Objective: To explore whether the association between bottle-feeding during the first year and childhood obesity at age 6 years is mediated by change in weight-for-age z-score (ΔWAZ) during infancy. Methods: Participants were 1062 mother-child pairs who participated in the Infant Feeding Practices Study II and Year 6 Follow-Up. Mothers completed 1 prenatal questionnaire, 10 postpartum questionnaires during the first year, and 1 follow-up questionnaire at 6 years. Child weights were reported by mothers and used to determine ΔWAZ from birth to later infancy. Causal mediation analysis was used to determine the average direct effect of bottle-feeding on obesity at age 6 years and the average causal mediation effect of bottle-feeding through ΔWAZ. Results: Infants who experienced rapid increases in bottle-feeding frequency during the first 6 months had significantly greater ΔWAZ compared with infants who had consistently low bottle-feeding frequency until 12 months of age [β = 0.45, 95% confidence interval (CI) 0.11-0.80]. Significant predictors of obesity at age 6 years were larger birth weight [adjusted odds ratio (AOR) = 4.27, 95% CI 2.42-7.64] and larger ΔWAZ (AOR = 1.59, 95% CI 1.29-1.98). An indirect effect of bottle-feeding, mediated via ΔWAZ, accounted for 59% of the total effect of bottle-feeding on obesity at age 6 years. Conclusions: Rapid increases in bottle use during the first 6 months postpartum predicted greater risk for obesity at age 6 years via an indirect mediation effect of larger ΔWAZ from birth to later infancy.
Keywords: bottle-feeding; breastfeeding; infant feeding practices; obesity; rapid weight gain.