BACKGROUND: Prior research on the restaurant environment and obesity risk is limited by cross-sectional data and a focus on specific geographic areas. OBJECTIVE: To measure the impact of changes in chain restaurant calories over time on body mass index (BMI). DESIGN: We used a first-difference model to examine whether changes from 2012 to 2015 in chain restaurant calories per capita were associated with percent changes in BMI. We also examined differences by race and county income, restaurant type, and initial body weight categories. SETTING: USA (207 counties across 39 states). PARTICIPANTS: 447,873 adult patients who visited an athenahealth medical provider in 2012 and 2015 where BMI was measured. MAIN OUTCOMES MEASURED: Percent change in objectively measured BMI from 2012 to 2015. RESULTS: Across all patients, changes in chain restaurant calories per capita were not associated with percent changes in BMI. For Black or Hispanic adults, a 10% increase in exposure to chain restaurant calories per capita was associated with a 0.16 percentage-point increase in BMI (95% CI 0.03, 0.30). This translates into a predicted weight increase of 0.89 pounds (or a 0.53% BMI increase) for an average weight woman at the 90th percentile of increases in the restaurant environment from 2012 to 2015 versus an increase 0.39 pounds (or 0.23% BMI increase) at the 10th percentile. Greater increases in exposure to chain restaurant calories also significantly increased BMI for Black or Hispanic adults receiving healthcare services in lower-income counties (0.26, 95% CI 0.04, 0.49) and with overweight/obesity (0.16, 95% CI 0.04, 0.29). LIMITATIONS: Generalizability to non-chain restaurants is unknown and the sample of athenahealth patients is relatively homogenous. CONCLUSIONS: Increased exposure to chain restaurant calories per capita was associated with increased weight gain among Black or Hispanic adults.
