Objective: We studied the prevalence and stability of overweight and obesity in a cohort of adolescents, and the effects of sedentary behaviour and physical activity on changes in body mass. The study also examined the extent to which physical activity mediated the relationship between sedentary behaviour and body mass.
Design: Four-year cohort study.
Methods: The study was part of the Health Behaviour of School-aged Children Study that took place in Wales between 1994 and 1998. Body height and weight measurements and self-report data on sedentary behaviour, physical activity and psychosocial adjustment were collected from 355 adolescents on two occasions 4 years apart. The mean age of the sample at baseline was 12.30 (SD=6.30) years.
Results: Weight conditions (underweight, overweight and obesity) and body mass were moderately stable over the interval. Regression analyses showed that sedentary behaviour at Time 1 predicted body mass at Time 2, while physical activity predicted a change in body mass over time. The influence of sedentary behaviour on body mass was not found to be mediated by physical activity. However, weight problems in Year 7 coincided with getting bullied, bullying others, and feeling left out of things. Obesity was also related to snacking and skipping breakfast.
Conclusions: Sedentary behaviour and physical activity in early adolescence both influenced body mass in late adolescence. Results indicated that promoting healthy diets and physical activities may have long-term health benefits for young people.