Absorption Rates and Mechanisms of Avenanthramides in a Caco-2 Cell Model and Their Antioxidant Activity during Absorption

Avenanthramides (AVNs) are a unique kind of polyphenols that were only detected in the oats and have been demonstrated to exhibit strong antioxidant activities but low bioavailability. The purpose of the present research was to evaluate the absorption rates and mechanisms of AVNs (AVN 2c, AVN 2f, and AVN 2p) using a human colon adenocarcinoma cell line (Caco-2) cell model and clarify the influence of the absorption process on the antioxidant capacities of AVNs. Furthermore, the absorption rates and antioxidant activities of ferulic acid and caffeic acid were compared with those of AVNs. Results showed that the apparent absorption rates (P_app) of AVN 2c, AVN 2f, and AVN 2p were 0.65 ± 0.05 × 10^-6, 1.18 ± 0.16 × 10^-6, and 1.44 ± 0.09 × 10^-6 cm/s, respectively, which were significantly lower than those of caffeic acid (3.76 ± 0.31 × 10^-6 cm/s) and ferulic acid (1.69 ± 0.13 × 10^-5 cm/s). Moreover, the metabolites (caffeic acid, ferulic acid, and AVN 2f) of AVNs after absorption were detected and quantified by high-performance liquid chromatography-mass spectrometry. Before absorption, although the antioxidant capacities of AVNs were significantly stronger than those of ferulic acid and caffeic acid, there was an opposite result after absorption. In addition, AVNs transported the Caco-2 monolayer by paracellular diffusion and were affected by monoamine oxidase and efflux transporters (P-gp, MRP2) during absorption. The co-administration of quercetin could significantly improve the absorption rates of AVNs.