PROJECT SUMMARY The obesity epidemic has fueled major increases in cardiometabolic diseases. A growing body of evidence suggests that cardiometabolic diseases are associated with an altered gut microbiota. Observational and intervention data agree that there is suggestive evidence that a heart-healthy Mediterranean-style diet may be associated with beneficial changes in gut microbiota. However, the literature has yet to reach consensus. More research needs to be done on how the gut microbiota may mediate dietary effects on cardiometabolic risk within community-based dietary interventions. It is important that scholarship take place within community- based dietary interventions so that the dietary changes being measured are of whole diets (rather than a specific dietary component) and thus are more sustainable by participants. To address these gaps, this grant proposes a diversity supplement to the Randomized Explanatory Trial of a Mediterranean Dietary Pattern Weight Loss Intervention for Primary Care Practices (R33HL142680). The overarching purpose of the parent R33 is to test whether a weight loss intervention emphasizing a healthful eating pattern (Med-South) can yield long-term weight loss and improved CVD risk profiles over usual care (WWTM—formerly Weight Watchers). The study will test the impact of the Med-South intervention by assessing outcomes at 4, 12, and 24 months in a sample of 360 patients. Fecal samples will also be collected at each timepoint. The current proposal leverages the patients recruited in the parent R33 to investigate the impacts of the Med-South intervention on the gut microbiota over time. To do this, we will complete 16S rRNA sequencing on stool DNA using baseline and 4-month intervention samples. Our goals are to (1) assess cross-sectional association between participants' baseline pre-intervention diet and microbiota composition, and (2) test the changes in the gut microbial community composition from baseline through four months of intervention follow-up. For each study participant, microbial measures for analysis include: (1) alpha-diversity, (2) beta-diversity, and (3) abundances of distinct taxa (genera).