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

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. 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. 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, = 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, = 0.03 and B: -3.414, = 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. 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.