"Breastfeed your baby till the age of one to boost your child's IQ" is the advice in the Metro.
The story comes from a study looking at the links between breastfeeding and children's later mental abilities. It found that children whose mothers breastfed for longer did better in language tests at age three and IQ tests at seven than children who were weaned earlier.
Breastfeeding is known to have many benefits, such as reducing the risk of developing ear infections, chest infections and constipation, as well as helping to build a strong bond between mother and baby.
However, this study does not prove that breastfeeding can make a child more intelligent – it only highlights a possible association. As the authors acknowledge, there may be other factors associated with breastfeeding that influence a child's IQ, such as the home environment and maternal intelligence and education.
Previous studies found that in developed nations, mothers who choose to breastfeed tend to be from the middle or upper classes. It could be socioeconomic factors, rather than breastfeeding, that may explain the influence on IQ.
The researchers attempted to adjust their findings for these other influencing factors (confounders), but any adjustment is a statistical best guess. It is always possible that these or other unmeasured factors could have influenced the results.
Aside from these limitations, breastfeeding (when possible) is the healthiest way to feed a baby. The Department of Health currently recommends breastfeeding exclusively for six months and continuing for at least one year. For more information, visit the NHS Choices breastfeeding pages.
Where did the story come from?
The study was carried out by researchers from Harvard Medical School and Harvard School of Public Health in the US and was funded by the US National Institutes of Health.
It was published in the peer-reviewed Journal of the American Medical Association Pediatrics.
The study got a big splash on the Mail Online website, accompanied by large photos of the Duchess of Cambridge, who has reportedly decided to breastfeed.
But the website's coverage of the study was misleading and potentially harmful. It claimed that British "experts" warned that delaying the introduction of solid foods for six months might leave some babies hungry, a theory that is not backed by the evidence.
It was also reported that breastfeeding exclusively for six months might put babies at risk of allergies, food aversions and obesity. These claims fly in the face of the established evidence.
However, the Metro's summary of the study, although brief, is more accurate.
What kind of research was this?
This was a prospective cohort study that looked at the association between breastfeeding and children's mental abilities at three and seven years of age. It also examined whether the mother's intake of fish during breastfeeding had any effect on this association.
The authors point out that while some studies have reported an association between breastfeeding and later intelligence, the link is still uncertain.
They also say that nutrients in breast milk, such as n-fatty acid docosahexaenoic acid (DHA), may benefit the developing brain. Breast milk DHA content is said to be determined by the mother's diet, which is in turn determined by fish intake.
The main limitation of such a cohort is being able to take account of all possible health, lifestyle and environmental factors that may be associated with both a parent's decision to breastfeed and the child's future mental ability.
What did the research involve?
The authors used data from a large study of pregnant women in the US designed to examine factors related to pregnancy and child health. The study recruited pregnant women attending antenatal care between 1999 and 2002.
The women were first followed up after they had given birth and then when their child reached six months, three years and seven years of age. The sample used for this analysis involved 1,312 mothers and children with complete data available on breastfeeding and child mental ability, out of a total of 2,128 women who delivered a live baby.
When their child was six months, each mother was asked whether they had ever breastfed the baby and whether they were now giving them infant formula or breast milk. The mothers of babies who had been weaned by six months were asked how old the baby was when breastfeeding stopped.
At 12 months, mothers were asked whether they had ever breastfed the child and whether they were still breastfeeding. For weaned babies, mothers were asked how old the baby was when breastfeeding stopped.
When the children were three years old, they were given an established vocabulary test (the Peabody Picture Vocabulary test) and tests of their hand/eye co-ordination.
At the age of seven, the children were again tested on hand-eye co-ordination through a test of their drawing skills. They were also tested for memory and learning skills.
Staff administering the tests were unaware of the children's breastfeeding status to reduce the risk of any bias.
The researchers collected data from mothers on their social and economic background and their health. When the babies were six months, mothers were also given a validated food frequency questionnaire, which included questions about the mother's average weekly fish intake (canned tuna, shellfish, oily fish and other fish, such as cod, haddock and halibut). Maternal intelligence was also measured using vocabulary and intelligence tests.
The researchers used other established tests to measure mental stimulation and emotional support in the child's environment.
They looked specifically at:
- duration of any breastfeeding in months
- duration of exclusive breastfeeding in months
- breastfeeding status at six months (categorised as formula only, never breastfed, mixed formula and breast milk, and breast milk only, no formula)
The researchers analysed whether the mother's duration of breastfeeding and whether she had exclusively breastfed was associated with the results of the children's mental ability tests.
They developed various models that adjusted their results for other factors that might have had an influence, including:
- the child's birth weight
- maternal age
- maternal smoking status
- maternal depression at six months
- child care
- household income
- parental education
They also took account of the mother's scores on intelligence tests and the tests on the home environment.
To look at the possible role of maternal fish intake, the researchers stratified their results according to whether mothers reported two or more servings or less than two servings of fish a week.
What were the basic results?
The researchers found that the children of mothers who breastfed for longer:
- scored higher on the vocabulary test at age three (0.21 points, 95% confidence interval [CI] 0.03 to 0.38 points per month breastfed)
- scored higher on the intelligence test at age seven (0.35, 95% CI 0.16 to 0.53 verbal points per month breastfed; and 0.29, 95% CI 0.05 to 0.54 non-verbal points per month breastfed)
Length of time breastfeeding was not associated with the test of drawing skills or of memory and learning at age seven. It was also not associated with hand-eye co-ordination at the ages of three and seven.
On sub-analysis, the researchers did observe a trend for an effect of breastfeeding for women who consumed two or more servings of fish per week, but the effect fell short of statistical significance.
How did the researchers interpret the results?
The researchers conclude that longer duration of breastfeeding and greater exclusivity of breastfeeding were associated with better language skills at three years and higher IQ results at the age of seven.
They say their results are consistent with previous research and that their findings support current recommendations to promote exclusive breastfeeding of children up to the age of six months and the continuation of breastfeeding to at least one year.
This study had several strengths, including its large sample size and detailed measurements of how long mothers breastfed and breastfed exclusively.
However, as the researchers acknowledge, in this type of study there is always a risk that confounders – both measured and unmeasured – can influence the results.
The researchers did take account of various possible confounders, including the home environment and maternal IQ. But there remains the possibility that other health, lifestyle and environmental factors could influence both the parents' decision to breastfeed and the child's future mental ability.
The researchers only followed up a subset of the study's original cohort who had complete data available on their breastfeeding status and the child's mental ability test results. Those with this information available tended to be of higher socioeconomic status and less likely to be of minority ethnicity, which means that the results may not be generalisable to all groups.
There was also the potential for bias when mothers reported information about breastfeeding. It is possible that in giving their answers, mothers felt they should give a "correct" rather than an accurate response, and estimated a longer duration of breastfeeding than actually took place.
The score differences seen in vocabulary and intelligence tests were also very small. It is not known whether these had any meaningful difference in terms of the child's everyday life and academic ability.
While this study cannot answer with certainty whether breastfeeding has beneficial effects on a child's intelligence, the benefits of breastfeeding for both baby and mother are well established.