Objective: Associations between diet and physical activity may identify behaviours that could be changed together to prevent childhood obesity. The present study examines associations between physical activity and obesogenic dietary behaviours in a large UK adolescent cohort.

Design: Cross-sectional analysis of a UK cohort. Adolescents aged 10-11 years completed three 1 d diet diaries. Average daily energy consumption, percentage energy from fat and carbohydrate, energy density and grams of fruit and vegetables were estimated. To assess physical activity participants wore an accelerometer for three or more days. Regression models were run by sex to examine the extent to which dietary variables predicted physical activity before and after controlling for pubertal status, maternal education and adiposity.

Setting: The Avon Longitudinal Study of Parents and Children (ALSPAC), south-west England.

Subjects: Adolescents who provided diet data at age 10 years and physical activity data at age 11 years.

Results: Among boys, percentage energy from fat was consistently negatively associated with accelerometer-determined indicators of physical activity (standardized beta (beta) = -0.055 to -0.101, P < 0.05) while total energy (beta = 0.066 to 0.091, P < 0.05) and percentage energy from carbohydrate (beta = 0.054 to 0.106, P < 0.05) were positively associated before and after adjustment for confounders. For girls fruit and vegetable intake was consistently positively associated with physical activity (beta = 0.056 to 0.074, P < 0.005). However all associations were weak. Associations were broadly comparable when participants with non-plausible dietary reports were included or excluded from the analyses.

Conclusions: Obesogenic diet and physical activity behaviours were weakly associated, suggesting that interventions should focus on implementing strategies that are independently successful at changing diet or physical activity behaviours either separately or in combination.