Increasing evidence has shown that gut microbiota plays important roles in metabolizing large molecular polyphenols to bioavailable and bioactive microbial metabolites. Theaflavin (TF) is one of the major color compounds in black tea and has demonstrated anti-inflammation, antioxidant, and anticancer effects properties. However, little is known about the metabolism of TF by gut microbiota in vivo. In this study, following the administration of TF to mice, the C-ring cleavage metabolites, dihydro- and tetrahydro-theaflavin (DH-TF and TH-TF) were detected in mouse feces by LC-MS and validated by authentic standards from in situ chemical reaction. The observation of the C-ring cleavage metabolites in TF-treated conventionalized mice but not in germ-free (GF) mice confirmed the role of gut microbiota in cleaving the C-rings of TF. The detection of DH-TF from the anaerobic incubation of TF with catechin-converting gut bacteria, Eggerthella lenta (Eggerth), suggested that the microbes with the capacity to cleave the C-ring of catechins were able to metabolize TF following the same mechanism. Additionally, three small phenolic metabolites were detected in mouse feces, and one of them was primarily detected in SPF mice not GF mice, which revealed that TF, subsequent to the cleaved C-ring, can be further metabolized into smaller phenolic metabolites by gut microbiota. Dose-dependent production of these metabolites were observed from the administration of 100 mg/kg to 400 mg/kg body weight of TF. In conclusion, gut microbiota can metabolize TF to the open-ring metabolites and the phenolic metabolites through the C-ring cleavage in mice.

Type 2 diabetes (T2D) is one of the most prevalent metabolic disorders in the United States. Increased blood glucose levels and improper crucial metabolism ensuing from insulin action, insulin secretion defect, or both are characteristics of this disease. The risk of developing T2D is associated with many factors, including obesity, race, inactivity, and genetics. Increased whole-grain (WG) consumption has been reported to lower the risk of obesity and T2D. Among WGs, barley shows a comparative advantage in its fiber content, especially the soluble fiber, beta-glucan (β-glucan), an active component credited for this benefit. Barley also contains important phytochemicals, mostly intertwined with its fiber, reported to offer glycemic response benefits. The mechanism by which barley exerts these changes in glycemic response is not entirely understood. However, the physical properties of barley fiber, the function of microbial metabolites of fiber, short chain fatty acids, and the beneficial effects of its phytochemicals through multiple pathways have all been reported as the potential mechanisms for its antidiabetic effects. This review summarizes recent studies concerning the health-promoting benefit of barley in preventing and moderating the risk factors associated with diabetes and the potential underlying mechanisms.