Using Electronic Health Records to Estimate the Impact of Seattle’s Sweetened Beverage Tax on Adult BMI

• In Event: Dietary and Health Impacts of Nutrition Policies

Abstract

City-level sweetened beverage taxes have emerged as a policy response to rising rates of obesity and associated chronic disease outcomes in the U.S. Research has found that these taxes raise prices and reduce sales of taxed beverages within the taxed cities; however, these findings do not necessarily imply tax impacts on sugar intake in affected households. In this paper, we look for evidence of sweetened beverage tax impacts on overall dietary intake of sugar by examining trends in Body Mass Index (BMI) growth in Seattle before and after the introduction of that city’s Sweetened Beverage Tax in 2018. Using electronic health records from a major medical system in Washington State, we analyze medical records from 2014 to 2019 for 98,000 adult patients to estimate the association between the introduction of Seattle’s Sweetened Beverage Tax in 2018 with the change in BMI growth among Seattle residents aged 18 to 65. We estimate changes in BMI growth for all patients with at least one BMI measurement before and after the tax.

 

We employ several methods to deal with compositional changes in the data from period to period, which arise due to patients appearing irregularly in electronic health records. First, we estimate an “annualized change” model using two-period difference in differences with only one pre-tax and one post-tax observation per person. Next, we impute missing observations using the Random Forest method, then use an event study to both estimate dynamic treatment effects and to test for parallel pre-treatment trends in the data.

 

In preliminary results, we find small, but statistically significant post-tax decreases in BMI trajectory for adults residing in Seattle, relative to a matched comparison group residing in King County outside of Seattle. Our preliminary estimates suggest an effect size of approximately 0.4 pounds less weight gain among Seattle patients per year, after the tax. These results are driven by lower-income, White, and Male patients.

 

Author

• Melissa Knox, University of Washington
  Presenting Author