The study suggests that breastfeeding can lead to benefits in adult life such as increased educational attainment and income.
The study, published in The Lancet Global Health, followed 3,493 infants born in Pelotas, Brazil. After an average of 30 years, the researchers measured their IQs and collected further information about their educational achievement and income.
"The effect of breastfeeding on brain development and child intelligence is well established, but whether these effects persist into adulthood is less clear," says lead author Dr. Bernardo Lessa Horta of Federal University of Pelotas in Brazil.
"Our study provides the first evidence that prolonged breastfeeding not only increases intelligence until at least the age of 30 years but also has an impact both at an individual and societal level by improving educational attainment and earning ability."
In the short-term, breastfeeding is known to reduce the prevalence of infectious diseases and mortality from them among infants. Exclusive breastfeeding is commonly recommended for the first 6 months after birth, to be continued alongside solid foods until at least the age of 1 year.
The Mayo Clinic describe breast milk as "the gold standard for infant nutrition" as it contains the right balance of nutrients for the baby while boosting its immune system.
Many previous observational studies involving breastfeeding have been limited due to social patterning. Mothers that breastfeed for longer durations have typically held high socioeconomic positions, and their improved access to health care may lead to overestimating the health benefits of breastfeeding.
While previous studies have been criticized for this potential confounding factor, the authors of the new study address this issue.
"What is unique about this study is the fact that, in the population we studied, breastfeeding was not more common among highly educated, high-income women, but was evenly distributed by social class," Dr. Horta explains.
Results also suggest the amount of milk consumed could play a role
For the study, the subjects were divided into five groups based on the amount of time they were breastfed for as infants. The participants were also controlled for 10 variables that may have contributed to increases in IQ, such as family income at birth, maternal age and parental schooling.
The researchers found that breastfeeding led to increases in adult intelligence, longer schooling and higher adult earnings, but also that the magnitude of the benefits was greater the longer a child was breastfed for, up to 12 months.
Compared with infants who were breastfed for less than one month, infants breastfed for 12 months had four more IQ points, 0.9 years more schooling and earned $104 per month more on average.
Dr. Horta believes there is a biological mechanism for the study's findings:
"The likely mechanism underlying the beneficial effects of breast milk on intelligence is the presence of long-chain saturated fatty acids (DHAs) found in breast milk, which are essential for brain development. Our finding that predominant breastfeeding is positively related to IQ in adulthood also suggests that the amount of milk consumed plays a role."
Although the researchers did not measure the characteristics of the infants' home environment or maternal-infant bonding, the researchers state that previous research suggests breastfed subjects have demonstrated improved cognitive functioning even after controlling for home environment and stimulation.
"Our results suggest that breastfeeding not only improves intelligence up to adulthood, but also has an effect at both the individual and societal level, by increasing educational attainment and earning ability," the authors conclude.
In contrast to the new research, Medical News Today reported on a study last year that suggested that breastfeeding may be no more beneficial than bottle-feeding for many long-term health outcomes.