Physical Activity, Sedentary Time, and Obesity in Adolescents: Differences by School Grade

ABSTRACT Background To better understand the key behavioral factors for adolescent obesity, this study investigated the independent and combined associations of physical activity (PA) and sedentary time (ST) with obesity prevalence across different school grades. Methods Cross‐sectional data from 614,943 adolescents in the Korea Youth Risk Behavior Web‐based Survey were analyzed. Independent associations of PA and ST with obesity were examined based on weekly PA frequency and daily nonacademic ST. Combined associations were assessed using adherence criteria (≥ 60 min/day of moderate‐intensity PA plus ≥ 3 days/week of vigorous‐intensity PA) and ST classification (≤ 2 h/day). Results In independent analysis, adolescents meeting PA guidelines consistently showed lower odds of obesity across all grades. For sedentary time, the ≤ 2 h/day group had the lowest odds, and a dose–response relationship was observed up to 6–8 h/day across grades. In combined analysis, adolescents meeting both PA and ST guidelines had the lowest obesity prevalence (OR = 0.54, 95% CI: 0.50–0.59). Implications for School Health Policy, Practice, and Equity Schools should promote daily PA and reduce ST. Since behavioral risk factors differ by developmental stage (e.g., academic stress in higher grades), grade‐tailored strategies are essential for effective obesity prevention. Conclusions Higher PA levels and lower ST are independently and jointly associated with lower obesity prevalence among Korean adolescents across all school grades.