Last summer Angela Garbes interviewed me for an incredible essay "The More I Learn About Breast Milk, The More Amazed I Am." A subject we discussed was one of those things you can't unlearn: Baby Spit Backwash. During the interview, I emphasized that the specific "baby saliva triggers immunofactors to increase in breastmilk" remains a hypothesis. But key links in this pathway have been empirically demonstrated:
1) Moms increase the concentration of some immunofactors in breastmilk when babies are sick (but moms are not) (Hassiotou et al. 2013; Breakey et al. 2015).
2) When babies suckle, nipple diameter increases and there is a vacuum with negative pressure, delivering fluids from the infant oral cavity— a cocktail of milk and saliva— back into the ducts of the breast. For the record the technical term for "baby spit backwash" is "retrograde milk flow" (Geddes et al. 2008; Geddes 2009; Geddes et al. 2012; Ramsey et al. 2004).
3) Milk is alive— with immune cells, stem cells, and microbes of maternal and infant origin (Hassiotou et al. 2013, Funkhouser and Bordenstein 2013)
Moreover, what we know about mammary gland anatomy & physiology and immune response indicates that this "baby saliva stimulates immune response" is not so far-fetched.
Depending on the illness, babies shed pathogens from mucus membranes— upper respiratory nasties hitching a ride through runny noses and coughing mouths— mixing together in saliva and snot as babies latch on to suckle at the breast. These naso-oral secretions from the baby backwash into the nipple duct system, bringing the pathogens with them.
And a fundamental component of the appropriately functioning immune system is that exposure to pathogens triggers immune responses. As any mother who has tangled with mastitis can tell you, immune response in the mammary gland is hecka important. And natural selection may have favored an increase in immune molecules in the mammary & milk because they functioned to protect the mammary gland, protect the baby, or both the mammary gland AND the baby.
Mother's milk is chock-a-block packed with immune molecules. Babies rely on the immunological umbrella of their mother's breast milk while the baby's own immune system is naive and developing- this is what makes milk not just food, but
But get ready to be even more blown away!
When breast milk mixes with baby saliva a chemical reaction happens that produces hydrogen peroxide. Yep- the combination of breast milk with baby saliva produces a strong enough reaction to "inhibit growth of the opportunistic pathogens Staphylococcus aureus and Salmonella spp" (Al-Shehriet al. 2015) while also promoting the growth of beneficial bacteria. Does adult saliva mixed with breast milk cause this reaction?
just baby spit.
This is just one of the myriad ways that "breastfeeding is a dynamic biological process, not simply a meal at the breast" (Raju 2011). So move over peanut butter chocolate cups, breast milk and baby spit is the real together at last!