Background Research has found breastfeeding to be protective of obesity; however, this link remains contentious. We examined longitudinal associations between exclusive breastfeeding duration, growth trajectories and obesity at 4 years among children participating in the Special Supplemental Nutrition Program for Women, Infants and Children (WIC), and whether these associations differed in the context of the 2009 WIC food package change, implemented to improve alignment with dietary guidelines and promote breastfeeding.

Methods Longitudinal data from 260 935 WIC-participating children in Los Angeles County, California, 2003–2016, were used to assess the relationship between duration of receipt of the fully breastfeeding package (an exclusive breastfeeding proxy) with childhood growth and obesity using mixed effects and Poisson regression models.

Results Children exclusively breastfed for longer duration had healthier growth trajectories and lower obesity risk at age 4. Compared with infants with no fully breastfeeding package receipt, any receipt (a breastfeeding initiation proxy) was associated with reduced obesity risk. Obesity risk was lowest for boys and girls exclusively breastfed for 7 (risk ratio (RR)=0.73, 95% CI=0.64 to 0.82) and 13 months (RR=0.63, 95% CI=0.58 to 0.69), respectively. Exclusive breastfeeding duration increased, but associations between exclusive breastfeeding duration and growth and obesity were not modified, following the 2009 WIC food package change.

Conclusion Increased duration of exclusive breastfeeding was associated with reduced obesity risk. The greatest incremental benefit was observed going from none to any exclusive breastfeeding, and the maximum cumulative benefit was among children receiving the fully breastfeeding package for more than 6 months. Breastfeeding promotion in WIC remains important for obesity prevention.