ABSTRACT Among young adults (18-24 years), a concerning 10-20% develop advanced but asymptomatic atherosclerotic lesions, yet cardiometabolic disease (CMD) prevention is understudied. In the U.S. two-thirds of young adults attend college, during which they adopt poor lifestyle behaviors that contribute to CMD risk. For example, college-based young adults (CBYA) engage in sedentary behavior (SB), defined as very low intensity behaviors in a seated, reclined, or supine postures, for ~7 hr/day. National and international public health agencies have recently called on the research community to better understand how to implement SB-reduction strategies. However, while CBYA present a captive but understudied population, we have scant understanding as to how to target this multi-dimensional behavior. Our long-term goal is the primary prevention of CMD in CBYA. To support this goal, the overarching aim of this proposal is to develop an evidence-based, multi-level intervention to target SB reduction. This longitudinal observational study will recruit a cohort of 500 CBYA (18-24 years) from a representative state college population. On each of the two visits (12 months apart) we will measure traditional and novel CMD risk biomarkers and administer participant questionnaires to collect information on lifestyle behaviors (diet, chronotype, substance use) and socio-ecological (individual, intra-individual, inter-individual) predictors of SB. Over the next 7 days, SB quantity will be captured using a thigh-worn inclinometer, and physical activity and sleep behaviors using accelerometry. SB context (television, transportation, academic/occupational, leisure computer, other) will be measured using ecological momentary analysis. Aim 1 will identify whether total SB (T-SB) or context-specific SB (CS-SB) is more strongly associated with CMD risk. The degree to which SB confers CMD risk likely depends on the context in which it occurs. For example, in the general adult population, television viewing is more strongly associated with CMD than other CS-SB. The reasons are unknown but may include the greater likelihood of co-occurring unhealthy behaviors (e.g., processed foods) while watching television. Therefore, Aim 2 will identify how much CMD risk is explained directly by SB, versus indirectly mediated by co-occurring lifestyle behaviors. Aim 3 will then identify multi-level intervention targets. To date, the existing longitudinal research has predominantly focused on non-modifiable socio-demographic variables. We will use a socioecological model to identify modifiable predictors. Last, will use the Intervention Mapping protocol to design an intervention implementation and evaluation plan. This innovative proposal will enhance our understanding of the importance of CS-SB and co- occurring behaviors to CMD risk. The final product will be an evidence-based CS-SB reduction intervention to target primary CMD prevention in CBYA. The likelihood of future high impact is e...