Total energy intake, adolescent discretionary behaviors and the energy gap

To estimate total energy intake and the energy gap-the daily imbalance between energy intake and expenditure-associated with discretionary behaviors of adolescents, namely their leisure active behaviors (playing or participating in sports and heavy chores), leisure sedentary behaviors (television (TV) viewing and playing video and computer games), productive sedentary behaviors (reading or doing homework).

Prospective observational study.

A total of 538 students (mean age at baseline=11.7 years) from public schools in the Boston area studied prospectively from the fall of 1995 to the spring of 1997.

Anthropometric assessment including height and weight, dietary assessment using a youth food frequency questionnaire and measures of TV, video, reading/doing homework and youth physical activity.

We estimate the change in total energy intake for each hour change in discretionary activity using regression methods. A 1-h increase in watching TV is associated with a 106 kcal h(-1) increase in total energy intake (95% confidence interval (CI): 61-150 kcal day(-1)). A similar change of 92 kcal h(-1) (95% CI: 37-147 kcal day(-1)) is seen with playing video and computer games. The change in energy intake associated with an hour change in physical activity is 292 kcal h(-1) (95% CI: 262-321 kcal day(-1)). No significant change is associated with reading/doing homework. Assuming that typical energy expenditures are associated with these behaviors, reading/doing homework appears to be an 'energy neutral' activity, whereas watching TV and playing video and computer games is associated with an energy surplus. If we assume that physical activity levels are moderate (3.5 METs), then this is also an energy surplus activity. If physical activity is assumed to be vigorous for the entire time allotted (>6.0 METs), an energy deficit could be achieved. We validated these estimates by calculating regressions predicting change in weight. Results indicate that each hour increase in TV viewing is associated with a weight increase of 0.38 kg (95% CI: 0.17-0.59 kg), with no significant associations for the other behaviors. A model with change in BMI as the dependent variable produced similar results.

Watching TV is an activity associated with a daily energy surplus. Although physical activity is thought of as an energy deficit activity, our estimates do not support this hypothesis. Reading/doing homework is the only discretionary activity examined which appears to be clearly energy neutral. The differential impacts of these discretionary behaviors on energy intake and the energy gap are discussed in relation to food-related advertisements aimed at children and adolescents.