Alterations of the gut microbiota induced by diet exert a strong influence on the development of metabolic syndrome. In this study, we prove the hypothesis that the long-term high-fat diet (HFD) may influence gut microbiota directly and/or indirectly by changing the redox state. Lipoic acid (LA), as a universal antioxidant, was used to improve the redox state. Reactive oxygen species (ROS), total antioxidant capacity (T-AOC), and malondialdehyde (MDA) were analyzed to profile oxidative stress states. PCR-denaturing gradient gel electrophoresis (DGGE) was used to describe gut flora structures, while plate count was employed for the quantitative analysis of Escherichia coli, lactobacilli, and enterococcus. The influence of redox state on the vitality of gut-derived bacteria was measured in vitro. ROS and MDA, which significantly decreased in LA mice compared with HFD mice, showed a strong positive association with E. coli and enterococcus (P < 0.05) and a negative association with lactobacilli (P < 0.05). Increased T-AOC in LA mice showed a high positive association with lactobacilli (P < 0.05) and a negative correlation with E. coli and enterococcus. These correlations implied that the dietary effects on the gut microbiota were conferred, at least in part, through an effect on oxidative stress. This study provides evidence that modulation of the redox state by an antioxidant has the potential to improve gut microbiota, which has relevance for metabolic health.

Oxidized tyrosine (O-Tyr) products have been detected in commercial food and have been demonstrated to induce liver injury in our previous study, but the precise mechanisms of the impact induced by dietary O-Tyr are still unclear. Kidney plays an important role in the metabolism of protein. Accumulation of O-Tyr products, especially the dityrosine (Dityr) and advanced oxidation protein products (AOPPs), in vivo was shown to be associated with many kidney diseases. Therefore, this study determined whether chronic exposure to dietary O-Tyr impaired renal function in rats. After O-Tyr treatment for 24 weeks, rats exhibited oxidative stress and protein oxidation in the kidneys, accompanied with inflammatory reaction and renal dysfunction. Elevated extracellular matrix (ECM) contents and the histological examination (HE and Masson stain) results indicated renal fibrosis. The Real-time PCR and Western blotting assay showed that O-Tyr activated phosphorylation of JNK/p38 and up-regulated the expression of transforming growth factor-β1 (TGF-β1) and Smad 2/3. These results suggest that dietary O-Tyr could induce oxidative stress, inflammation and renal fibrosis through JNK/p38/TGF-β1 signaling pathway. Dityr (accounting for 22 % of the total O-Tyr material) may be responsible for the O-Tyr-induced injury. This study also provides a modified procedure for separation and purification of Dityr, the main oxidized product in O-Tyr.

The findings suggested that decreased insulin secretion triggered by OTPs may be mediated by oxidative stress and mitochondrial damage in pancreatic β cells.