Evidence that breastmilk helps determine our behavior.

mother's milk knows bestDr. Katie Hinde, professor of human evolutionary biology at Harvard University, is no stranger to the benefits of breastfeeding.  “The milk that a mammalian mother produces for her young is a complete and comprehensive diet,” Hinde, and her coauthor, J Bruce German, stated in their 2012 paper on breastmilk, a view supported by the recent return to exclusive breastfeeding by many Western mothers.  But Hinde’s 2013 study on lactation and infant behavior provides compelling evidence that breastmilk plays an even greater role in human development.

Hinde analyzed the amounts of fats, proteins, vitamins, sugars, minerals, and hormones in the milk of different species of mammals in order to determine its impact on the baby’s physical and psychological development.  These nutrition levels- referred to as energetic density- varied drastically by individual, but one trend remained constant.  In both human and rhesus monkeys, the breastmilk produced for a male offspring was consistently different than milk produced for a female child.  But why, exactly, does this difference occur, and what does it mean for the neonatal community?

Is Milk Smart Enough to Have “Personality?”

Scientists have already established that each mammalian species produces a unique blend of nutrients that reflect and cater to the needs of their environments and the development of their offspring.  Animals with a short infancy period, such as Hooded seals, create breastmilk with high fat content in order to transfer the most energy in a short amount of time.  Conversely, primates like chimps and humans produce more diluted milk, since prolonged infancy means they must ration enough energy to lactate over a much longer period of time.  But more recently, Hinde tells us, researchers proved that “milk composition varies [not only] among species, it also varies among mothers within species.  In this way, infants receive ‘personalized’ milk from their mother.”

But this designer combination of calories and antibodies only explains half the puzzle.  Like so many things in life, timing is everything.  The gut-to-brain pathway is sensitive to hormonal and nutrient signals, particularly during the period of accelerated neurodevelopment during an infant’s first year, when babies are- you guessed it- breastfeeding.  The milk that babies ingest helps establishes the makeup of their gut bacteria, a finding corroborated by Dr. William Parker, a researcher at Duke University Medical Center.  In their 2012 study, Parker’s team found that infants given breastmilk instead of formula had faster immune response against E. coli infection, all thanks to their well-developed microbiotica.  These healthier gut flora, in turn, influence neurobiological functioning, as the baby’s brain is rewarded when ingesting carbohydrate-heavy breastmilk.

But not every newborn is rewarded in the same way.  Baby female rhesus monkeys reacted most positively to being fed calcium rich, but less fatty milk, over a longer period of time, while baby males caused their mothers to produce fattier, protein rich milk but give fewer feedings.  The differences in milk makeup seem to play to the unique growth needs of males versus females.  This highly segmented system of reward and healthy growth not only fosters more successful offspring, but may actually affect an infant’s temperament for the rest of its life.

Is it Really True that Mother’s Milk Knows Best?

So what does all this mean in the world of infant care?  Scientists aren’t sure, but if definitive lines can be drawn between breastmilk and baby behavior, doctors may be able to provide more targeted, personalized treatment to sick or underdeveloped preemies by synthesizing “matching” microbiotic formulas when breastfeeding isn’t an option.  If we stop treating breastmilk as a product of the mother, and instead see it, as Hinde writes, as an integrated “behavioral negotiation between the mother and infant,” we may come closer to understanding this perfect form of nutrition.  In Hinde’s opinion, mother’s milk knows best.

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