Objective: To assess developmental trends in physical activity and sedentary behaviour in British adolescents in relation to sex, ethnicity and socioeconomic status (SES). Design: A 5-year longitudinal study of a diverse cohort of students aged 11-12 years at baseline in 1999. Setting: 36 London schools sampled using a stratified random sampling procedure. Participants: A total of 5863 students categorised as white, black or Asian, and stratified for SES using the Townsend Index. Main outcome measures: Number of days per week of vigorous activity leading to sweating and breathing hard. Hours of sedentary behaviour, including watching television and playing video games. Data were analysed using multilevel, linear, mixed models. Results: Marked reductions in physical activity and increases in sedentary behaviour were noticed between ages 11-12 and 15-16 years. Boys were more active than girls, and the decline in physical activity was greater in girls (46% reduction) than in boys (23%). Asian students were less active than whites, and this was also true of black girls but not boys. Black students were more sedentary than white students. Levels of sedentary behaviour were greater in respondents from lower SES. Most differences between ethnic and SES groups were present at age 11 years, and did not evolve over the teenage years. Conclusions: Physical activity declines and sedentary behaviour becomes more common during adolescence. Ethnic and SES differences are observed in physical activity and sedentary behaviour in British youth that anticipate adult variations in adiposity and cardiovascular disease risk. These are largely established by age 11-12 years, so reversing these patterns requires earlier intervention.

Objective To examine the developmental trajectory of obesity in adolescence in relation to sex, ethnicity, and socioeconomic status. Design Five year longitudinal cohort study of a socioeconomically and ethnically diverse sample of school students aged 11-12 years at baseline. Setting 36 London schools recruited to the study in 1999 by a stratified random sampling procedure. Participants 5863 students participated in one or more years. Main outcome measures Weight, height, and waist circumference measured annually by trained researchers; overweight and obesity defined according to International Obesity Task Force criteria; adiposity and central adiposity indexed by body mass index (BMI) and waist standard deviation scores relative to 1990 British reference values. Results In school year 7 (age 11-12), the prevalence of overweight and obesity combined was almost 25%, with higher rates in girls (29%) and students from lower socioeconomic backgrounds (31%) and the highest rates in black girls (38%). Prevalence of obesity increased over the five years of the study at the expense of overweight, but no reduction occurred in the proportion of students with BMIs in the healthy range. Waist circumferences were high compared with 1990 norms at age 11 (by 0.79 SD in boys and by 1.15 SD in girls) and increased further over time. Both BMI and waist circumference tracked strongly over the five years. Conclusions Prevalence of overweight and obesity was high in London school students, with significant socioeconomic and ethnic inequalities. Little evidence was found of new cases of overweight or obesity emerging over adolescence, but few obese or overweight adolescents reduced to a healthy weight. The results indicate that persistent obesity is established before age 11 and highlight the need to target efforts to prevent obesity in the early years.

A multidimensional approach to understanding the context of physical activity in early adolescence is needed because factors in several domains are relevant. The correlates of physical activity and sedentary behaviors are distinct in this age group, and there are also important gender differences.