A number of studies suggest a positive association between breastfeeding and cognitive
development in early and middle childhood. However, the only previous study that investigated
the relationship between breastfeeding and intelligence in adults had several methodological
shortcomings.
To determine the association between duration of infant breastfeeding and intelligence
in young adulthood.
Prospective longitudinal birth cohort study conducted in a sample of 973 men and women
and a sample of 2280 men, all of whom were born in Copenhagen, Denmark, between October
1959 and December 1961. The samples were divided into 5 categories based on duration
of breastfeeding, as assessed by physician interview with mothers at a 1-year examination.
Intelligence, assessed using the Wechsler Adult Intelligence Scale (WAIS) at a mean
age of 27.2 years in the mixed-sex sample and the Børge Priens Prøve (BPP) test at
a mean age of 18.7 years in the all-male sample. Thirteen potential confounders were
included as covariates: parental social status and education; single mother status;
mother's height, age, and 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.
Duration of breastfeeding was associated with significantly higher scores on the Verbal,
Performance, and Full Scale WAIS IQs. With regression adjustment for potential confounding
factors, the mean Full Scale WAIS IQs were 99.4, 101.7, 102.3, 106.0, and 104.0 for
breastfeeding durations of less than 1 month, 2 to 3 months, 4 to 6 months, 7 to 9
months, and more than 9 months, respectively (P =.003 for overall F test). The corresponding
mean scores on the BPP were 38.0, 39.2, 39.9, 40.1, and 40.1 (P =.01 for overall F
test).
Independent of a wide range of possible confounding factors, a significant positive
association between duration of breastfeeding and intelligence was observed in 2 independent
samples of young adults, assessed with 2 different intelligence tests.