Determinants of Dietary Transition and Its Nutritional Consequences Among Adolescents in Rapidly Urbanising Communities of South West Nigeria: A Mixed-Methods Longitudinal Study

Dietary transition, characterised by a shift from traditional plant-based diets toward energy-dense, processed, and animal-source food consumption, is accelerating in rapidly urbanising communities across Nigeria. Adolescents represent a critical but underexplored population in this transition, facing unique developmental nutritional requirements alongside profound social, economic, and commercial influences on food choice. This study investigated the determinants and nutritional consequences of dietary transition among adolescents aged 13 to 18 years in four rapidly urbanising communities across Ogun, Oyo, and Lagos states using a mixed-methods longitudinal design. A cohort of 540 adolescents was recruited at baseline and followed for eighteen months. Quantitative data collection included three-day weighted food records, anthropometric measurements, serum lipid panels, serum ferritin, and haemoglobin at baseline, nine months, and eighteen months. Qualitative inquiry through in-depth interviews with adolescents, caregivers, and food vendors provided explanatory depth. Structural equation modelling was used to test a conceptual causal model integrating socioeconomic, environmental, and psychosocial determinants. Results showed a significant longitudinal increase in ultra-processed food energy share from 28 to 41 percent over the study period, associated with urbanisation pace indices. Increased ultra-processed food consumption was independently associated with rising body mass index z-scores and declining serum ferritin. Qualitative data revealed peer norms, affordable junk food availability near schools, and media advertising as primary psychosocial drivers. The study provides original longitudinal evidence on dietary transition dynamics and produces a validated causal model for intervention design. Keywords: dietary transition, adolescents, urbanisation, ultra-processed foods, South West Nigeria.