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Background: Cardiovascular diseases (CVD) remain the most common cause of death globally. While these diseases usually manifest in adulthood, several studies have demonstrated that an unfavourable cardiometabolic profile in childhood is predictive of risk status in adulthood. Given the established cardiometabolic health benefits from physical activity and higher cardiorespiratory fitness (CRF), and that most adolescents are insufficiently active, a call has been made for more prospective studies in this age interval – particularly among older adolescents and young adults to gain more knowledge about physical activity behaviour through major life transitions. Aims: We aim to examine I) whether the association between physical activity and cardiometabolic risk factors in youth differ by level of CRF, II) changes in device-measured physical activity and sedentary time from childhood to young adulthood, III) prospective associations between physical activity and sedentary time with various risk factors for developing CVD, and IV) tracking of sedentary time, CRF, and other cardiometabolic risk factors from childhood to young adulthood. Methods: I) Whether the association between physical activity and cardiometabolic risk factors in youth differ by CRF level was examined with a systematic review of published literature. II) Levels, trends, and associations of physical activity and sedentary time, CRF, and cardiometabolic risk markers were investigated in the Physical Activity among Norwegian Children Study (PANCS) follow-up, consisting of three study waves at age 9 years (n=731), age 15 years (n=731), and age 24 years (n=258). Physical activity was measured with accelerometery at all time-points. Main results: I) Most published studies indicate that physical activity is more beneficial for cardiometabolic health in children and adolescents with lower CRF levels compared to their more fit peers. However, the evidence quality is weak due to potential selective reporting and a high risk of bias in most of the included studies. II) We observed a significant decrease in device-measured physical activity and an increase in sedentary time from childhood to adolescence. These variables remained relatively stable during the transition from adolescence to young adulthood. The absolute tracking of physical activity over time was weak, but individuals in the lowest quartile of moderate-to-vigorous physical activity (MVPA) and total physical activity at age 9 were significantly more likely to stay in that quartile by age 24 compared to peers in the higher quartiles at baseline. III) Sedentary time during adolescence was linked to lower peak oxygen uptake (VO2peak) in young adulthood, while MVPA and vigorous physical activity (VPA) were associated with higher VO2peak over this period. Furthermore, higher levels of VPA during adolescence was associated to favourable changes in visceral fat, insulin levels, and clustered cardiometabolic risk in young adulthood. The relationship between VPA and cardiometabolic risk may vary based on childhood BMI tertiles, with those in the highest tertile demonstrating the most favourable association. IV) Several cardiometabolic risk markers tracked of moderate to strong magnitude from childhood into young adulthood, including BMI, waist circumference, VO2peak, LDL-cholesterol, and clustered cardiometabolic risk. Systolic blood pressure demonstrated moderate to weak stability, while sedentary time, insulin, and high-sensitivity CRP exhibited weak to no tracking over this period. Tracking of waist circumference was slightly stronger in girls than boys, while VO2peak had higher stability among boys. Tracking patterns for the other risk markers were similar between boys and girls. Conclusions: Significant changes in physical activity and sedentary time occurred between ages 9 and 15, with only smaller changes between ages 15 and 24. Less active children were more likely to remain less active young adults, suggesting early intervention may be necessary. Furthermore, key cardiometabolic risk factors remained stable from ages 9 to 24. However, sedentary time appear less stable and may be more modifiable than risk factors of high stability. Supporting current recommendations, being more active and less sedentary in adolescence is associated with better cardiometabolic health in young adulthood. Particularly, VPA shows benefits for various cardiometabolic health markers. Additionally, physical activity could be most beneficial among youth “at risk” (e.g., low CRF, obese).
