Introduction: In several studies a positive association between breastfeeding and intellectual development in childhood has been suggested. However, the association between breastfeeding and adult intelligence is unknown, and the purpose of the present study was to investigate the relationship between duration of breastfeeding and intelligence in young adulthood.
Material and methods: A prospective longitudinal study based on the Copenhagen Perinatal Cohort. A mixed-sex sample comprising 973 individuals with a mean age of 27.2 years was assessed with a clinical intelligence test (WAIS) and an all-male sample comprising 2280 individuals was assessed with a military intelligence test (BPP) at the mean age of 18.7 years. Based upon duration of breastfeeding, the samples were divided into five categories. Thirteen potential confounders were included as covariates: Parental social status and education, single mother status, mother's height, age, weight gain during pregnancy, and cigarette consumption during the third trimester, number of pregnancies, estimated gestational age, birth weight, birth length, and indexes of pregnancy and delivery complications.
Results: Duration of breastfeeding was associated with significantly higher scores on both the verbal and performance parts of the WAIS. With regression adjustment for potential confounding factors, the full scale IQs were 99.4, 101.7, 102.3, 106.0, and 104.0 for breastfeeding durations of < or = 1 month, 2-3 months, 4-6 months, 7-9 months, and > 9 months (p = 0.003). The corresponding mean scores for the BPP were 38.0, 39.2, 39.9, 40.1, and 40.1 (p = 0.01). Thus, the mean test scores suggested a dose-response relationship for breastfeeding during the first nine months of life and adult intelligence.
Discussion: Independent of a wide range of possible confounding factors, a significant positive association between duration of breastfeeding and intelligence was observed in two independent samples of young adults, assessed with two different intelligence tests. Although duration of breastfeeding may correlate with maternal intelligence and with the quality of mother-child interaction, these findings suggest that nutrients in breastmilk may have long-term positive effects on cognitive and intellectual development.