Biobehavioral Mechanistic Model of Alcohol Use Following Bariatric Surgery: The BREW ME Model Abstract Metabolic and bariatric surgery (MBS) is currently the most efficacious and durable intervention for Class III obesity. While weight loss and metabolic outcomes after surgery are very good, unfortunately one deleterious complication is a markedly increased risk of hazardous alcohol use after MBS (i.e., almost 1 in 5 patients develop Alcohol Use Disorder). The understanding of mechanisms of hazardous alcohol use following MBS are developmentally young, but there is growing evidence of the unique role of the microbial environment. Our transdisciplinary research team has been at the forefront of advancing understanding of how the microbiome and the rewarding effects of alcohol change following MBS. Although the mechanisms through which changes in the microbiome influence post-surgical outcome remains unclear, the gut microbiome is increasingly recognized for its bi-directional communication with the brain and the Microbiome-Gut-Brain Axis may explain the complex relationships between the gut microbiome, subsequent changes in the rewarding effects of alcohol, and resulting risk for hazardous alcohol use. For example, the microbiome has been shown to impact neurobiological reward systems and alter reward processing. Further, patients who experience strong reward value of alcohol engage in more drinking behavior. Therefore, the rewarding effects of alcohol may be influenced through the gut microbiome, leading to significant impacts on the post-surgical alcohol consumption. Unfortunately, the extant literature is methodologically limited due to traditional assessments further limiting understanding of these relationships. Thus, we propose a novel, multi-method, longitudinal design to investigate the Biome-Reward as Mechanisms for EtOH Use (BREW ME) Model to assess the mechanistic role of post-surgical changes in reward in the relationship between the gut microbiome and hazardous alcohol use. This study will address many of the methodological limitations in the literature (e.g., cross sectional design, reliance on self-reported drinking behavior) and will be the first to combine a microbiome and ecologically valid, real-time assessment of the reward value of alcohol in MBS patients. Understanding key relationships between these variables will aid the development of interventions that can be tailored to individual patients based upon their risk profile and can then be implemented early to optimize surgery-related outcomes.