[No one can define it, much less fully replicate it. ]
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GO AHEAD, TRY TO EXPLAIN MILK  
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Katherine J. Wu
June 22, 2023
The Atlantic
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_ No one can define it, much less fully replicate it. _

, Miya, CC BY-SA 4.0, via Wikimedia Commons

 

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
time figuring out what, exactly, humans and other mammals are feeding
their kids.

The trouble is, no one can really describe what milk is—least of all
the people who think most often about it_. _They can describe,
mostly, who makes it: mammals (though arguably also some other
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secretions from their throat or their skin). They can describe,
mostly, where it comes from: mammary glands via, usually, nipples
(though please note the existence of monotremes
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into abdominal grooves). They can even describe, mostly, what milk
does: nourish, protect, and exchange chemical signals with infants to
support development and growth.

But few of these answers get at what milk, materially,
compositionally, is actually _like_. Bridget Young, an
infant-nutrition researcher at the University of Rochester, told me
milk was an “ecological system”; Alan S. Ryan, a clinical-research
consultant, called it a “nutritional instrument.” Bruce German, a
food scientist at UC Davis, told me milk was “the result of the
evolutionary selective pressure on a unique feeding strategy,”
adding, by way of clarification, that it was “a biological
process.” A few researchers defaulted to using milk to explain
something else. “It’s the defining feature of mammals,” says
Melanie Martin, an anthropologist at the University of Washington.
None of these characterizations were bad. But had I been that alien, I
would have no idea what these people were talking about.

What these experts were trying to avoid was categorizing milk as a
“food”—the way that most people on Earth might, especially in
industrialized countries where dairy products command entire
supermarket aisles. “Overwhelmingly, when we think about milk, when
we talk about milk, we think of nutrition,” says Katie Hinde, an
evolutionary biologist at Arizona State University. That’s not the
wrong way to think about it. But it’s also not entirely right.

The milk that mammals make is undoubtedly full of the carbs, fat,
protein, vitamins, and minerals newborn mammals need to survive. And,
across species, much of it does resemble the creamy, tart-tangy,
lactose-rich whitish liquid that billions of people regularly buy. But
to consider only milk’s nutrient constituents—to imply that it has
a single recipe—is to do it “a disservice,” German told me.
Mammalian milk is a manifestation of hundreds of 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; it paints some of an infant’s
first portraits of its mother, and telegraphs chemical signals to the
microbes that live inside the gut. Milk can sustain echidnas that
hatch from eggs, and wildebeest that can gallop within hours of birth
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it can support newborn honey possums that weigh just three milligrams
at birth, and
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calves clocking in at up to 20 tons. Among some
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it influences
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and may shape their sleep habits
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them [[link removed]] toward certain foods
[[link removed]]. Some of its ingredients
are found nowhere else in nature; others are indigestible, still
others are alive.

Milk is also dynamic in a way that no other fluid is. It remodels
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and months after birth [[link removed]];
it changes from the beginning of a single stint of feeding to the end
[[link removed]]. In humans, scientists
have identified “morning” milk that’s high in cortisol, and
“night” milk that’s heavy in melatonin
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certain primates have “boy milk and girl milk,” German told me,
which support subtly different developmental needs. Tammar wallabies,
which can nurse two joeys of different ages at once, even produce
milks tailored to each offspring’s developmental stage
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Kevin Nicholas, a biologist at Monash University, has found that when
the joeys swap teats, the younger sibling’s growth accelerates. And
when mothers and their offspring change, milk changes in lockstep. It
reflects the mother’s
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health
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taking on new flavors as her diet shifts
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its fat content fluctuates
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depending on how far apart bouts of nursing are spaced. Scientists are
just beginning to understand how made-to-order milk might be:
Some evidence
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that maternal tissues may register
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when infants catch infections—and modify milk in real time to
furnish babies with the exact immune cells or molecules they need.

“It’s a triad: mother, milk, and infant,” says Moran Yassour, a
computational biologist at Hebrew University of Jerusalem. “Each one
of them is playing a role, and the milk is active.” That dynamism
makes milk both a miracle, and an enduring mystery—as unique and
unreplicable as any individual parent or child, and just as difficult
to define.

