Income and Obesity: What Is the Direction of the Relationship? a Systematic Review and Meta-Analysis.

Background: There is evidence that socio-economically disadvantaged groups experience worse health and higher morbidity, including obesity. However, obesity does not only constitute a risk of the economically disadvantaged (causation hypothesis). By switching perspectives, people with obesity can also be seen as a disadvantaged group, since they are prone to labor market discriminations that include decreased chances for higher education and job security, higher unemployment and lower income (health selection hypothesis). By exploring obesity as a risk of the socially disadvantaged and the obese as a socially disadvantaged group, this review aims to examine the relative importance of causation and selection processes in the linkage between income and obesity. In particular, we raise the question on the direction of the link between income and obesity, while also discussing the contribution of both causation and selection processes to this relationship.

Methods: A systematic literature search was conducted that focused on medical, psychological and sociological databases (i.e. Medline, PsychInfo, Sociological Abstracts, International Bibliography of Social Sciences and Sociological Index).

Results: 14 studies on causation and 7 studies on selection were found. Meta-analyses revealed that lower income is associated with subsequent obesity (odds ratio: 1.27, CI-95: 1.10 to 1.47; risk ratio: 1.52, CI-95: 1.08 to 2.13), though the significance weakened once adjusted for publication bias. Studies on selection indicated a more consistent relation between obesity and subsequent income, even after taking publication bias into account (standardized mean difference: -.15, CI-95: -.30 to -.01). Sensitivity analyses implied that the association is influenced by obesity measurement, gender, length of observation and study quality.

Conclusion: Findings suggest that the association between income and obesity is bidirectional. Therefore, both causation and selection processes should be addressed to fully understand the relation between income and obesity.