Improving Healthy Eating Behaviour among In-School Adolescents through Collaborative Instructional Intervention: A Theory-Driven Quasi-Experimental Study

Background Adolescence is a critical window for establishing dietary behaviours that shape long-term health outcomes. In Nigeria and other Sub-Saharan African countries, school-based nutrition education has been largely ineffective when delivered through conventional didactic approaches. This study evaluated the effect of a Health Belief Model–guided collaborative learning intervention on diet quality, nutrition knowledge, and perceived dietary behaviour among in-school adolescents in Ogun State, Nigeria. Methods A quasi-experimental, pre-test–post-test control group design was used. Three hundred adolescents aged 13–19 years from two public secondary schools in Odeda Local Government Area were recruited, with 150 assigned to each group. The intervention group received eight weeks of structured nutrition education delivered through collaborative learning techniques, while the control group received the same content through conventional didactic instruction. Diet quality was assessed using the Global Diet Quality Questionnaire (DQQ), nutrition knowledge using a validated 20-item questionnaire, and perceived dietary behaviour using an HBM-based scale. Data were analysed using paired t-tests, independent t-tests, Difference-in-Differences analysis, and ANCOVA. Results The intervention group demonstrated statistically significant improvements in dietary diversity (DDS: 33.74%), NCD-Protect score (153.22%), and GDR score (39.86%), while the NCD-Risk score showed no significant change in either group (p = 0.967). Nutrition knowledge improved significantly in the intervention group (DiD = 7.94; p = 0.007), with no meaningful change in the control group. All six HBM constructs improved significantly in the intervention group, including self-efficacy (13.46%) and perceived severity (18.96%). ANCOVA confirmed that the intervention independently predicted improvements in both dietary diversity and nutrition knowledge after adjusting for baseline scores and sociodemographic covariates. Conclusions Collaborative learning–based nutrition education guided by the Health Belief Model is effective in improving diet quality, nutrition knowledge, and health beliefs among in-school adolescents. These findings support the integration of participatory and theory-driven instructional approaches into school-based nutrition programming in Nigeria and similar low-resource settings.