RESUMO
INTRODUCTION: Adolescent eating patterns are characterised by high consumption of unhealthy foods, which has resulted in an increasing prevalence of overweight and chronic diseases. It is crucial to promote healthy eating habits, and nutritional interventions based on the transtheoretical model have been found to be especially effective. Mobile health strategies also seem promising for adolescents. This study aims to outline a smartphone intervention via WhatsApp for adolescents to promote healthy eating consumption, better nutritional knowledge, self-efficacy in the adoption of healthy practices and progress through the stages of change. METHODS AND ANALYSIS: There will be three distinct groups in this randomised study: a general intervention group (GG), in which the participants will receive the same healthy eating messages, based on the Brazilian food guide; a tailored intervention group (TG), in which the participants will receive healthy eating messages based on their stage of change; and a control group (CG), in which participants will receive messages on a different theme. Possession of a smartphone, use of WhatsApp and being a senior student (16-19 years) from a public school of the Federal District of Brazil will be the study's inclusion criteria. Rural schools will be excluded. The sample size estimated is 390 individuals: 38 in the GG, 314 in the TG and 38 in the CG. The intervention will last 6 weeks, with a daily message sent to the students. We will investigate nutritional knowledge, self-efficacy in the adoption of healthy eating practices, food consumption and stages of change using preintervention and postintervention questionnaires. Memorisation of the messages will be also assessed. ETHICS AND DISSEMINATION: The study was approved by the University of Brasília, School of Health Sciences and Research Ethics Committee. At the end of the study, the participating schools will receive a printed report with the main results of the intervention. TRIAL REGISTRATION NUMBER: RBR-5b9jk7.