. Some dietary fibers, such as pectins or some gums, also have physiological effects due to their influence on the rheology of the intestinal content; a high viscosity is generally connected with a delayed gastric emptying and increased small intestinal transit time. A viscous environment in the small intestine may also inhibit absorption of nutrients with its physiological consequences.

It should be noted that the evidence for much of the health benefits described below comes from epidemiological associations. In many cases it is not easy to carry out trials for such benefits with isolated dietary fibers, as these trials will take too long (e.g., for the lowered risk for colon cancer, or a lowered death rate from cardiovascular disease) and thus are very costly to carry out. For many of these diseases the lack of suitable and validated biomarkers also plays an important role; as an example, whereas the serum lipid level of cholesterol is an accepted biomarker for the risk for cardiovascular disease, such markers are not available for colon cancer, or obesity.

1.1.2 Effect on bowel movement

The best known effect of dietary fiber is its influence on stool: it decreases the time for food passage through the entire gastrointestinal tract and increases fecal bulk. In fact, this feature is used by some authorities as a basis for their guidelines for fiber intake (Health Council of the Netherlands, 2006; Institute of Medicine, 2005). Moreover, these effects can also be investigated easily in intervention studies with isolated fibers. As shown in Table 1 insoluble fibers have the strongest effect on bowel habit as they act as fecal bulking agents; e.g., what bran provides from 2.6 to 4.9 g/g (Cummings, Beatty, Kingman, Bingham, & Englyst, 1996; Maki et al., 2009), whereas soluble fibers such as pectin or inulin only provide about 1–2 g/g (e.g. Den Hond, Geypens, & Ghoos, 2000; Salminen et al., 1998). Their main effect is on stool consistency and frequency of defecation as they increase the softness of fecal matter.

1.1.3 Favorable colonic fermentation

Dietary fibers reach the colon intact and there they can be fermented by specific colonic bacteria and converted into short-chain fatty acids (SCFA), lactic acid and gas. The extent of the fermentation depends very much on the type...