Re-evaluating associations between the Supplemental Nutrition Assistance Program participation and body mass index in the context of unmeasured confounders

•Studies show a link between Supplemental Nutrition Assistance Program participation and BMI.•Near-far matching is an analytical technique that may help control for unmeasured confounding.•Unmeasured confounding is plausible if not likely in the SNAP and BMI association.•Adjusted regression showed a SNAP-BMI association of +1.23 kg/m2 (0.84, 1.63).•Near-far matching showed a null SNAP-BMI association of +0.21 kg/m2 (−3.88, 4.29).

ObjectiveTo evaluate the association between participation in the Supplemental Nutrition Assistance Program (SNAP) and body mass index (BMI) in the presence of unmeasured confounding.MethodsWe applied new matching methods to determine whether previous reports of associations between SNAP participation and BMI were robust to unmeasured confounders. We applied near-far matching, which strengthens standard matching by combining it with instrumental variables analysis, to the nationally-representative National Household Food Acquisition and Purchasing Survey (FoodAPS, N = 10,360, years 2012-13).ResultsIn ordinary least squares regressions controlling for individual demographic and socioeconomic characteristics, SNAP was associated with increased BMI (+1.23 kg/m2, 95% CI: 0.84, 1.63). While propensity-score-based analysis replicated this finding, using instrumental variables analysis and particularly near-far matching to strengthen the instruments' discriminatory power revealed the association between SNAP and BMI was likely confounded by unmeasured covariates (+0.21 kg/m2, 95% CI: −3.88, 4.29).ConclusionsPrevious reports of an association between SNAP and obesity should be viewed with caution, and use of near-far matching may assist similar assessments of health effects of social programs.