Intestinal Adaptation to Repeated Exposure of Flavonoid-rich Foods: In Vitro and Clinical Data
Interest in application of flavonoids for chronic disease prevention has grown significantly, but the low oral bioavailability of these compounds from acute doses is commonly highlighted as a limitation when considering their biological significance. Still, the impact of broad dietary patterns such as repeated exposure on flavonoid’s absorption, metabolism, and eventual efficacy is critical to consider since evidence suggests that their bioavailability may be enhanced with repeated exposure. To fill this gap in knowledge, this dissertation will focus on three major areas including characterization of flavonoid metabolites, in addition to use of in vitro models and clinical work to test the effect of repeated exposure on flavonoid bioavailability. Though flavan-3-ols undergo Phase II metabolism in humans and rodents, researchers have generally not been able to utilize fully characterized standards for these metabolites. Thus, collaborators synthesized flavan-3-ols metabolites after which liquid chromatography/time-of-flight mass spectrometry (LC-TOF-MS) and nuclear magnetic resonance (NMR) spectroscopy were used to characterize their structure and match the synthesized metabolites to those found in rodent plasma. To explore changes occurring in the upper small intestine from flavonoid repeated exposure, Caco-2 cells were differentiated in the presence of isolated flavan-3-ols, green tea, grape seed, or blackberry extracts. EGCG and EC pretreatment altered formation rate of Phase II metabolites, in addition, green tea and grape seed extract pretreatment both resulted in increased flavan-3-ol transport. In contrast, blackberry extract pretreated monolayers displayed decreased transport of phenolic compounds. Finally, alterations in mRNA expression of select transport and metabolizing genes were observed in cells pretreated with blackberry extract. To determine if flavonoid absorption changes with repeated exposure to blackberry in humans, a controlled feeding study was performed to assess the effect of three-week daily blackberry exposure on flavonoid pharmacokinetics. The results showed increased plasma AUC of peonidin glucoside after blackberry treatment in lean volunteers. Accumulation of total anthocyanins in urine was greater in the lean group after blackberry exposure. This difference may be driven by increased Phase II anthocyanin metabolites since there was greater accumulation of anthocyanin metabolites in urine in the lean group after blackberry exposure. Taken together, these data suggest that the small intestine may be a key regulator of the observed adaptive phenomena occurring in vivo. These results demonstrate that there is both differential transport, absorption, and metabolism of flavonoids, including select flavonoids and phenolic acids, after repeated exposure to flavonoid-rich blackberry and that this response appears to differ with BMI. These studies provide a basis for future work on the effect of chronic flavonoid exposure on their bioavailability and metabolism in a range of interventions.
Ferruzzi, Purdue University.
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