Importance: Decreasing the amount of caloric beverages consumed and simultaneously increasing water consumption is important to promoting child health and decreasing the prevalence of childhood obesity.

Objective: To estimate the impact of water jets (electrically cooled, large clear jugs with a push lever for fast dispensing) on standardized body mass index, overweight, and obesity in elementary school and middle school students. Milk purchases were explored as a potential mechanism for weight outcomes.

Design, Setting, and Participants: This quasi-experimental study used a school-level database of cafeteria equipment deliveries between the 2008-2009 and 2012-2013 and included a sample of 1227 New York, New York, public elementary schools and middle schools and the 1 065 562 students within those schools.

Intervention: Installation of water jets in schools.

Main Outcomes and Measures: Individual body mass index (BMI) was calculated for all students in the sample using annual student-level height and weight measurements collected as part of New York’s FITNESSGRAM initiative. Age- and sex-specific growth charts produced by the Centers for Disease Control and Prevention were used to categorize students as overweight and obese. The hypothesis that water jets would be associated with decreased standardized BMI, overweight, and obesity was tested using a difference-in-difference strategy, comparing outcomes for treated and nontreated students before and after the introduction of a water jet.

Results: This study included 1 065 562 students within New York City public elementary schools and middle schools. There was a significant effect of water jets on standardized BMI, such that the adoption of water jets was associated with a 0.025 (95% CI, −0.038 to −0.011) reduction of standardized BMI for boys and a 0.022 (95% CI, −0.035 to −0.008) reduction of standardized BMI for girls (P < .01). There was also a significant effect on being overweight. Water jets were associated with a 0.9 percentage point reduction (95% CI, 0.015-0.003) in the likelihood of being overweight for boys and a 0.6 percentage reduction (95% CI, 0.011-0.000) in the likelihood of being overweight for girls (P < .05). We also found a 12.3 decrease (95% CI, −19.371 to −5.204) in the number of all types of milk half-pints purchased per student per year (P < .01).

Conclusions and Relevance: Results from this study show an association between a relatively low-cost water availability intervention and decreased student weight. Milk purchases were explored as a potential mechanism. Additional research is needed to examine potential mechanisms for decreased student weight, including reduced milk taking, as well as assessing impacts on longer-term outcomes.