Researchers have long known that breast-feeding protects against infant respiratory infections. But a new study hints that, at least with premature babies, this protective effect may be more pronounced in girls than boys.

For the study, published 2 June, scientists followed 119 premature babies in Buenos Aires for their first year of life. An analysis of the results revealed that half of the formula-fed girls needed hospitalization for their first respiratory infection after leaving the intensive care unit. By comparison, only about 6% of breast-fed girls required such treatment for respiratory infection. There was no difference between breast-fed and formula-fed boys: both groups had an 19% rate of hospitalization for respiratory infection (Pediatrics, doi:10.1542/peds.2007-1757; 2008).

Many scientists had thought that breast milk encourages similar immune protection in male and female infants thanks to immunoglobulin A, a type of maternal antibody that it contains. “But if that were the case, then there would be no reason to see a gender difference because everybody would be protected equally,” says Fernando Polack, an immunologist at the Johns Hopkins Bloomberg School of Public Health in Baltimore, who led the new study.

Instead, Polack suggests that breast-feeding activates some kind of protective immune system response in infants, one that is apparently more easily activated in girls than boys.

Because this sex difference has never been documented before, other researchers remain skeptical. Epidemiologist Julius Atashili of the University of North Carolina at Chapel Hill says that the sample size of the study was too small to make any definitive conclusions. Moreover, “there may be different reasons why the infants were very low birth weight to begin with,” he says.

Others note that only four of the 119 infants in the study were exclusively breast-fed. “They're combining all kinds of feeding and calling it 'breast-fed',” says Miriam Labbok, director of the University of North Carolina's Center for Infant and Young Child Feeding and Care.

Polack is currently looking for a biological mechanism behind this observed sex difference.