ABSTRACT
Background
The relationship between food environment exposures and diet behaviors is unclear, possibly because the majority of studies ignore potential residual confounding.
Methods
We used 20 years (1985–6; 1992–3; 2005–6) of data from the Coronary Artery Risk Development in Young Adults (CARDIA) study across four U.S. cities (Birmingham, AL; Chicago, IL; Minneapolis, MN; Oakland, CA) and instrumental-variables (IV) regression to obtain causal estimates of longitudinal associations between the percentage of neighborhood food outlets (per total food outlets within 1-km network distance of respondent residence) and an a priori diet quality score, with higher scores indicating higher diet quality. To assess the presence and magnitude of bias related to residual confounding, we compared results from causal models (IV regression) to non-causal models, including ordinary least squares regression, which does not account for residual confounding at all; and fixed-effects regression, which only controls for time- invariant unmeasured characteristics.
Results
The mean diet quality score across follow-up was 63.4 (SD=12.7). A 10% increase in fast food restaurants (relative to full-service restaurants) was associated with a lower diet quality score over time using IV regression (β=−1.01; 95% CI=−1.99, −0.04); estimates were attenuated using non-causal models. The percentage of neighborhood convenience and grocery stores (relative to supermarkets) was not associated with diet quality in any model, but estimates from non-causal models were similarly attenuated compared to causal models.
Conclusion
Ignoring residual confounding may generate biased estimated effects of neighborhood food outlets on diet outcomes, and may have contributed to weak findings in the food environment literature.