Breast-feeding, breast-milk feeding, and intelligence quotient

32 Love Walk, London SE5 8AD, United Kingdom

Dear Sir:

Anderson et al (1) performed a valuable review of infant feeding studies, showing a consistent 3-point higher intelligence quotient (IQ) in the breast-fed infants. The authors presumed that this was due to breast-milk nutrients, ignoring the theory that adversities associated with infant feeding depend not on what is fed, but on how it is fed. Maternal stress or time shortages predispose to supine feeding (eg, propped bottles and prolonged nocturnal breast-feeding), leading to positional latent otitis media and secondary aerodigestive tract infections (2). Otitis media can produce temporary deafness, a low verbal IQ, and behavior problems. Feeding difficulties, such as those in premature infants (3) and twins, maximize IQ deficits. Of the formula-fed infants, those with low birth weight had an IQ that was 5.2 points lower and those of normal weight had an IQ that was 2.7 points lower than that of the weight-matched, breast-fed infants; twins had a low verbal IQ (4). Partially breast-fed children had more infections as infants, worse behavioral problems, and lower educational achievements than did fully breast-fed infants with similar socioeconomic backgrounds (3). Full control for maternal behavior and infections in infants probably would eliminate the higher IQs observed in breast-fed than in formula-fed infants. Anderson et al's meta-analysis was incomplete or misleading for the following reasons:

1. The higher IQ in the breast-fed infants vanished after correction for parenting skills (not social class or education) (5). Bottle-fed infants with excellent home care do not have excessive infections (3). Breast-feeding did not result in higher cognitive scores than did standard or fortified formula feeding when it did not reduce infections (6).
   
2. Many studies show lower verbal than nonverbal IQs or lower mental than psychomotor quotients in nonbreast-fed infants (6–13). This is consistent with infant deafness, not with developmental brain disorders, which are associated with low global or nonverbal IQs, or both. Hence, verbal and performance IQs should be analyzed separately.
   
3. Anderson et al state that the observed dose effect (an increasing benefit with an increasing duration of breast-feeding) suggests specific advantage of breast milk. In contradiction, there was no IQ benefit from the first 2 mo of breast-feeding. This is inconsistent with an expected immediate boost from breast milk, but supports a delayed deficit as infections develop in the artificially fed. In a study by McMichael et al (12), there was only a 1-point difference in the verbal IQ in those breast-fed from 1 to 6 mo, but a 6-point advantage in those breast-fed for >6 mo compared with fully bottle-fed infants.
   
4. Anderson et al ignored the disadvantages of breast-feeding beyond 12 mo. Soranus (14), 1900 y ago, condemned night feeding and Erasmus Darwin (15) said that infants artificially fed, while lying on their backs got diarrhea. "Nursing bottle" caries occurs in children fed from propped bottles, and prolonged breast-feeding in bed leads to tooth decay from dependent fluid pooling. A large body of literature clearly shows that primennial breast-feeding reduces infections and malnutrition, whereas during the second year it predisposes to malnutrition (16), and, presumably, to a low IQ. Supine bed feeding is its only plausible cause. Anderson et al stated, "Greater duration of breast-feeding may enhance its effect on cognitive development," citing the comprehensive study of Hoefer and Hardy (17), who actually found a large drop in IQ in infants breast-fed beyond 10 mo of age. There was also considerably more breast-feeding in Terman's gifted cohort (18) than in control subjects, but only from 2 to 12 mo and not after 12 mo. Even in this sample of infants with a very high IQ, exclusive bottle feeding was twice as common in those with IQs <160.
   
5. Breast-feeding was confounded with breast-milk feeding. In very-low-birth-weight children, those who were directly breast-fed had a 5-point higher IQ than did the non-breast-fed children, whereas those who were fed expressed breast milk had a 2-point lower IQ (19).
   
6. Why was the study by Taylor and Wadsworth (20) omitted from Figure 1 of Anderson et al's study, which depicted covariate-adjusted differences for the matched composite observations? This study of a UK cohort had the largest sample of the studies reviewed.
   
7. Some studies (3, 5, 6, 12, 13, 18) were not reviewed by Anderson et al. An update including an analysis of all relevant studies conducted over the past century would greatly improve the review.
   

There is a great problem in the literature with unreported data sets. Thus, the first report (1978) from the excellent Dunedin cohort (4) stated that there was no feeding effect on cognition, but it gave no raw data to support this finding. The second report (1982) stated that there was a feeding effect, but still omitted separate verbal and performance IQs. It would be surprising if verbal IQs were not lower than performance IQs, given that in this cohort male sex and no breast-feeding were risk factors for otitis media, which in turn predicted poor verbal IQs, reading, and behavior at age 13 y.

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