Mediators and moderators of the effects of a school-based intervention on adolescents' fruit and vegetable consumption: the HEIA study.

Objective: To examine whether targeted determinants mediated the effects of the HEalth In Adolescents (HEIA) intervention on fruit and vegetable (FV) consumption and explore if these mediating effects were moderated by sex, parental education or weight status.

Design: Cluster-randomised controlled trial.

Setting: The HEIA study (2007-2009) was a Norwegian 20-month multi-component school-based intervention to promote healthy weight development. FV consumption and targeted determinants were self-reported at baseline, mid-way (8 months) and post-intervention (20 months).

Participants: Adolescents (11-13-year-old) in twenty-five control schools (n 746) and twelve intervention schools (n 375).

Results: At post-intervention, more adolescents in the intervention group compared with the control group had knowledge of the FV recommendations (OR: 1·4, 95 % CI 1·1, 1·9) and reported a decreased availability of vegetables at home (β: -0·1, 95 % CI -0·2, 0·0). Availability/accessibility of FV at home, availability of vegetables at dinner, taste preferences for different types of FV and knowledge of the FV recommendations were positively associated with the consumption of FV. However, none of the post-intervention determinants significantly mediated the intervention effects on FV consumption. Although no moderating influences by sex, parental education or weights status were observed on the mediating effects, exploratory analyses revealed significant moderations in the b-paths.

Conclusions: Since none of the targeted determinants could explain the increase in FV consumption, it remains unclear why the intervention was effective. Reporting on a wide range of mediators and moderators in school-based interventions is needed to reveal the pathways through which intervention effects are achieved.