Interactions between diet, gut microbiota and endocannabinoid system Isabelle Bourdeau-Julien1,3, Sophie Castonguay-Paradis1,3, Gabrielle Rochefort1,3, Sébastien Lacroix1,2,3, Lydiane Parent1,3, Julie Perron1,3, Cyril Martin2,3, Élisabeth Demers-Potvin1,3, Benoît Lamarche1, Nicolas Flamand2,3, Vincenzo Di Marzo1,2,3, Alain Veilleux1,3 and Frédéric Raymond1,3 1. Centre Nutrition, santé et société (NUTRISS), INAF, École de nutrition, Université Laval, Québec, Canada; 2. Centre de recherche de l’Institut de cardiologie et de pneumologie de Québec, Faculté de médecine, Département de médecine, Université Laval, Québec, Canada; 3. Canada Excellence Research Chair in the Microbiome-Endocannabinoidome Axis in Metabolic Health. The endocannabinoid system is involved in the many metabolic processes altered in metabolic diseases. Recent evidences suggest that the endocannabinoidome interacts with the gut microbiota. Furthermore, dietary intakes are also recognized as key determinants of both the gut microbiota and the endocannabinoidome. Despite those documented relationships, the mechanisms regulating the interactions between the diet, the endocannabinoidome and the microbiota remain undefined. In a longitudinal fixed sequence study, 21 participants received a Mediterranean diet (MedDiet) for 3 days, a 13-day control diet reflecting the average Canadian dietary intake, and once again a MedDiet for 3 consecutive days. Fecal and blood samples were collected at each dietary change during the protocol to evaluate alterations in gut microbiota composition and plasma levels of endocannabinoidome mediators and short-chain fatty acids (SCFAs). The endocannabinoidome mediators EPEA, 2-EPG, DHEA, 2-DHG, OEA and 2-OG significantly increased following MedDiet periods but decreased after the control diet. Inversely, the SCFA valerate, isovalerate and isobutyrate decreased after the MedDiet compared to control diet. Plasmatic propionate, acetate and butyrate levels were only moderately or not altered by the dietary interventions. The relative abundance of the genera Bacteroides, Butyricicoccus, Lachnoclostridiumand Lachnospira significantly increased following MedDiet compared to control diet. We found that specific gut microbiota genera correlated with the levels of circulating endocannabinoids or their congeners. Specifically, Intestinibacter positively correlated with circulating 2-AG, 2-DPG and 2-DHG levels while Paraprevotella negatively correlated with LEA but positively correlated with 2-OG and 2-LG. We also observed that the relative abundance of Butyricicoccus was positively correlated with acetate while Haemophilus was correlated with acetate and butyrate. Taken together, our results support the hypothesis that both systems respond rapidly to drastic dietary changes and that specific gut microbiota taxa are correlating with circulating endocannabinoidome mediators. This study further enhances our understanding of the interactions between diet, gut microbiota and endocannabinoidome.