BACKGROUND: The superiority of an intervention combining two sets of theory-based behavior change techniques targeting planning and self-efficacy over an intervention targeting planning only or self-efficacy only has rarely been investigated. PURPOSE: We compared the influence of self-efficacy, planning, and self-efficacy+planning interventions with an education-based control condition on adolescents' body fat, assuming mediating effects of respective social cognitive variables and moderate-to-vigorous physical activity (MVPA). The moderating role of the built environment was examined. METHODS: Participants (n = 1,217, aged 14 to 18 years) were randomly assigned to four conditions: planning (n = 270), self-efficacy (n = 311), self-efficacy+planning (n = 351), and control (n = 285). The measurement was conducted at baseline (T1), two-month follow-up (T2), and fourteen-month follow-up (T3). Interventions/control group procedures were delivered at T1 and T2. Percent of body fat tissue (measured at T1 and T3) was the main outcome. Social cognitive mediators (self-efficacy and planning) were assessed at T1 and T2. The behavioral mediator (MVPA) and the presence of built MVPA facilities (the moderator) were evaluated at T1 and T3. RESULTS: Similar small increases of body fat were found across the three intervention groups, but the increment of body fat was significantly larger in the control group. On average, differences between control and intervention groups translated to approximately 1% of body fat. Effects of the interventions on body fat were mediated by relevant social cognitive variables and MVPA. A lower increase of body fat was found among intervention group participants who had access to newly-built MVPA facilities. CONCLUSIONS: We found no superiority of an intervention targeting two social cognitive variables over the intervention targeting one cognition only.

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