Over the years, various studies have demonstrated the many advantages of breastfeeding, findings that have led the American Academy of Pediatrics to recommend that all mothers breastfeed their children for the first year of the child’s life. Several studies have also examined the relationship between breastfeeding and intellectual development in younger children and have demonstrated better cognitive functioning in children who were breastfed. The extent to which breastfeeding during infancy has an effect on later intellectual development has been more difficult to assess.

In a recent study from Denmark, a prospective longitudinal birth cohort study included a sample of 1575 adults born in Copenhagen, Denmark, between October 1959 and December 1961. The sample was divided into 5 groups based on duration of breastfeeding, as assessed by physician interview with the participants’ mothers one year after the child’s birth: (1) 1 month or less; (2) 2 to 3 months; (3) 4 to 6 months; (4) 7 to 9 months; and (5) more than 9 months. Intelligence was assessed using the Wechsler Adult Intelligence Scale (WAIS) in 973 subjects (490 males and 483 females), with a mean age of 27.2 years (range 20-34 years). In addition, the Børge Priens Prøve (BPP) was used in another sample of 602 men drafted into the Danish military (mean age at testing of 18.7 years). Thirteen potential confounders were included as covariates: parental social status and education; single mother status; mother’s height, age, and weight gain during pregnancy; cigarette consumption during the third trimester; number of pregnancies; estimated gestational age; birth weight; birth length; and indexes of pregnancy and delivery complications.

Longer duration of breastfeeding was significantly correlated with higher scores on the Verbal, Performance, and Full Scale WAIS IQs. Duration of breastfeeding was also positively correlated with birth weight and length and with mother’s age, social class, and education level and was negatively associated with being single and cigarette consumption during pregnancy. With regression adjustment for these 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 =0.003). Similar results were observed in the all-male sample using the BPP assessment of intelligence.

This study indicates a significant positive association between duration of breastfeeding and intelligence in two independent samples of young adults, assessed with 2 different intelligence tests. Exactly how breastfeeding may promote cognitive development and intelligence is not clear. It has been hypothesized that breast milk may contain special nutrients that foster brain development. Long-chain polyunsaturated fatty acids, especially docosahexaenoic acid (DHA), are present in human milk but not in infant formula or cow’s milk and are important constituents of cell membranes in the central nervous system. Several randomized controlled trials in infants have demonstrated improved mental development in those receiving a formula supplemented with DHA. Others have pointed to the importance of the increased physical contact and facilitated mother-infant bonding that stems from breastfeeding and their central role in promoting a child’s cognitive and emotional development. Duration of breastfeeding may also reflect a willingness and ability on the mother’s part to invest significant time and energy in her child, a pattern of behavior that is likely to persist throughout the child’s life and may have significant impact on the child’s later intellectual development.

Ruta Nonacs, MD PhD

Mortensen EL, Michaelsen KF, Sanders SA, Reinisch JM. The association between duration of breastfeeding and adult intelligence. JAMA. 287(18):2365-71, 2002.

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