Gut Fermentation of Dietary Fibres: Physico-Chemistry of Plant Cell Walls and Implications for Health

Abstract

The majority of dietary fibre (DF) originates from plant cell walls. Chemically, DF mostly comprise carbohydrate polymers, which resist hydrolysis by digestive enzymes in the mammalian small intestine, but can be fermented by large intestinal bacteria. One of the main benefits of DF relate to its fermentability, which affects microbial diversity and function within the gastro-intestinal tract (GIT), as well as the by-products of the fermentation process. Much work examining DF tends to focus on various purified ingredients, which have been extracted from plants. Increasingly, the validity of this is being questioned in terms of human nutrition, as there is evidence to suggest that it is the actual complexity of DF which affects the complexity of the GIT microbiota. Here, we review the literature comparing results of fermentation of purified DF substrates, with whole plant foods. There are strong indications that the more complex and varied the diet (and its ingredients), the more complex and varied the GIT microbiota is likely to be. Therefore, it is proposed that as the DF fermentability resulting from this complex microbial population has such profound effects on human health in relation to diet, it would be appropriate to include DF fermentability in its characterization-a functional approach of immediate relevance to nutrition.

Keywords: cereals; fruit; large intestinal fermentation; microbiota; plant cell walls; polyphenols; short-chain fatty acids; vegetables.

Figure 1

Schematic depiction of key soluble and insoluble dietary fibre structures which form the chemical components comprising the plant cell wall. The backbone structures for cellulose, mixed-linkage glucans, xyloglucan and arabinoxylan are (1,4)-β-linked, while the backbone of pectin is comprised of (1,4)-α-linked chains of galacturonosyl residues. In the pectin structure, the left hand part containing alternating rhamnose and galacturonic acid in the backbone is rhamnogalacturonan I, the middle section without long branches is homogalacturonan, and the right hand section with complex multi-sugar branches is rhamnogalacturonan II. Chain aggregation is prevented for xyloglucan, arabinoxylan and pectic non-cellulosic wall polysaccharides, due to the presence of short oligosaccharide-, monosaccharide- or acetyl group side chains. For mixed-linkage glucans, on the other hand, it is the irregular conformation of this polysaccharide which prevents main chain aggregation (Adapted from Burton et al., 2010 [50]).

Figure 2

Basic structure of (A). Some of the simplest phenols and flavonoids (adapted from Khoddami, Wilkes et al., 2013 [68]), and (B). The common classes of polyphenols found in fruits and vegetables (adapted from Singh et al., 2011 [69]).

Figure 3

A comparison of in vitro fermentability of arabinoxylan as a pure dietary fibre component versus wheat bran, showing differences in the cumulative gas volumes over time for each substrate, with arabinoxylan readily fermented compared to the more complex wheat bran dietary fibre (Adapted from Williams et al., 2011) [33].