Breastfeeding and Children’s Cognitive Development up to the Age of 4 Years: The Japan Environment and Children’s Study

<b><i>Objective:</i></b> Breastfeeding, depending on its duration, has been suggested to benefit children's cognitive development. We aimed to examine this issue by using the Japan Environment and Children's Study, a nationwide prospective birth cohort study. <b><i>Methods:</i></b> We evaluated the relationship between feeding methods and cognitive development in 2- and 4-year-old children. We classified the children based on the following feeding method during the first 6 months postpartum: (1) exclusive breastfeeding group, solely breastfeeding; (2) partial breastfeeding group, solely breastfeeding for ≤5 months and combination of breastfeeding/formula for the rest of months; (3) formula-fed group, solely formula feeding for ≥4 months; and (4) others. Cognitive development was assessed by trained testers using the Kyoto Scale of Psychological Development 2001. <b><i>Results:</i></b> Data on 1,329 boys and 1,398 girls were analyzed. Multiple regression analysis was performed using the exclusive breastfeeding group as the reference. Boys of age 2 years in the formula-fed group had significantly lower developmental quotients (DQs) in the language-social developmental (L-S) area (partial regression coefficient [B]: -4.624, <i>p</i> = 0.01), whereas no significant difference was observed in those of age 4 years. Girls of age 2 and 4 years in the formula-fed group had significantly lower L-S area DQ (B: -3.637, <i>p</i> = 0.03 and B: -3.414, <i>p</i> = 0.03, respectively). In the partial breastfeeding group, no significant differences in the L-S area DQ were observed in 2- and 4-year-old boys and girls. <b><i>Conclusions:</i></b> Exclusive breastfeeding for the first 6 months postpartum may be more beneficial for verbal cognitive development in 4-year-old girls than solely formula feeding for ≥4 months. Furthermore, breastfeeding combined with formula for the first 6 months postpartum may not have a disadvantage on cognitive development in boys and girls of age ≥ 2 years, when compared with that observed with exclusive breastfeeding during the first 6 months postpartum.