PROJECT SUMMARY / ABSTRACT Cardiovascular disease (CVD) is the leading cause of death in the United States and physical activity is a potent way to decrease this risk. Serious gaps remain in our understanding of the optimal duration of physical activity and sedentary behavior during adolescence and young adulthood if risk for concurrent and future CVD is to be averted, and were specifically identified as priority research areas in the 2018 Department of Health and Human Services (HHS) Physical Activity Guidelines for Americans. Adolescence and young adulthood are critical transition periods that set physical activity patterns for the rest of adulthood and may represent a critical window for early intervention. The overall objective of this K08 is to develop evidence to bridge the HHS-articulated gaps by describing independent effects of sedentary behavior in adolescence and thresholds of physical activity in young adulthood on the prevention of later-life CVD to inform future HHS guidelines. The central hypothesis is that highly sedentary adolescents and young adults will require higher physical activity levels to reduce CVD risk in later adulthood. This project will analyze longitudinal cohort data from the Adolescent Brain Cognitive Development (ABCD) Study, the National Longitudinal Study of Adolescent to Adult Health (Add Health), and the Coronary Artery Risk Development in Young Adults (CARDIA) Study. The Specific Aims will: (1) determine independent associations of contemporary forms of sedentary behavior (screen time) in adolescents with CVD risk factor incidence; (2) evaluate associations of attaining HHS-recommended adolescent (60 mins/day, 7 days/week) versus adult (30 mins/day, 5 days/week) physical activity levels during specific age ranges in young adulthood with incidence of CVD risk factors; and (3) estimate population-level lifetime effects of adolescent and young adult sedentary behaviors, physical activity dose, and their interactions on later-life CVD mortality. The candidate’s long-term career objective is to become an independent physician-epidemiologist generating actionable evidence to optimize adolescent and young adult behaviors to prevent CVD later in life. The proposed career development plan will provide training and mentorship in: (1) physical activity, sedentary behavior, and CVD epidemiology and measurement; (2) longitudinal analyses and modeling methods; (3) guideline development and implementation; and (4) career development and plan for independence. Training will be achieved through coursework, seminars, tutorials, conferences, hands-on experience, and active mentoring. The mentoring team includes nationally recognized experts in cardiovascular epidemiology, adolescent and young adult medicine, physical activity/sedentary behavior, biostatistics, guideline development, and implementation science. Completion of this K08 will inform specific guidance for adolescents and young adults in the next edition of the Phys...