In its earliest forms, milk probably didn’t have much nutritional
value at all. Scientists think the substance’s origins date
back about 300 million years
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before the rise of mammals, in a lineage of creatures that hatched
their young from very delicate eggs. The structures that would later
develop into mammary glands started out similar to the ones we use to
sweat; the substance that would become proper milk pooled on the
surface of skin and was slathered onto shells. The earliest milks
probably had few calories and almost none of its hallmark lactose. But
they were deeply hydrating, and teeming with immunity.

As our ancestors jettisoned egg laying for live birth, they began to
extrude milk not just as a defensive shield for their offspring, but
as a source of calories, vitamins, and minerals. The more that milk
offered to infants, the more that it demanded of those that produced
it: Mothers “dissolve themselves to make it,” German told me,
liquefying their own fat stores to keep their babies fed, “which is
impressive and scary at the same time.” In its many modern
manifestations, milk is, in every mammal that produces it, a one-stop
shop for newborn needs—“the only real time in life where we have
hydration, nutrients, and bioactive factors that are all a single
source,” says Liz Johnson, an infant-nutrition researcher at
Cornell.

Each time mammals have splintered into new lineages, taking on new
traits, so too has their milk. While most primates and other species
that can afford to spend months doting on their young produce dilute,
sugary milks that can be given on demand, other mammals have evolved
milk that encourages more independence and is calorific enough to
nourish in short, ultra-efficient bursts. Hooded seals, which have to
wean their pups within four days of birth, churn out goopy milk
that’s nearly sugar-free, but clocks in at about 60 percent
fat—helping their offspring nearly double in weight
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the time they swim away. Marsupial milk, meanwhile, is ultra-sweet
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or triple the sugar content
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and cottontail rabbits pump out a particularly protein-rich
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thing milk _can’t _do? Be high in both sugar and fat, says Mike
Power, a biological anthropologist at the Smithsonian Conservation
Biology Institute, where he maintains a large repository of mammalian
milk: “Nature has never been able to produce ice cream.”) Each
species’ milk even has its own microbiome—a community of helpful
bacteria that goes on to seed the newborn infant’s gut. Mammal milks
are now so specialized to their species that they can’t substitute
for one another, even between species that otherwise live similar
lives.

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
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third-most-common solid ingredient is a group of carbohydrates known
as human milk oligosaccharides, or HMOs, which aren’t digestible by
our own cells but feed beneficial bacteria in the colon while keeping
pathogens out
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Roughly 200 types of oligosaccharides have been found in human
milk—an inventory
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complexity, and nuance than
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of any other mammalian species described to date, says Concepcion
Remoroza, a chemist who’s cataloging the HMOs of different mammalian
milks at the National Institute of Standards and Technology.

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 were basically
concentrating our pathogens and our parasites,” Power told me, in
ways that put infants at risk. Perhaps the millennia modified our milk
in response, making those unsanitary conditions possible to survive.

Mammals would not exist without their milk. And yet, “we don’t
actually know that much about milk,” down to the list of its core
ingredients in our own species, says E. A. Quinn, an anthropologist at
Washington University in St. Louis. Even for the breast-milk
components that scientists can confidently identify, Quinn told me,
“we don’t really have a good handle on what normal human values
are.” Many studies examining the contents of breast milk have
focused on Western countries
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population skews wealthier, well nourished, and white. But so much
varies from person to person, from moment to moment, that it’s tough
to get a read on what’s universally _good_; likely, no such
standard exists, at least not one that can apply across so many
situations, demographics, and phases of lactation, much less to each
infant’s of-the-moment needs.

