Abstract
BACKGROUND: The transition to secondary school is a critical period for adolescents, marked by increased autonomy and substantial changes in their physical and social environments, which can negatively influence lifestyle habits. The COVID-19 pandemic also affected these routines, but it remains unclear how adolescents' behaviors shifted as they moved to secondary school during the pandemic. OBJECTIVES: We examined changes in adolescents' lifestyle behaviors (screen time, sleep, sedentary time, physical activity (PA), and diet) during the transition to secondary school and explored whether changes were moderated by the pandemic or child gender. METHODS: A sample of 689 adolescents had their health behaviors measured via self-report, 24-hour dietary recalls, and accelerometry at two time points: the final year of elementary school (grade 7) and the first year of secondary school (grade 8). 42% of the sample completed all data collection before the pandemic, and 58% during the pandemic. Covariate-adjusted mixed-effects models assessed behavioral trajectories over time, including 3-way interactions between time, pandemic exposure, and gender. RESULTS: The transition to secondary school was generally associated with increased sedentary time and screen time, and reduced light PA, MVPA, likelihood of meeting sleep recommendations, and fruit and vegetables intake. However, some effects varied significantly by gender and cohort. For example, girls consistently had lower odds of meeting sleep recommendations and engaged in less MVPA than boys, while the pandemic cohort showed decreases in sedentary time and screen use (but remained above pre-pandemic levels), and increases in weekday fruit and vegetables intake. CONCLUSIONS: The transition to secondary school was associated with less healthy lifestyles, with variations by gender and pandemic exposure. To develop targeted interventions that promote positive habits during this critical life stage, it is necessary to identify the mechanisms driving these changes, as they may also reflect other developmental or environmental influences coinciding with the transition. Importantly, interventions should also address behavioral changes that occurred during the pandemic, particularly increases in recreational screen time that may persist into adulthood.