Effects of in vitro simulated digestion on the antioxidant activity of different Camellia sinensis (L.) Kuntze leaves extracts

The stability of tea phenolic compounds is influenced by pH value and digestive processes. However, the complex mixture of constituents in tea may modulate the stability of these compounds during digestion. In this study, tea infusions obtained from green, black, and Oolong tea leaves were exposed to in vitro simulated gastrointestinal digestion, and the stability of ( +)-catechin, caffeine, (-)-epicatechin, epigallocatechin-3-gallate (EGCG), and gallic acid was compared to that of isolated compounds. Changes in antioxidant activity were also evaluated by means of DPPH assay and in a H2O2-induced in vitro oxidative stress model, using Caco-2 cells. The stability of teas antioxidant constituents was different when using teas extract, compared to the reference compound alone, with the total phenolic content being more stable in extracts containing them in higher amount. EGCG degradation correlated well with changes in the DPPH inhibition assay, confirming its pivotal role in the antioxidant activity of tea. Differently, the antioxidant effect in the in vitro cell-based model was much more related to the initial total phenolic content of the extracts, with green tea being more effective than black tea and Oolong tea. Moreover, the antioxidant activity of teas was strongly affected by gastrointestinal digestion. Taken together, these findings suggest a protective role of teas phytocomplex against gastrointestinal digestion of antioxidant constituents. In conclusion, the effect of gastrointestinal digestion on the antioxidant activity of tea should be taken into account, as this may be different from one extract to another and information on the stability of active constituents cannot be extrapolated from data obtained using single compounds.