Abstract Body: Background and aims: Obesity is recognized as an important cardiovascular risk factor. Consequently, weight loss is regarded as a protective factor. Nevertheless, the effects of weight loss on the epigenome and metabolome are not well understood. Our objective is to analyze changes in the epigenome (DNA methylation) and metabolome (serum/plasma) after 1-y of a lifestyle intervention in overweight/obese subjects at high cardiovascular risk. Methods: We conducted a randomized controlled trial for weight loss in high risk subjects (55-75 years; BMI: 27-40 kg/m2; and metabolic syndrome). Subjects were randomized 1:1 to the active intervention group (IG) consisting of an energy-restricted Mediterranean diet (MedDiet) and increased physical activity, or to the control group (CG) (minimal intervention with MedDiet). This is a 6-year intervention multicenter study, but we present here data from the Valencia-field center (n=465 at baseline). Details about the intervention have been registered and published. DNA methylation was measured in all participants at baseline with the EPICv1 array, as well as serum metabolites using the NMR-Nightingale platform. In a subsample (n=92) including responders and non-responders in order to maximize sample size we analyzed DNA methylation at baseline and 1-y (EPICv2) and metabolomics (in plasma) with the Global Discovery Panel of Metabolon. Normalization, feature selection and multivariable adjusted regression models were fitted to analyze changes. Results and conclusions: After 1y intervention we detected statistically significant differences (P<0.001) in weight loss between the IG (mean:-3.4+/-3.8% weight; range: -16.8 to 4.4 %) and the CG (mean:-0.42+/-3.1%; range:-14.7 to 6.3). At baseline, a methylation-wide signature was significantly associated (P<5x10-8) with weight loss in the IG, including cg22010309-intergenic, cg24800130-FAM108A1 (related to fat browning and lipid metabolism in adipocytes) and cg24978424-YTHDC1 (autophagy-related), among others. After 1-y intervention, we detected several statistically significant changes in epigenetic biomarkers (both in differentially methylated loci and composite biomarkers related to aging). Also, we found significant changes in some metabolites (stearoylsphingomyelin(d18:1/18:0), 5-oxoproline; cysteine, linoleate(18:2n6), stearate (18:0); 3-indoxyl sulfate; and 3-methyl-2-oxobutyrate, among others) showing an impact of weight loss on these markers, but more studies are needed.