Effect of a multilevel blended intervention based on social cognitive theory on fruit and vegetable consumption in students

Inadequate fruit and vegetable (F&V) consumption among adolescents remains a critical global public health challenge. This cluster-randomized controlled trial evaluated the effectiveness of a SCT-based multilevel blended intervention in increasing F&V consumption-frequency scores among female students in grades 7–9 in Seydun, Iran, during 2023–2024. Four public schools were randomized to the intervention ( n = 104) or comparison group ( n = 101). The intervention comprised eight weekly 60-minute sessions delivered through a combination of face-to-face and WhatsApp-based activities, with complementary components involving parents and school staff. Outcomes were assessed at baseline and two months after the intervention using a researcher-developed questionnaire with reported evidence of face validity, content validity, internal consistency, and test–retest reliability. Analysis of covariance (ANCOVA) models compared post-intervention scores between groups while adjusting for baseline scores. The intervention group showed significantly greater improvements in fruit (1.84 to 3.00, p < 0.001) and vegetable (1.72 to 2.75, p < 0.001) consumption-frequency scores and SCT-related constructs than the comparison group. These findings suggest the utility of SCT in addressing individual, social, and environmental determinants of adolescent nutrition behavior. The study provides supportive evidence for the potential effectiveness of integrated, theory-based, multilevel school-based interventions in enhancing healthy dietary habits among adolescents.