Abstract Body: Reducing caloric intake is a recommended first-line strategy for weight loss and CVD risk reduction in adults with overweight/obesity. Reduced endothelium-dependent vasodilation is a central factoring underlying the heightened risk and prevalence of CVD with increased adiposity. In addition, with increased adiposity there is a shift in the phenotype and biology of adipose tissue resulting in an adipocyte secretome, particularly adipocyte-derived extracellular vesicles (Ad-EVs), with proatherogenic effects. Circulating Ad-EVs are elevated with overweight/obesity and linked to CVD risk. The aim of this ongoing study is to determine whether intentional diet-induced weight loss improves endothelium-dependent vasodilation and reduces circulating Ad-EVs in adults with overweight/obesity. Forearm blood flow (FBF) responses to intra-arterial infusions of acetylcholine (ACh: 4.0, 8.0 and 16.0 μg/100 mL tissue/min) and sodium nitroprusside (NTP: 1.0, 2.0 and 4.0 μg/100 mL tissue/min) and circulating Ad-EV (perilipin A⁺ vesicles; via flow cytometery) were measured in 20 normal weight and overweight/obese adults: 10 normal weight (age 54+2 yr; 4 M/6 F; body mass: 60.6+2.8 kg; BMI 21.4+0.6 kg/m2) and 10 overweight/obese (55+3 yr; 4 M/6F; 98.9+7.7 kg; 32.4+1.3 kg/m2). FBF response to ACh was ~30% lower (P=0.001) in the overweight/obese (from: 4.5+0.3 to 10.9+0.7 mL/100 mL tissue/min) vs normal weight group (4.5±0.3 to 17.0±1.1 mL/100 mL tissue/min). Circulating Ad-EVs were markedly higher (P=0.02) in overweight/obese (212±40 Ad-EV/µL) compared with normal weight (109±9 Ad-Ev/µL) adults. All 10 overweight/obese adults completed a 12-week hypocaloric diet intervention (~1200-1500 kcal/d; 15-20% protein, 30-35% fat, 45-55% carbohydrate, 2300 mg sodium). The weight loss intervention resulted in a 7% (range: 3-11%) reduction in body mass (92.5+7.9 kg). FBF to ACh increased ~25% (4.0+0.3 to 13.7+ 0.8 mL/100 mL tissue/min; P=0.04) and circulating Ad-EVs decreased ~30% (150+28 Ad-EV/µL; P=0.005) in response to the weight loss intervention. The initial findings of this ongoing trial indicates that a hypocaloric diet consistent with current dietary recommendations for weight loss improves endothelium-dependent vasodilation and lowers circulating levels of Ad-EVs in overweight/obese adults. Enhanced vascular function and, potentially, improved adipose tissue biology represent important mechanistic factors underlying reduced cardiometabolic risk with diet-induced weight loss.