TABLE 1.
Structural characterization of tannins and their metabolism.
| Gallotannins | Ellagitannins | Procyanidins | |
|---|---|---|---|
| Structure | Esters of gallic acid with monosaccharide (mostly beta-D-glucose) | Esters of ellagic acid with monosaccharide (mostly beta-D-glucose) | Oligomers and polymers of flavan-3-ols |
| • B-type procyanidins: dimer procyanidins consist of two flavon-3-ol units with C4–C8 (procyanidins B1–B4) or C4–C6 (procyanidins B5–B8) interflavan bond | |||
| • A-type procyanidins: dimer procyanidins with C4–C8 interflavan bond and an additional C2–C7 ether bond | |||
| • C-type procyanidins: trimer procyanidins with interflavan bond | |||
| Bioavailability | • Low | • Low | • Degree of polymerization >3: not absorbed |
| Metabolism | • Hydrolysis in stomach to a lesser extent | • Metabolism by gut microbiota to urolithins | • Breakdown of procyanidins in the gastric environment |
| • Mainly by gut microbiota | • Procyanidins are metabolized by colon microflora to low-molecular-weight compounds: phenyl valerolactone, phenylacetic acid, and phenylpropionic acid | ||
| Metabolites | • Low-weight gallotannins, pyrogallol, catechol, gallic acid: absorbed | • Urolithins well absorbed | • Dimers and trimers of procyanidin: absorbed to a small degree |
| • Phenyl valerolactone, phenylacetic acid, and phenylpropionic acid: well absorbed |