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.

The experimental procedures were integrated into a health promotion and education program.

Pre-manipulation education: Across the study groups, participants received a common healthy lifestyle education program, focusing on nutrition and physical activity, which was a part of the school curriculum. The combination of nutrition and PA interventions is in line with best practice guidelines for interventions promoting healthy body weight. The education program was delivered by teachers and a group format was used. The groups discussed food composition, safe food handling, food labeling, nutrient needs for age and gender groups, dietary guidelines, and clinical nutrition issues. The program did not include behavior change techniques and was not accompanied by changes in policies.

The intervention conditions were delivered via a combination of printed forms with paper-and-pencil exercises and face-to-face sessions. All experimental conditions included an initial session (completing the forms individually in the groups + face-to-face component) and sets of handouts for three following weeks. The face-to-face component was delivered within three days of completing the initial forms. The initial session was followed by a booster session (group + face-to-face components), delivered at 2-month follow-up. Across the groups, completing the forms (individual component) took approximately 30 minutes and was conducted in classrooms. The face-to-face components took 45-60 minutes and were conducted in the offices of school nurses or school psychologists.

Group intervention component. At the baseline participants completed the intervention materials individually, using self-copy paper; the copies were collected for fidelity analysis and originals were left for participants. The paper-and-pencil materials followed a similar format in four groups in terms of word count, visual format, and the number and length of sections requiring participants filling in the blanks.

Face-to-face intervention component. During the face-to-face component (at the baseline and during the booster sessions) all participants received feedback on their body weight, information regarding their physical activity levels and energy expenditure based on participant's age, gender, body weight, followed by moderate-to-vigorous physical activity recommendations (body mass and body fat measured during the booster session was not recorded). Next, all participants and experimenters jointly reviewed the forms completed during the group component. Adolescents reported included plans and were prompted to provide even more detailed responses to questions included in the forms. Experimenters asked participants to read the content of the form loudly; looked for sections which were incomplete and encouraged adolescents to complete these sections; prompted participants to provide detailed responses.

Eligibility Criteria:
Inclusion Criteria:

* Healthy adolescents
* Adolescents with chronic conditions but without contraindications for moderate-to-vigorous physical activity

Exclusion Criteria:

* Adolescents who were younger than 14 years old
* Adolescents who declared plans for changing schools during the following year (e.g., due to graduation or moving to another region)
* No parental consent at the baseline
* Existing diseases with contraindications for moderate-to-vigorous physical activity