Effect of Gut Microbiota Biotransformation on Dietary Tannins and Human Health Implications

Abstract

Tannins represent a heterogeneous group of high-molecular-weight polyphenols that are ubiquitous among plant families, especially in cereals, as well as in many fruits and vegetables. Hydrolysable and condensed tannins, in addition to phlorotannins from marine algae, are the main classes of these bioactive compounds. Despite their low bioavailability, tannins have many beneficial pharmacological effects, such as anti-inflammatory, antioxidant, antidiabetic, anticancer, and cardioprotective effects. Microbiota-mediated hydrolysis of tannins produces highly bioaccessible metabolites, which have been extensively studied and account for most of the health effects attributed to tannins. This review article summarises the effect of the human microbiota on the metabolism of different tannin groups and the expected health benefits that may be induced by such mutual interactions. Microbial metabolism of tannins yields highly bioaccessible microbial metabolites that account for most of the systemic effects of tannins. This article also uses explainable artificial intelligence to define the molecular signatures of gut-biotransformed tannin metabolites that are correlated with chemical and biological activity. An understanding of microbiota-tannin interactions, tannin metabolism-related phenotypes (metabotypes) and chemical tannin-metabolites motifs is of great importance for harnessing the biological effects of tannins for drug discovery and other health benefits.

Keywords: biotransformation; gut microbiota; molecular motifs; mutual interaction; polyphenols; tannins.

Figure 1

Representative examples of hydrolysable tannins in nature.

Figure 2

Representative examples of condensed tannin structures in nature for monomeric (A), dimeric (B) and trimeric (C) tannin types.

Figure 3

Microbiota-mediated biotransformation of ellagitannins to urolithins as detailed in [69].

Figure 4

Gut microbial-mediated degradation pathway of flavan-3-ol.

Figure 5

Examples of the reported biological activities of bacterial metabolites of CTs and their proposed mechanisms of action: (A) 5-(3′,5′-dihydroxyphenyl)-γ-valerolactone; (B) 5-(3′,4′-dihydroxyphenyl)-γ-valerolactone. Plants in the bottom are (left-to-right): strawberry, pomegranate, tea.

Figure 6

Reported action mechanisms of tannin-derived urolithins, highlighted in blue, with their related pharmacological activities.

Figure 7

Importance of tannin metabolite chemical substructures vs. biological effects.

Figure 8

Examples of chemical motifs indicating biological effects. Blue: ani-inflammatory; orange: antioxidant; green: anticancer; red: anti-atherosclerotic. Coloured motifs represent structural positive contributions towards health effect.