Gut Commensal Microbes Metabolize Flavonoids to Attenuate Key Parameters of the Human Metabolic Syndrome in A Mouse Model of Diet-Induced Obesity Lucas J. Osborn1,2, Karlee Schultz1,2, Beckey DeLucia1,2, Naseer Sangwan1,2, William Massey1,2, Zeneng Wang1,2, Ibrahim Choucair1,2, Danny Orabi1,2, Daniella Allende3, J. Mark Brown1,2*, Jan Claesen1,2* 1Department of Cardiovascular and Metabolic Sciences, Lerner Research Institute, Cleveland Clinic; 2Center for Microbiome & Human Health, Cleveland Clinic; 3Robert J. Tomsich Pathology & Laboratory Medicine Institute, Cleveland Clinic; *Co-Senior Authors More than 100 million Americans are living with metabolic syndrome, increasing their propensity to develop heart disease—the leading cause of death worldwide. Several recent seminal studies have demonstrated the pivotal role of gut microbes contributing to cardiovascular disease in a diet-dependent manner. Furthermore, dietary flavonoid consumption has been shown to abrogate the disease burden of metabolic syndrome in humans. Given the central contributions of diet and gut microbiota to cardiometabolic disease, we hypothesized that microbial metabolites originating postprandially after flavonoid consumption may contribute to cardiometabolic disease prevention and regression. To test this hypothesis, we gave conventionally raised or antibiotic treated (gut microbe-suppressed) mice a single oral gavage of a control chow diet slurry or a human-relevant flavonoid-rich fruit and vegetable slurry and sacrificed them four hours later to quantify flavonoid-derived gut microbial metabolites. Here we used a targeted mass spectrometry approach to measure the concentration of microbial flavonoid metabolites in portal plasma. As expected, several flavonoid metabolites were enriched in conventionally raised mice fed the fruit and vegetable slurry and significantly suppressed in antibiotic treated mice. Moreover, there was no observed difference in the flavonoid-depleted control diet group. Next, we administered one of the microbial flavonoid metabolites identified in the acute gavage study subcutaneously in obese mice for six weeks. Strikingly, this single microbial flavonoid catabolite was sufficient to abrogate hepatic steatosis in a mouse model of metabolic syndrome. Collectively, these data suggest that microbial flavonoid catabolism plays a central role in the attenuation of diet-induced obesity and these data can be attributed to a single microbial flavonoid metabolite.