Review Article

Review Article: Dietary Fibre in the era of Microbiome Science

John O'Grady; Eibhlís M. O'Connor; Fergus Shanahan


Aliment Pharmacol Ther. 2019;49(5):506-515. 

In This Article

Defining Dietary Fibre

Fibre is not a single substance but rather a heterogeneous group of materials, each with different biologic effects. It is comprised of plant-derived carbohydrate which evades typical human amylase-driven digestion in the absence of cellulase availability required for its breakdown. Instead, consumption by humans requires digestion by microbes, using anaerobic fermentation, the end products of which are short chain fatty acids (SCFA). Consequently, terms such as microbiota accessible carbohydrates (MACs) to describe fibre have emerged.[4,5] While the fermentation process separates most forms of dietary fibre from digestible carbohydrates, such as sugar and starch,[6] the distinction requires a more comprehensive definition. In addition, the fibre analysis methods of the Association of Official Analytical Chemists (AOAC) and those of Englyst lead to variability in measuring fibre content of food which impacts both recommended intake and standardised definitions.[7]

Definitions of dietary fibre have been offered by The Codex Alimentarius Alinorm, an internationally recognised food standards programme (a joint commission by the Food and Agriculture Organization of the United States [FAO] and the World Health Organization [WHO]), with apparent acceptance.[8,9,10] Dietary fibre may be defined as polymers with 10 or more monomeric units which are neither digested nor absorbed in the human intestine. The decision whether to include monomeric unit counts of 3-9 is left to national authorities.[8,10] In Europe, a minimum count of 3 is accepted and includes resistant (nondigestible) oligosaccharides (monomeric unit 3-9), nonstarch polysaccharides (monomeric unit ≥10) and resistant starch (monomeric unit ≥10)[8] (Figure 1). Lignin and other compounds associated with polysaccharides in plant cell walls are included in the definition as long as they remain associated with the oligosaccharide or polysaccharide fraction.[8,9]

Figure 1.

Fibre subtypes by polymer length. This is not exhaustive list of fibre examples

Dietary fibre may be subtyped by properties of solubility, viscosity and fermentation and, although there is significant overlap, this is of practical use to correlate certain dietary fibre characteristics to observed health outcomes (Figure 2).[2,11,12] Solubility refers to dissolution in water, but it is the viscosity (capacity to gel with water) of certain soluble fibres that influences chyme consistency and slows digestion of consumed nutrients to absorbable components by digestive enzymes.[13,14,15,16,17,18]

Figure 2.

Overlapping properties of fibre by solubility, viscosity and fermentation

Oligosaccharides are highly soluble and fermentable fibres and include fructo-oligosaccharides (FOS) and galacto-oligosaccharides (GOS). These short chain fibres are highly fermentable due to their small size and solubility.[6] The remaining subtypes, polysaccharides and resistant starch are broadly categorised as long chain fibres according to their properties (Figures 1 and 2).

Soluble, nonviscous, readily fermentable fibres (inulin, wheat dextrin) dissolve in water and are rapidly and completely fermented. Soluble, viscous, readily fermentable fibres (β-glucan, gums, pectin) are similar but form a gel-like consistency with water. These characteristics are then lost following fermentation.[10,13,14,15,16,17] Soluble, viscous, slowly fermented fibres (psyllium) also form a gel-like consistency, but do not undergo extensive fermentation.[13] As such, the capacity to interact with water is preserved throughout the colon. This allows softening of stools in those suffering from constipation and adds form to loose stools.[13,16,17]

Insoluble fibres (wheat bran, lignin, cellulose) exert a laxative effect by stimulation and irritation of gut mucosa to increase secretion and peristalsis. Large and coarse dietary fibre particles have a greater effect on stool bulking and transit time.[16,19,20,21]