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.
Highlights
The strategy adopted in this article is to delineate the chemical structure and varieties of either hydrolysable or condensed tannins, mention their natural sources and abundance, describe their absorption and metabolism by gut microbiota-related metabolites, discuss the reverse effect of tannins or their metabolites on the microbial community itself and, identify past research findings considering the pharmacological effects of tannin metabolites on the host, with emphasis on microbiota-mediated metabolites
Tannins are ubiquitous in most plant foods and beverages, and most of the reported health benefits of these products are due to tannins
The ability of tannins to exert such health effects mainly depends on the bioavailability of these compounds, which varies greatly among different sites of the gastrointestinal tract
Summary
Tannins are metabolised by microbial enzymes, and the combined in vivo activity of enteric and hepatic enzymes leads to the formation of conjugated derivatives, which have different pharmacological profiles and are rapidly excreted through urine or via bile secretions back into the gastrointestinal tract (GIT) [6]. These pharmacokinetic and metabolic effects question the relevance of in vitro studies investigating the effects of polyphenols on cell lines. The strategy adopted in this article is to delineate the chemical structure and varieties of either hydrolysable or condensed tannins, mention their natural sources and abundance, describe their absorption and metabolism by gut microbiota-related metabolites, discuss the reverse effect of tannins or their metabolites on the microbial community itself and, identify past research findings considering the pharmacological effects of tannin metabolites on the host, with emphasis on microbiota-mediated metabolites
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