Physical activity rather than sedentary behaviour is socially determined in French adolescents with overweight and obesity.

Social differences in prevalence of overweight and obesity among adolescents, known as the weight social gradient, could be explained by differences in behaviours between social classes. This study examined the respective association of physical activity (PA), sedentary behaviour (SB) and weight status with adolescents' socioeconomic status. We used cross-sectional data for 1935 adolescents (13-18 years old) with overweight or obesity who participated in the PRALIMAP-INÉS (PRomotion de l'ALImentation et de l'Activité Physique - INÉgalités de Santé) trial conducted in northeastern France between 2012 and 2015. Adolescents completed the International Physical Activity Questionnaire for PA and SB and the Family Affluence Scale for socioeconomic status. Weight status was assessed by the body mass index (BMI) and BMI z-score. Social gradient of weight status, PA and SB were described according to the Family Affluence Scale (slopes) and evidenced by the linear trend test (p). Adolescents' socioeconomic status was positively associated with PA practice (frequency, vigorous PA and leisure-time PA), but there was no association with their SB. The results confirmed a significant weight social gradient: BMI (β = 0.37, p < .0001) and BMI z-score (β = 0.07, p = .0001). The weight social gradient in adolescents was mostly associated with PA (5.7% to 8.1%) rather than SB (2.7% to 5.7%). Nearly 14% of BMI z-scores could be related to a combined PA and SB effect. PA was found an important factor of weight social gradient in adolescence. Actions aimed at preventing weight social inequalities among adolescents could include PA promotion as lever. CLINICAL TRIALS REGISTRY AND NUMBER: ClinicalTrials.gov (NCT01688453).