The control group received two one-hour general sessions on health issues and printed general advices regarding healthy diets.

Intervention was focused on the reduction in consumption of sugar-sweetened carbonated beverages by students. During seven months of one school year, a healthy lifestyle education programme was implemented using simple messages encouraging water consumption instead of sugar-sweetened carbonated beverages. Education was delivered via classroom activities; banners were hung promoting water consumption, and water bottles with the logo of the campaign were given to children and schoolteachers.

The centre of the campaign was to encourage the exchange of sugar-sweetened beverages for water. Ten one-hour sessions of activities facilitated by four trained research assistants were assigned for each class. The activities required 20-30 min and teachers were encouraged to reiterate the message during their lesson. Classroom quizzes and games using water v. sugar-sweetened carbonated beverages as the theme, as well as song and drawing competitions, were promoted. In addition, a musician using a tambourine helped each class to collectively develop songs related to drinking water and reducing the consumption of sugar-sweetened carbonated beverages. This musical activity was conducted during three one-hour sessions.

We have calculated changes in body mass index (BMI in follow-up minus BMI on baseline) and compared mean changes between control and intervention groups, in order to address differences in BMI gain among groups.

Prevalence of overweight was assessed in both groups (intervention and control) in baseline and at the end of follow-up, according to standard definition proposed by Cole et al., 2000. Then, we evaluated changes in prevalences among groups.