Introduction Drinking water has been associated to weight loss in experimental controlled studies and also with reduced weight gain in observational studies, whereas sugar sweetened beverage (SSB) consumption is associated with weight gain, mainly among adolescents. We examined whether water consumption displace SSB and whether the changes in body mass index (BMI) was associated with water and beverage intake.

Methods A randomised school trial of 4^th graders from 22 public schools of the city of Niterói, Rio de Janeiro, Brazil, followed students during one school year. Most of the 1134 participants were 10–11 years old. Baseline consumption of water and SSB were evaluated using a drinking frequency questionnaire and one 24-h recall.

Results At baseline, a higher frequency of drinking water was associated with a greater intake of juices (p=0.02) and sodas (p<0.0001). Baseline frequency of drinking water was negatively associated with one year weight change (regression coefficients of −0.21) comparing more than three glasses of water per day with <3 (p=0.04), whereas for an increase in one glass of juice there was an increase of 0.15 units of BMI (p=0.002). After adjustment for physical activity and mutually adjustment for water and SSB results were materially unchanged.

Conclusion Our findings confirm the protective effect of drinking water while drinking juices was a risk factor for BMI gain. There was no compensation between water and SSB consumption, therefore the marketing of increasing water consumption would not prevent the excessive weight gain.