Danish and American researchers have found that babies who nurse longer tend to score slightly but significantly higher on IQ exams as adults. The effect is strongest for those who breast-feed for between seven and nine months; after that it appears to fade.
"It's not the difference between Einstein and a mentally handicapped child, but it could make quite a difference," says June Machover Reinisch, director emerita of Indiana University's Kinsey Institute and co-author of the study. The work appears in tomorrow's issue of the Journal of the American Medical Association.
Breast milk is known to help babies build defenses against infections and allergies, and even fend off chronic diseases like diabetes and cancers later in life. Human milk is stocked with antibodies -- immune system proteins -- that infant formula doesn't have.
It's also rich in cholesterol and fatty acids, especially two molecules, docosahexaenoic acid (DHA) and arachidonic acid (AA), key players in healthy brain and eye development.
Two of the nation's largest formula makers, Ross Products and Mead Johnson Nutritionals, acknowledging the advantages of these compounds, announced recently they would begin adding them to select brands.
In the latest work, scientists in Copenhagen and at the Kinsey Institute compared duration of breast-feeding with scores on two standardized adult intelligence tests in 973 men and women and another 2,280 men. All of the subjects were born in Denmark between 1959 and 1961, putting them in their early 40s now.
One of the IQ tests was a one-on-one exam measuring a wide range of aptitudes. The other was a more brute force exam taken by all new recruits in the Danish armed forces.
Duration of breast-feeding was assessed for a year after birth, and broken into five categories: less than a month; two to three months; four to six months; seven to nine months; and longer than nine months.
The researchers tried to account for more than a dozen factors known to influence intelligence. These included parental behaviors and traits such as a mother's age at delivery, her social status, her history of smoking, and the education level of the family breadwinner. They also considered variables specific to the babies themselves: their length and weight at birth and whether they were premature.
Yet even after controlling for these factors, nursing longer clearly predicted higher IQ in early adulthood. The difference was greatest on the civilian test -- considered a more sensitive and inclusive tool -- reaching nearly six points, on average, for those who nursed seven to nine months compared with the briefest nursers (99.4 versus 105). It was consistent across both exams.
"The trend is very clear and linear. The more [breast milk] the better, up to nine months," Reinisch says. "At that point, what's probably the case is that whatever the factor is that is enhancing neural development has done its work."
In addition to containing nutrients critical for brain growth, breast-feeding could boost intelligence in other ways, too. Women who nurse may interact more with their babies during the first year of life. Similarly, mothers who prolong breast-feeding may be more intensely invested in their children throughout their childhood and beyond, the researchers suggest. Some evidence ties family environment to intellectual development, though studies are mixed, the researchers say.
The U.S. Surgeon General would like to see that 75 percent of women start breast-feeding upon giving birth, and that 50 percent continue to do so six months later.
However, Carol Huotari, manager of the center for breast-feeding information at La Leche League, says formula industry figures show that about 68 percent began breast-feeding at childbirth, and only 31 percent of women stuck with nursing six months later.
Huotari says $3.6 billion in health-care costs could be saved each year -- largely through the prevention of ear infections and stomach ailments -- if the government's guidelines could be achieved.
What's more, she says, breast-feeding carries both emotional and health benefits for mothers, providing not only a sense of fulfillment and achievement but some protection against osteoporosis and certain cancers.
"It's not just a matter of benefit to the baby," she says. "It's a benefit to the mother; it's a benefit to the whole family."