The secrets of breast milkProviding nutrients turns out to be only part of what milk does. And it might not even be the most important part.
By: Nicholas Day, Slate
CHICAGO — When we come out of the womb, we make our way to the breast. We enter the world knowing we’re mammals, with milk on our minds.
But even as grown-ups, we have never known exactly what’s in that milk — or, as strange as it may sound, what the point of it is. For decades, milk was thought of strictly in terms of nutrients, which makes sense — milk is how a mother feeds her baby, after all. But providing nutrients turns out to be only part of what milk does. And it might not even be the most important part.
“Mother’s milk is food; mother’s milk is medicine; and mother’s milk is signal,” says Katie Hinde , an assistant professor of human evolutionary biology at Harvard. (She also writes the fascinating blog Mammals Suck.) “When people find out I study milk, they automatically think we already know about it, or it’s not important. And I’m like, ‘No, we don’t know about it, and it’s super important.’”
But first, a disclaimer. In my new book “Baby Meets World,” I write about how, contrary to myth, not nursing has never been a death sentence. Hundreds of years before halfway-decent formula, infants were fed gruesome substitutes for breast milk (mushed bread and beer, say) — and although many more died than those who were nursed, many also survived. So the lesson of the new science of milk isn’t that formula is some sort of modern evil. It’s that milk is really complicated — and evolutionarily amazing.
Here’s how complicated: Some human milk oligosaccharides — simple sugar carbohydrates — were recently discovered to be indigestible by infants. When my son was nursing, those oligosaccharides weren’t meant for him. They were meant for bacteria in his gut, which thought they were delicious. My wife was, in a sense, nursing another species altogether, a species that had been evolutionarily selected to protect her child. (A relationship immortalized in the paper titled “Human Milk Oligosaccharides: Every Baby Needs a Sugar Mama.”) In effect, as Hinde and UC-Davis chemist Bruce German have written, “mothers are not just eating for two, they are actually eating for 2 times 1011 (their own intestinal microbiome as well as their infant’s)!” That’s what’s meant by milk serving as medicine, and that’s only scratching the surface.
But Hinde primarily studies the food and the signal elements of milk. “The signal is in the form of hormones that are exerting physiological effects in the infant,” she explains. “Infants have their own internal hormones, but they’re also getting hormones from their mother. They’re binding to receptors in the babies, and we’re just starting to understand what those effects are.”
Hinde works with rhesus macaques, and she’s tracked the effects of the hormone cortisol in their milk. Cortisol is often thought of as the stress hormone, but its function is far more varied, and Hinde has found that the amount and especially the variation of cortisol successfully predicts how the infant macaques go on to behave. It’s a stunning finding: The composition of early milk seems to mold infant temperament. But — and here’s the twist — the males were much more sensitive than the females. Roughly, the more cortisol, the more bold and exploratory the male rhesus macaques were.
Such sex-specific variations in milk, possibly “programmed” by the placenta during gestation, may be common. In humans, there’s early data suggesting that mothers produce fattier milk for boys than girls. But that may be only part of the story, as Hinde has found with rhesus macaques. “Just because sons are getting better milk doesn’t mean they’re getting more. It looks like they’re getting very similar total calories.” So why do sons get fattier milk? “In rhesus macaques, daughters stay in their social groups their whole lives,” Hinde notes. “They form a bond with their mother that only ends when one of them dies. So it might be that mothers are nursing their daughters more frequently and that helps establish this bond.” In contrast, the sons end up leaving the group — and fattier milk means they nurse less often, which means they can spend more time playing with strangers, developing skills they’ll need later in life. The milk, in other words, reflects and cements the social structure of rhesus macaques.
We think of milk as a static commodity, maybe because the milk we buy in the grocery store always looks the same. But scientists now believe that milk varies tremendously. It varies from mother to mother, and it varies within the milk of the same mother. That’s partly because the infants themselves can affect what’s in the milk. “Milk is this phenomenally difficult thing to study because mothers are not passive producers and babies are not passive consumers,” Hinde says. Instead, the composition of milk is a constant negotiation, subject to tiny variables.