Fate and Bioavailability of Tannins in Human and Animals: Gut Microbial Transformations

Abstract

Very little is known about the metabolic fate and bioavailability of tannins. To exert their biological properties, tannins have to be available to some extent in the target tissue. Therefore, the biological properties of tannins may depend on their absorption in the gut and their bioavailability. Absorption, bioavailability and metabolism of monomeric phenols have been extensively studied in both animals and humans, but little is known about the bioavailability of polymeric tannins and the results are controversial. It is unlikely that tannins with high molecular weight are absorbed intact and thus the degree of tannin polymerisation may have a major impact on their fate in the body. The biological properties of both conjugated derivatives and microbial metabolites have rarely been examined. Their study will be essential to better assess the health effects of dietary polyphenols. Despite potential adverse effects of dietary tannins, it is believed that source, levels and molar composition of tannins and its interactions in the gastro-intestinal ecology may have a variable degree of pro-nutritional and pro-health effects. It is thus proposed that a gradient relationship exists between feeding niche, dietary intake and physiological ability of humans/animals to deal with tannins. It is believed that tannins may exert their biological effects in two different ways: (i) as an unabsorbable, complex structure with binding properties which may produce local effects in the gastrointestinal tract (antioxidant, radical scavenging, antimicrobial, and antinutrient effects), or (ii) as absorbable tannins (probably low-molecular-weight) and absorbable metabolites from gut microbial fermentation of tannins that may produce systemic effects in various organs. The review tries to focus the unexplored area of research, particularly its beneficial nutrition and health effects in animals and human.