Prevalence of breastfeeding
The present study investigated the association between breastfeeding and cognitive function in children from 5.5 months to 8 years of age using multiple assessment tools and a prospective design. The prevalence of breastfeeding in our study is comparable to previous studies. Despite evidence of the beneficial effects of breastfeeding on the health of mother and child, the prevalence of breastfeeding was substantially different between countries, with a clear tendency of lower breastfeeding duration and prevalence in wealthier countries [17]. For instance, the proportion of children who were ever-breastfed in our study was 97.4%. The previously reported proportion of ever-breastfed children in most countries was over 90% and was especially high in low-income countries. However, some high-income countries such as France (63%), Spain (77%), Ireland (55%), and the United States (79%) had substantially lower proportions of ever-breastfed children [17]. The proportion of children who continued breastfeeding after six months was 61.8% in our study. The average proportion of children who continued breastfeeding after six months was lower than 50% in high-income countries, with especially low proportions in Denmark (13%), France (23%), Canada (30%), and the United Kingdom (34%) [17]. The previously reported proportion of breastfeeding at six months in Korea was 61%, which is consistent with the present findings [18]. The prevalence of breastfeeding in Korea is reported to have increased remarkably since the lowest prevalence in 2000, which is encouraging news for the health of children [18].
Early development
Early development of infants at T1, T2, and T3 assessed by the Denver II showed no significant differences in odds ratios for developmental delay between the groups of breastfeeding duration. These are inconsistent findings with previous studies. Barros et al. reported significantly higher suspected developmental delay at the one year assessment in children breastfed for less than one month (42.4%) compared to those breastfed for nine months or more (25.5%) [19]. Wang and Wu also reported significantly higher developmental delay in the personal-social domain of the Denver II assessed at one year of age in non-exclusively breastfed children (36%) compared to exclusively breastfed children (21%) [20].
The results of the early development assessment with the K-ASQ presented different aspects than the Denver II assessment. The odds ratios to have atypical scores in at least one subdomain of the K-ASQ at T2 was significantly higher by 2.63-fold in children breastfed for one to three months than the reference group (children breastfed for three to six months). However, there were no significant differences in odds ratios for developmental delay assessed with the K-ASQ at T1 and T3.
In the comparison of the K-ASQ score as a continuous variable among breastfeeding groups, scores on communication and problem-solving subdomains at T2 and T3 in children breastfed for more than three months were significantly higher than the children breastfed for three months or less. These are consistent with the findings of previous studies on early development using the ASQ, which have reported the benefits of breastfeeding on early development. An Irish study of 11,134 children that assessed early development with the ASQ at nine months old reported the positive effect of breastfeeding on gross motor, fine motor, problem-solving and personal-social skills [21]. A French study with 1,999 three-year-old children also reported that ever-breastfed children scored 6.2 points higher on the ASQ than never-breastfed children [22]. The study also reported a significant positive association between exclusively breastfed infants and higher scores on the problem-solving domain of the ASQ. An Australian cohort study with 2,868 children reported that infants breastfed for four months or longer had higher scores in fine motor skills and communication assessed at age one and three years. Infants who were breastfed for less than four months were also more likely to have at least one atypical score across the subdomains compared to children breastfed for four months or longer [23].
Cognitive function in middle childhood and school-age
There were significant differences in cognitive function assessed using the vocabulary test (REVT) among the groups of breastfeeding duration. There was no difference in receptive language score among the six groups of breastfeeding duration. However, when grouped by children who were breastfed for more than three months or three months or less, those breastfed for more than three months scored significantly higher on the vocabulary test. This is consistent with previous findings for language development in middle childhood based on breastfeeding duration. An Australian cohort study with 1,195 children assessed language ability with the Peabody Picture Vocabulary Test (mean=100; SD=15) and reported that children who were breastfed for over six months presented higher mean scores (3.56 points at five years and 4.04 points at ten years, respectively) than children who were never-breastfed [24].
Our results indicate an advantageous association between breastfeeding and cognitive function during school days. Scores on the M-FIT subscales of vocabulary and language inference in children breastfed for more than three months were significantly higher than children breastfed for three months or less. These findings are consistent with previous studies on the cognitive function of school-age children based on breastfeeding duration [25-27]. For instance, children born preterm who were breastfed had higher IQ scores by 7.6 points (about half a standard deviation) at eight years than never-breastfed children [25]. An Irish study with 8,226 nine-year-old school children also reported that ever-breastfed children scored significantly higher percentage points on reading and mathematics than never-breastfed children[27]. Huang et al. also reported that breastfeeding had a significant association with higher intelligence and that the association remained significant during the schooling and adolescent period [28].
Limitations
The present study has some limitations to note. First, due to the characteristics of longitudinal cohort studies, a substantial number of subjects did not participate in the follow-up assessments. Notably, some participants were excluded from the adjusted model analysis due to missing covariate data. The missing data may bias the relationship between breastfeeding and children’s cognitive function. Thus, future study of a more complete dataset with covariate analysis is warranted. Second, although we tried to include important socio-demographic covariates, not all covariates could not be included. For instance, previous studies indicated that maternal IQ is a major moderating factor for the association between breastfeeding and children’s intelligence, which was not included in our study [6, 29]. Despite these limitations, the present study has the strength of using multiple tools at multiple time points to assess children’s cognitive development using a prospective design.