Metabolic health is emerging as an important risk and prognostic factor for metabolic diseases, including cardiovascular disease (CVD), diabetes and cancer. (1-12) Metabolic health has been defined according to the presence or absence of a minimum number of cardiometabolic risk factors defined according to clinical cut-points (i.e. elevated waist circumference, triglycerides, blood pressure, and fasting glucose, low HDL-cholesterol, insulin resistance (HOMA-IR) and inflammatory biomarkers (hsCRP), or pharmacological treatment for these parameters). Obesity is associated with poor metabolic health, but metabolically unhealthy individuals may be at high risk of metabolic diseases, independent of BMI (metabolically unhealthy normal-weight phenotype, MUHNW) and obese individuals that are metabolically healthy may have similar disease risk to their lean counterparts (metabolically healthy obese phenotype, MHO). (1) Analysis of data from the US National Health and Nutrition Examination Survey showed that a substantial proportion of overweight or obese individuals are metabolically healthy and over 30% of normal weight individuals are cardiometabolically unhealthy. (13) The etiologies of the associations between these phenotypes and cardiometabolic disease risk are unclear. Recent studies have demonstrated the utility of using a metabolomics approach to explore the concepts of MUHNW and MHO in relation to cardiometabolic disease endpoints. For example, an 'obese metabolome' (metabolites related to insulin resistance, branched chain and aromatic amino acids, nucleotide metabolites, and several lipid classes) associated with a 2 to 5-fold increased risk of cardiovascular events, independent of BMI, in the TwinsUK cohort. (14) Matched for BMI, MUHNW individuals had higher visceral adiposity, higher blood pressure and were also more likely to become obese over time compared with their metabolically healthy lean counterparts. Such findings point towards the potential of metabolomics for disease risk stratification, where BMI alone can fail to identify a significant proportion of individuals that may be at risk for cardiometabolic disease. We propose to leverage the unique resources of COMETS to determine metabolites associated with metabolic health, how this profile associates with obesity-relevant diseases (i.e. obesity-related cancers and cardiovascular disease), and the extent to which metabolites mediate the association of body mass index with these outcomes.