The neighborhood food environment modifies the association between infant feeding and childhood obesity.

Background: The Special Supplemental Nutrition Program for Women, Infants and Children (WIC) issues infant formula to infants who are not fully breastfed, and prior research found elevated obesity risk among children receiving lactose-reduced infant formula with corn syrup solids (CSSF) issued by WIC. This study was conducted to evaluate associations between a broader set of specialty infant formulas issued by WIC and child obesity risk, whether neighborhood context (e.g. neighborhood food environment) modifies associations, and whether racial/ethnic disparities in obesity are partly explained by infant formula exposure and neighborhood context.

Methods: WIC administrative data, collected from 2013-2020 on issued amount (categorical: fully formula fed, mostly formula fed, mostly breastfed, fully breastfed) and type of infant formula (standard cow's milk formula, and three specialty formulas: any CSSF, any soy-based formula, and any cow's milk-based formula with added rice starch) and obesity at ages 2-4 years (defined as a Body Mass Index z-score ≥ 95th percentile according to World Health Organization growth standard) were used to construct a cohort (n = 59,132). Associations of infant formula exposures and race/ethnicity with obesity risk were assessed in Poisson regression models, and modification of infant feeding associations with obesity by neighborhood context was assessed with interaction terms.

Results: Any infant formula exposure was associated with significantly higher obesity risk relative to fully breastfeeding. Receipt of a CSSF was associated with 5% higher obesity risk relative to the standard and other specialty infant formulas (risk ratio 1.05, 95% confidence interval 1.02, 1.08) independent of breastfeeding duration and receipt of other specialty infant formulas. The association between CSSF and obesity risk was stronger in neighborhoods with healthier food environments (10% higher risk) compared to less healthy food environments (null). Racial/ethnic disparities in obesity risk were robust to adjustment for infant formula exposure and neighborhood environment.

Conclusions: Among specialty infant formulas issued by WIC, only CSSFs were associated with elevated obesity risk, and this association was stronger in healthier food environments. Future research is needed to isolate the mechanism underlying this association.