Doctors monitored 119 premature babies through their first year of life and found that girls who received formula were eight times more likely to be hospitalised with respiratory illnesses than those weaned on breast milk.
But breastfeeding did not have the same protective effect on boys. Nearly a fifth of boys developed respiratory infections severe enough to need hospital treatment, whether they were breastfed or not.
The study is the first to show gender differences in the health benefits of breastfeeding and overturns the previously held belief that breast milk contains protective chemicals from the mother’s immune system that pass directly to her baby. Were that the case, both boys and girls would benefit equally, the researchers write in the journal Pediatrics.
Dr Fernando Polack, an infectious disease specialist at Johns Hopkins children’s centre in Baltimore, said breastfeeding did not affect the number of infections baby girls caught, but reduced their severity, meaning breast milk helped them to cope with infections better.
In the study, the team investigated how babies fared after picking up their first respiratory infection. Only 6% of breastfed girls were hospitalised by their chest infection, compared with half of the girls who were weaned on formula. There was almost no difference between boys, with 18% breastfed and formula-fed boys hospitalised by their first chest infection.
The protective benefit of breastfeeding remained the same as babies picked up more infections throughout their first 12 months of life.
The findings are particularly relevant to families in developing countries where antibiotics and other drugs may be scarce and where an estimated quarter of premature babies are hospitalised with serious chest infections.
Dr Polack said the findings suggested that mothers of premature girls should be strongly urged to breastfeed their infants.