The results of this study from India support the null hypothesis, that there was no association between either duration of breast-feeding, or age at introduction of complementary foods, and cognitive abilities in 9-10 year old South Indian children.
Strengths of the study were that in a large sample of children, we had a battery of cognitive function tests specifically adapted for, and validated in, a South Indian population and collected data on a variety of potential confounders including birthweight, gestational age, maternal age, parity, height and BMI at pregnancy, SES, parents’ education and rural/urban residence. Breast-feeding duration was obtained by maternal recall at one year for 94% of children (of whom more than 60% of the children were still breast-fed making our data reasonably accurate) and for the remaining at two and three years of follow-up. Limitations are that the dataset was relatively homogeneous in terms of breast-feeding (majority of the children were breast-fed for a year or more) and we were unable to differentiate between exclusive and partial breast-feeding, and we did not have information on frequency of breast-feeding, the nutritional quality of breast-milk, or the type and nutritional quality of complementary foods.
To our knowledge, only two studies from developing countries have examined cognitive performance in relation to breast-feeding. One study in the Philippines reported that longer duration (12 months+) of breast-feeding was associated with higher cognitive performance among low-birthweight children aged 8 and 11 years.[6
] Another study (in Chile) found a non-linear association, with higher cognitive abilities in 5½ year old children exclusively breast-fed for 2-8 months compared to those breast-fed for <2 months or >8 months.[9
] Many observational studies in high-income countries have shown higher cognitive performance among children[4
] and adults[13
] who were breast-fed compared to those not breast-fed, and/or breast-fed for a longer duration. Adjusting for SES, parental education, intelligence and other confounders these associations lost their significance in some studies,[16
] while in some other studies tended to diminish although they remained significant.[7
] It is therefore unclear whether there is a genuine biological effect of breast-feeding upon cognitive development, or whether the association results entirely from confounding, and remains in the latter studies only because of an inability to measure and adjust for all the relevant confounding factors. One possible explanation for the negative findings in our study is that SES was only weakly related to breast-feeding duration in Mysore and confounding was not an issue, thus revealing a genuine lack of effect of breast-feeding on cognitive ability. An alternative explanation is that we failed to detect an effect because of a lack of heterogeneity in breast-feeding duration in our population. A striking difference between the studies in high-income countries and ours was that most infants in the former stopped breast-feeding <6 months,[7
] and ‘longer duration’ could mean anything from 2+ to 8+ months; few of our children were breast-fed for as short a duration as this and 65% were breast-fed for 12 months or more. If the first 6 months of life is a critical period in which breast-feeding can influence cognition, we may have had inadequate power to detect this because almost all our children were breast-fed during that time. It is possible that the nutritional quality of breast-milk is important in this context. We have no data on the docosahexanoic (DHA) and arachidonic acid (AA) (fatty acids important for infant brain development [1
]) concentrations in our study. Studies in India reporting fatty acid composition in breast-milk are scarce and only one recent study reported that milk DHA levels in Indian women (consuming predominantly vegetarian diet) were similar to milk DHA levels reported from Western and European women. Levels of plasma DHA and AA in Indian women were lower compared to the levels in American and European women. Maternal plasma omega-3 and omega-6 fatty acids levels were positively associated with their respective levels in milk though there was no direct association between maternal plasma and milk DHA or AA levels.[35
] The study suggest that levels of LCPUFA vary between different populations and may be dependent on their dietary intakes. Low levels of these or other nutrients in breast-milk could be another explanation for our negative findings.
Randomized intervention studies are a way of overcoming confounding, but it is impractical to randomise healthy babies to be breast-fed or formula-fed. In a large randomised trial of a breast-feeding promotion programme in Belarus, which led to significant differences in breast-feeding initiation and continuation between intervention and control groups, children in the experimental group had higher test scores of intelligence and teachers’ academic ratings compared to the control group at 6.5 years.[15
] However, this study was criticised[36
] because the paediatricians who administered the cognitive tests were not blind to the intervention status of the children, although the teacher ratings and results based on audit data were blinded. In another trial among preterm infants, 8 year old children who were breast-fed during infancy had higher test scores for IQ than formula-fed children.[37
] However, this trial involved non-randomized comparisons between breast milk fed and formula-fed infants, which might be biased by the socio-biological differences between these groups. The same authors, in a subsequent randomized multicentre study of preterm infants, reported no differences in psychomotor and developmental indices at 18 months between those receiving donor’s breast milk and those fed on nutrient enriched preterm formula.[23
] They concluded that considering the lower nutrient value of donor’s breast milk their data add significant support to the opinion that breast milk promotes neurodevelopment.
We found no association between the age at introduction of complementary foods and cognitive performance. Two studies have investigated associations between cognitive function and diet quality in infancy based on dietary history collected from/or after 4 months.[38
] One study reported that meat consumption from 4-12 and 4-16 months was positively associated with children’s psychomotor but not mental developmental indices up to 24 months of age.[38
] The other study reported that 4 year old children who consumed more fruits, vegetables and home-prepared foods at infancy (6 and 12 months) had higher full-scale and verbal intelligence scores, independent of confounding variables.[39
] Infancy is a period of rapid brain growth, and it seems likely that nutritive quality of complementary foods, including their timely introduction, is important for cognitive development. More research is required in this area.
To conclude, within this cohort, most of whom were breast-fed for >12 months, we found no evidence that longer duration of breast-feeding promotes cognitive development. We highlight the fact that prolonged breast-feeding was the norm among these children, and since brain development is most rapid in the first 6 post-natal months, it may be that breast-feeding at this time is beneficial but that we were unable to detect an effect since almost all children were breast-fed throughout that period. In support of this, cognitive scores tended to rise across the first 3 categories of duration of breast-feeding (although none of the trends was significant) suggesting that longer breast-feeding up to the age of 8 months may benefit cognitive development. There was no evidence of an association between the age at introduction of complementary foods and cognitive performance. Our study adds to a very small literature on this topic from developing countries. Despite the negative findings in relation to cognitive function, we strongly support WHO guidelines on infant feeding practices (exclusive breast-feeding for 6 months, introduction of nutritious complementary foods from 6 months, and continued breast-feeding up to 2 years), which have been clearly shown to reduce infant infections and mortality, and prevent stunting, in developing country populations.[40