Gut microbial catabolism of grape seed flavan-3-ols by human faecal microbiota. Targetted analysis of precursor compounds, intermediate metabolites and end-products

► We conducted fermentations of a grape seed extract with human faecal microbiota. ► Changes in flavan-3-ol precursors and microbial metabolites were monitored. ► 5-(Phenyl)-γ-valerolactones and other derivatives were produced up to 10 h. ► Final phases (10-48 h) involved further dehydroxylation of metabolites. ► Formation of intermediate metabolites seems a key step in flavan-3-ol bioavailability.