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. Originally conceived of and still regulated
as a food, formula fulfills only part of milk’s tripartite raison
d’etre. Thanks to the strict standards on carb, fat, protein,
vitamin, and mineral content
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by the FDA and other government agencies, modern formulas—most of
which are based on skim cow’s milk—do “the nourish part really
well,” helping babies meet all their growth milestones, Bridget
Young, the University of Rochester infant nutrition researcher, told
me. “The protect and communicate part is where we start to fall
short.” 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
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life, they might be less likely to develop certain metabolic issues
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To close a few of those gaps, some formula companies have set their
sights on some of milk’s more mysterious ingredients. For nearly a
decade, Abbott, one of the largest manufacturers of formula in the
United States, has been introducing a small number of HMOs into its
products; elsewhere, scientists are tinkering with the healthful punch
via live bacterial cultures
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A few are even trying a more animal-centric route. The company ByHeart
uses whole cow’s milk as its base, instead of the more-standard
skim. And Nicholas, the Monash University biologist, is taking
inspiration from wallaby milk—complex, nutritious, and stimulating
enough to grow organs of multiple species almost from scratch
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guide the development of formulas for premature human infants not yet
ready to subsist solely on mature milk.

All of these approaches, though, have their limits. Of the 200 or so
HMOs known to be in human milk, companies have managed to
painstakingly synthesize and include just a handful in their products;
the rest are more complex, and even less well understood. Getting the
full roster into formula will “never happen,” Sharon Donovan, a
nutritional scientist at the University of Illinois at
Urbana-Champaign, told me. Other protein- and fat-based components of
milk, specially packaged by mammary glands, are, in theory, more
straightforward to mix in
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But those ingredients might not always behave as expected when worked
onto a template of cow’s milk, which just “cannot be compared”
to the intricacies of human milk, Remoroza told me. (In terms of
carbs, fats, and protein, zebra milk
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match for us.)

A company called Biomilq is trying a radical way to circumvent cows
altogether: It’s in the early stages of growing donated
human-mammary-gland cells in bioreactors, in hopes of producing a more
recognizable analogue for breast milk, ready-made with our own
species-specific mix of lactose, fats, and proteins, and maybe even a
few HMOs, Leila Strickland, one of Biomilq’s co-founders, told me.
But even Strickland is careful to say that her company’s product
will never be _breast _milk. Too many of breast milk’s
immunological, hormonal, and microbial components come from elsewhere
in the mother’s body; they represent her experience in the world as
an entire person, not a stand-alone gland. And like every other milk
alternative, Biomilq’s product won’t be able to adjust itself in
real time to suit a baby’s individual needs. If true milk represents
a live discourse between mother and infant, the best Biomilq can
manage will be a sophisticated, pretaped monologue.

For all the ground that formula has gained, “no human recipe can
replicate what has evolved” over hundreds of millions of years,
Martin, of the University of Washington, told me. That may be
especially true as long as formula continues to be officially regarded
as a food—requiring it to be, above all else, _safe_, and every
batch the same. Uniformity and relative sterility are part and parcel
of mass production, yet almost antithetical to the variation and
malleability of milk, Cornell’s Johnson told me. And in regulatory
terms, foods aren’t designed to treat or cure, which can create
headaches for companies that try to introduce microbes and molecules
that carry even a twinge of additional health risk. Float the notion
of a very biologically active addition like a growth factor or a
metabolic hormone, and that can quickly “start to scare people a
bit,” Donovan, of the University of Illinois at Urbana-Champaign,
told me.

As companies have vied to make their formulas more milk-esque and
complex, some experts have discussed treating them more like drugs, a
designation reserved for products with proven health impact. But that
classification, too, seems a poor fit. “We’re not developing a
cure for infancy,” Strickland, of Biomilq, told me. Formula’s main
calling is, for now, still to “promote optimal growth and
development,” Ryan, the research consultant, told me. Formula may
not even _need _to aspire to meet milk’s bar. For babies that are
born full-term, who remain up-to-date on their vaccinations and have
access to consistent medical care, who are rich in socioeconomic
support, who are held and doted on and loved—infants whose
caregivers offer them immunity, resources, and guidance in many other
ways—the effect of swapping formula for milk “is teeny,” Katie
Hinde, of Arizona State University, told me. Other differences
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in the past between formula- and breastfed infants have also
potentially been exaggerated or misleading
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so many demographic differences exist between people who are able to
breastfeed their kids and those who formula-feed that tracing any
single shred of a person’s adult medical history back to their
experiences in infancy is tough.

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
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lines of race
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and socioeconomic status. “Where milk matters the most,
breastfeeding tends to be supported the least,” Hinde told me. 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.

KATHERINE J. WU
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writer at _The Atlantic._

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