Abstract Body (Do not enter title and authors here): Background: Poor diet quality accounts for approximately half of all deaths from cardiometabolic diseases. Food pattern modelling using nationally representative U.S. data suggests that replacing typical snacks consumed with tree nuts will improve diet quality. However, few clinical trials have assessed the potential for tree nuts to improve diet quality. Pecans are rich in polyphenols, which may improve endothelial and vascular function. This study aimed to examine how substitution of usual snack foods with 2 oz/day of pecans affects flow mediated dilation (FMD), carotid-femoral pulse wave velocity (cf-PWV), blood pressure and diet quality after 12 weeks in individuals at increased risk for cardiometabolic disease. Hypothesis: Intake of 2 oz/day of pecans in place of habitually consumed snacks will improve vascular health and diet quality compared to habitual intake. Methods: In this single-blind RCT, adults (46 ± 13 years, 29.8 ± 3.7 kg/m²) with ≥1 criterion for metabolic syndrome who were free from cardiovascular disease, type 2 diabetes and other chronic diseases were included. The experimental group was provided with 2 oz/day of pre-portioned raw, unsalted pecans (~392 kcal) and instructed to replace the snacks they currently consume with the provided pecans. The control group was instructed to continue their usual diet. FMD (primary outcome), cf-PWV, brachial and central blood pressure were measured at baseline and following the intervention period (~12 weeks). At three timepoints (baseline, week 6, and week 12) during the study, participants completed three 24-hour recalls (total 9 recalls). The Healthy Eating Index-2020 (HEI-2020) was calculated to assess diet quality. Results: A total of 138 (69 per group) participants were randomized. No between-group difference in FMD, cf-PWV, or blood pressure was observed. The HEI-2020 was 11.1 points higher (95% CI 5.1, 17.2) in the pecan group compared to the usual diet group at week 12. The following HEI-2020 components were improved in the pecan group vs. the usual diet group: total protein foods (0.5 points [95% CI 0.1, 1.0]), seafood and plant proteins (2.5 points [95% CI 1.6, 3.4]), fatty acids (4.1 points [ 95% CI 2.6, 5.5]), and sodium (1.8 points [95% CI 0.3, 3.2]). Conclusion: Replacement of usual snacks with 2 oz of pecans improved adherence to the Dietary Guidelines for Americans but did not improve endothelial function, blood pressure, or arterial stiffening.