BACKGROUND: Efficacious school-based interventions among ethnic minority youth are often intensive and difficult to disseminate. OBJECTIVE: This parallel, open-label-randomized controlled trial aimed to compare changes in adolescent-standardized body mass index (zBMI) from a school-based obesity intervention given 0, 1, 3, or 5 days a week. METHODS: Mexican-American youth (n = 243) with overweight or obesity were recruited from a Houston school district and randomized to receive an obesity intervention with established efficacy 0 (control), 1, 3, or 5 d/wk (respectively, equating to 0, 40, 120, or 200 min of contact each week). In each condition, 80% of intervention time was allocated to physical activity and 20% to nutrition, with behavioural modification overlaid throughout. zBMI was calculated from directly measured height and weight. A linear mixed model evaluated differences in zBMI over time between conditions. RESULTS: Participants were 12.02 +/- 0.57 years old with a zBMI of 1.80 +/- 0.46 at baseline. Among those with complete data at 1 year (n = 203), a significant condition by time interaction was indicated (F = 9.42, p < 0.001). Those who received the intervention 3 or 5 d/wk had significantly greater decreases in zBMI than control (respectively, -0.19 zBMI units/y; 95% CI -0.28 to -0.11; and -0.18 zBMI units/y; 95% CI -0.27 to -0.10, both p < 0.001). No differences were found between 3 and 5 d/wk (95% CI -0.09 to 0.08, p = 0.87). CONCLUSIONS: The intervention provided 3 d/wk resulted in similar zBMI improvements as the intervention provided 5 d/wk. This information can be used to develop scalable school-based obesity interventions.

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