The association between income inequality and adolescent body mass index: findings from the COMPASS study (2016-2019)

Income inequality has been linked to high and unhealthy body mass index (BMI), though there is a dearth of evidence in adolescents. Therefore, this study examines the association between income inequality and BMI in a large sample of Canadian adolescents.

A pooled cross-sectional design was used. Participants were adolescents (n = 101,901) from 157 Canadian secondary schools participating in the 2016-2017, 2017-2018, or 2018-2019 waves of the Cannabis, Obesity, Mental health, Physical activity, Alcohol, Smoking, and Sedentary behaviour (COMPASS) study. BMI was calculated from self-reported height and weight and converted to World Health Organization (WHO) z-BMI scores. Gini coefficients were calculated at the census division level (n = 49) using data from the 2016 Canadian Census. Multilevel modelling was performed to account for the clustering of students nested within schools, which were nested within census divisions. Interactions were included to determine whether associations were heterogeneous for males and females.

Income inequality demonstrated a non-linear association with WHO z-BMI score (z-Gini: β = 0.05, 95%CI: 0.02, 0.08; z-Gini²: β = -0.02, 95% CI: -0.04, -0.01) among adolescents after adjusting for student-, school-, and census division-level covariates. This association was more pronounced among females.

The association between income inequality and BMI, being overweight, or having obesity appears to be non-linear. Public health units and schools may benefit from incorporating upstream factors such as income inequality into their interventions attempting to promote healthy weights.