What is milk? Experts have lots of answers, none very clear; infant formula lacks what has evolved over millions of years

Explaining mammalian milk seems impossible.
(Photo by Marcel Christ, Gallery Stock via The Atlantic)

It's much easier to answer the question "Where's the beef?" than "What is milk?"

Milk is a mystery. "No one can really describe what milk is—least of all the people who think most often about it," reports Katherine J. Wu of The Atlantic. "They can describe, mostly, who makes it: mammals (though arguably also some other animals that feed their young secretions from their throat or skin). They can describe, mostly, where it comes from: mammary glands . . . They can even describe, mostly, what milk does: nourish, protect, and exchange chemical signals with infants to support development and growth."

“It’s the defining feature of mammals,” University of Washington anthropologist Melanie Martin told Wu, who begins her story this way: "If an alien life form landed on Earth tomorrow and called up some of the planet’s foremost experts on lactation, it would have a heck of a time figuring out what, exactly, humans and other mammals are feeding their kids," Wu writes. The composition of milk is elusive; from nutritionists and academics Wu got such vague descriptorsas "ecological system," "nutritional instrument, and "the result of the evolutionary selective pressure on a unique feeding strategy." 

 . . . None of these characterizations were bad. But had I been that alien, I would have no idea what these people were talking about."

Mammalian milk is unique, Wu writes:"Millions of years of evolutionary tinkering that have turned it into a diet, and a developmental stimulus, and a conduit for maternal-infant communication, and a passive vaccine. It builds organs, fine-tunes metabolism, and calibrates immunity. . . . Among some primates, it influences infants’ playfulness and may shape their sleep habits and bias them toward certain foods. Some of its ingredients are found nowhere else in nature; others are indigestible. Still, others are alive."

And few things are as adaptive as mammals' milk. "It remodels in the hours, days, weeks, and months after birth; it changes from the beginning of a single stint of feeding to the end," Wu reports. "Human milk—like other primate milk—is on the watery, sugary side. But its concentrations of immunity-promoting ingredients have no comparator. It bustles with defensive cells; it shuttles a stream of antibodies from mother to young, at levels that in some cases outstrip those of other great apes’ milk by a factor of at least 10. . . . The sheer defensive firepower in our species’ milk is probably a glimpse into the challenges in our past, as humans crowded together to plant, fertilize, and harvest mass quantities of food and invited domesticated creatures into our jam-packed homes."

We know all that, but "We don’t actually know that much about milk," such as its normal values in humans, E. A. Quinn, an anthropologist at Washington University, told Wu, who writes, "Milk’s enduring enigmas don’t just pose an academic puzzle. They also present a frustrating target—simultaneously hazy and mobile—for infant formulas that billions of people rely on as a supplement or substitute." Martin told her, "No human recipe can replicate what has evolved" over hundreds of millions of years.

Infant formula does "the nourish part really well. . . . The protect and communicate part is where we start to fall short,” University of Rochester infant-nutrition researcher Bridget Young told Wu, who writes: "Differences in health outcomes for breastfed and formula-fed infants, though they’ve shrunk, do still exist: Milk-raised babies have, on average, fewer digestive troubles and infections; later in life, they might be less likely to develop certain metabolic issues."

Wu concludes, "The biggest hurdles in infant feeding nowadays, after all, are more about access than tech. Many people—some of them already at higher risk of poorer health outcomes later in life—end up halting breastfeeding earlier than they intend or want to because it’s financially, socially, or institutionally unsustainable. Those disparities are especially apparent in places such as the U.S., where health care is privatized and paid parental leave and affordable lactation consultants are scarce, and where breastfeeding rates splinter unequally along the lines of race, education, and socioeconomic status." Katie Hinde of Arizona State University's School of Human Evolution and Social Change told Wu, “Where milk matters the most, breastfeeding tends to be supported the least. If milk is a singular triumph of evolution, a catalyst for and a product of how all mammals came to be, it shouldn’t be relegated to a societal luxury."