Effects of Self-efficacy, Planning, and Self-efficacy+Planning Interventions on Body Fat Among Adolescents

This longitudinal experimental study tested the effects of three brief interventions: (1) prompting the formation of plans (or implementation intentions), (2) prompting self-efficacy beliefs, and (3) prompting planning + self-efficacy in adolescents aged 14-18 years relative to an active 'education only' control group. It was hypothesized that participants assigned to the interventions would exhibit a smaller increase in body fat at 14-month follow-up compared to controls. The study also investigated whether the combined planning + self-efficacy intervention would have larger effects on the main outcome (body fat) than single-component interventions. Second, it was hypothesized that the effects of the intervention conditions on body fat at 14-month follow-up would be mediated by their respective psychological and behavioral constructs: self-efficacy and planning at T2 (Mediator 1), and by moderate-to-vigorous physical activity (MVPA) at T3 (Mediator 2). It was expected that the effects of the interventions including the planning component (i.e., planning intervention and self-efficacy + planning intervention) would be mediated by respective cognitions, i.e. planning, whereas the effects of the interventions including self-efficacy component (i.e., self-efficacy intervention and self-efficacy + planning intervention) would be mediated by self-efficacy. Finally, it was explored whether the effects of the intervention (both direct and indirect effects, via their respective psychological variables and MVPA) on body fat would be moderated by the presence of built PA facilities, located in the proximity of schools.