Bioavailability of Tannins and Other Oligomeric Polyphenols: a Still to Be Studied Phenomenon

Polyphenols are a group of plant secondary metabolites characterized with the presence of multiples of phenol structural units. Tannins are plant-originated phenolic substances, which play an important role in plant defense system as well as have found applications in medicine, industry, and other fields of human activity. Despite these, the mechanisms of bioavailability of these substances are poorly studied and even there are very few amounts of sources gathering recent studies concerning this issue. So the main purpose of this review is to focus on the known peculiarities and possible ways of the bioavailability of these oligomeric substances. Due to the ability of tannins to bind proteins and other macromolecules, they are responsible for astringency or bitterness of red wines and also for the anti-nutritional effects, which can be reduced by the action of tannase. In nutriceutical industry, tannins are used as natural preservers and antioxidants. They have also a wide range of medicinal application as anti-hemorrhagic and hemostatic, anti-malarial and antitumor, antimicrobial, antioxidant, anti-tyrosinase, anthelmintic, antiviral, and anti-inflammatory activities among others. It is suggested that absorption of tannins is correlated with their polymerization degree and the activity of these components depends on their absorption in the gut, metabolism and, therefore, the bioavailability. Despite numerous promising applications, there are still plenty of open spaces to fully understand determinants of the bioavailability of tannins and other oligomeric polyphenols pivotal for the expression of their biological activity.