Worldwide, low-income and middle-income countries are undergoing rapid demographic and nutritional changes. Populations tend to move from their traditional food habits towards dietary patterns characterized by a more energy-dense diet, while urbanization and technology (work, transport) play a role in the reduction of their physical activity. This shift towards energy-dense diets and lower levels of physical activity are key determinants of chronic diseases. Nowadays, childhood overweight and obesity are becoming a major threat to public health all in low-income and middle-income countries. Clearly, not properly addressing this health problem will only aggravate the already enormous impact of chronic diseases on the social and economic development of these countries. This study aims to develop a culturally relevant school-based prevention intervention among Ecuadorian adolescents (11-13 years old) in Cuenca. The intervention will target behavioural nutrition and physical activity. Intervention strategies will be developed based on theoretical behavioural change models integrated in two complementary frameworks, i.e. (i) the Intervention Mapping protocol, and (ii) the Comprehensive Participatory Planning and Evaluation protocol.

The study includes two phases:

1. A formative research, which will be conducted in order to understand the causality of the school environment and unhealthy eating and low physical activity levels in school-going adolescents, aged 12-13 years old.
2. A paired, cluster-randomized controlled trial including 1400 adolescents (12-13 years old) from 10 pairs of schools (i.e. clusters). Within each pair of clusters, one cluster will be randomly assigned to the intervention and the other to the control group.

Inclusion Criteria:

• 11-13 years old
• school-going
• willing to participate

Exclusion Criteria:

• pregnant
• severe medical/physical disorder