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, which I suspect is the only place on the Internet where you can fill out a Mammal Madness bracket.) “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—because conversations about lactation always seem to require disclaimers, especially if you happen to be someone who will never ever lactate. (I’m pretty sure.) 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 isn’t modern or 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 × 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.
For example, she notes, in humans skin-to-skin contact appears to trigger signals that are sent through the milk. “If the infant is showing signs of infection, somehow that’s being signaled back to the mother and she up-regulates the immune factors that are in her milk. Now is that her body’s responding to a need of the baby? Maybe. Is it that she also has a low-grade infection that she’s just not symptomatic for and so her body’s doing that? Maybe. Is it partially both? Maybe. We don’t know. It’s brand-new stuff.”
The new awareness of this sort of signaling is why there’s been a paradigm shift in the study of milk. Scientists have gone from seeing it only as food to seeing it far more expansively—as a highly sensitive variable that plays a wide range of developmental roles.
This new perspective should change how we look at formula, too, Hinde says. Instead of comparing breast milk and formula, we should accept how little we actually know about breast milk. “We need to go back to square one and look at all the variation in breast milk and where it’s coming from and what it does,” she says. “Because how could we possibly know what the difference between breast milk and formula is if we aren’t even keeping track of what the variation in breast milk is doing? And so the more that we understand about what is in milk, and what predicts how it varies, the more opportunity there is for formula to better emulate what breast milk is.”
Almost 150 years after the first infant formula, the splendidly named Liebig’s Soluble Food for Babies, was proclaimed to be “virtually identical” to human milk, we now know how much we don’t know about milk. It’s a deeply intimate mystery. And the scientists who study it are a lot like almost any parent gazing down at their sucking child: They too are full of wonder.