Using routinely collected growth data to assess a school-based obesity prevention strategy

Background: Studies of school-based anti-obesity interventions have yielded inconsistent results. Using growth screening data from a school administrative database, we re-evaluated a nutrition education intervention that appeared, in a sample of students in grades 5-7, to have a beneficial effect on weight status. Methods: Ten K-8 schools (5 control and 5 intervention) participated in a 2 year cluster-randomized trial of a multi-component obesity prevention strategy. We obtained student height and weight data for six consecutive school years and imputed any missing baseline and follow-up measurements (53% and 55%, respectively). We analyzed changes in BMI Z-scores via mixed-effects linear regression and in the prevalence of overweight/obesity via conditional logistic regression. Results: We analyzed data for 8,186 (96%) K-8 students in the 10 schools (4,511 in intervention, 3,675 in control). From baseline to the end of the intervention period, mean increases in BMI Z-score were 0.10 and 0.09 in the control and intervention groups, respectively (p = 0.671). The prevalence of overweight/obesity increased by 3% in both groups (p = 0.926). There was no significant intervention effect on the incidence or remission of overweight/obesity. Conclusions: Using routinely collected data for the entire target student population, we failed to confirm earlier findings of an intervention effect in a subset of students in grades 5-7. Volunteer bias in the prior evaluation and/or measurement error in the routinely collected data are potential reasons for the discrepant findings. Population-based interventions should be evaluated in the full target population, using the intent-to-treat principle to avoid bias.