According to the article in Nature, breast milk for boys has more fat and protein than breast milk for girls.
When asked about how such a difference could occur, coordinator of lactation services at the University of Washington Medical Service in Seattle, Ginna Wall said, “We know that boys grow faster than girls, and perhaps this is due to the milk, or the milk may be responding to the commands of the child.”
“It could be triggered by prenatal chemistry or by the infant's sucking style, although in real time they sure don't look different when they're nursing.”
While the reason may still be unclear, experts surmise the difference may be due to the reaction of the mother’s body to the way the baby sucks, so perhaps baby boys suck differently compared to baby girls, thought Barbara Holmes, a lactation specialist at the New York University Langone Medical Center.
SOURCES: • Denise Mann. December 23, 2010. “New Insight on Benefits of Breastfeeding” medscape.com • December 23, 2010. “Different breast milk for boys and girls, says University of Washington” examiner.com