“People tend to underestimate what milk is,” says Katie Hinde, a biologist and associate professor at the Center for Evolution and Medicine at the School of Human Evolution & Social Change at Arizona State University. She also runs the very funny, highly informative, and deeply nerdy blog Mammals Suck… Milk!
“That’s in part because you go to the store and there’s an entire aisle dedicated to buying milk that is literally a homogenized, standardized food. It leads us to take mother’s milk for granted.”
But right now, researchers like Hinde—a mix of evolutionary biologists, dairy scientists, microbiologists, anthropologists, and food chemists—are examining milk, and the more closely they look, the more complexities they find.
According to Hinde, when a baby suckles at its mother’s breast, a vacuum is created. Within that vacuum, the infant’s saliva is sucked back into the mother’s nipple, where receptors in her mammary gland read its signals. This “baby spit backwash,” as she delightfully describes it, contains information about the baby’s immune status. Everything scientists know about physiology indicates that baby spit backwash is one of the ways that breast milk adjusts its immunological composition. If the mammary gland receptors detect the presence of pathogens, they compel the mother’s body to produce antibodies to fight it, and those antibodies travel through breast milk back into the baby’s body, where they target the infection.