The Secret Life of Female Fat

by Cat Bohannon

For generations, women have been taught to hate our fat—sometimes going to great lengths to reduce it. But maybe we’d feel differently if we knew what it actually does. In this adapted excerpt from her book, Eve: How the Female Body Drove 200 Million Years of Human Evolution, researcher Cat Bohannon sheds light on some eye-opening facts about female fat that may leave you wondering, “Does this dress make my ass look smart?”

What do you see when you look in the mirror? If you’re like me, the first thing you see is the outline of your fat deposits—those subcutaneous ones, just under the skin. There they are, the bumps and curves and general padding that, together with your muscles, bone, cartilage, and keratin, make up the shape of you. You may or may not have deeply held opinions about your fat, but it’s unlikely you have no opinion. You likewise may or may not have opinions on semaglutide—drugs like Wegovy and Ozempic—though the media’s been breathlessly declaring this the “era of Ozempic,” which apparently means “the end of food,” which may or may not be code for “the end of ancient, long-evolved pleasure” or “the end of women feeling like crap for one reason and the beginning of another.” 

But unless you’re a scientist doing research in these areas, you probably don’t realize that your fat deposits are actually an organ system, as vital to your body’s well-being as your liver and immune system. They deeply impact how you go about being alive. And for people with female bodies, they may be even more vital. Too bad that while 81 percent of semaglutide users are female, in one recent study on its benefits, only 28 percent of the subject pool the drug was tested on was.

Not like that should shock you: all women are understudied and under-cared for. For half a century, biology and biomedical research essentially only studied male bodies. But that’s starting to change! And we’re learning all kinds of new things about the biology of sex differences. The more we study this stuff, the better medicine we’ll be able to provide for people of all genders. But there’s a massive knowledge gap to fill. Meanwhile, because adipose research has only recently started to expand our understanding of what all this weird, lumpy “fat” really is and how it works, we’re even more in the dark about what happens when those cells have two X chromosomes. And if they do, whether it matters where they happen to be housed in that female body.

Let me make it real for you. In 2011, The New York Times published an article about liposuction. It seems that women who have liposuction on their hips and thighs do grow back some of their fat, but they grow it back in different places. Basically, your thighs may stay thinner, but your upper arms will soon be fatter than they were before. It was a cute, fluffy article. But unlike most plastic surgeons, I’d guess, I’d just been reading the latest research on the evolution of adipose tissue—specifically female adipose tissue.

As it turns out, women’s fat isn’t the same as men’s. Each fat deposit on our body is a little different—the fat deposits around your heart, for example, behave differently than the ones under your chin. But of the many depots of fat throughout the human body, it also appears to be true that cis women’s hip, buttock, and upper thigh fat, or “gluteofemoral” fat, is chock-full of unusual lipids: long-chain polyunsaturated fatty acids, or LC-PUFAs. (Think omega-3. Think fish oil.) Our livers are bad at making these fats from scratch, so we need to get most of them from our diets. And bodies that can become pregnant need them so they can make baby brains and retinas. 

Most of the time, female gluteofemoral fat resists being metabolized. As many women know, these areas are the first places we tend to gain weight and the last places we lose it. They’re also popular sites for women to get liposuction. But in the last trimester of pregnancy—when the fetus ramps up brain development and fat stores—a pregnant body starts retrieving and dumping these special lipids by the boatload into the baby’s body. This specialized hoovering of that parent’s gluteofemoral fat stores continues throughout the first year of breast-feeding—the most important time for infant brain and eye development. Some scientists now believe that human females evolved to have fatty hips precisely because they’re specialized to provide the building blocks for human babies’ big brains. Since we can’t get enough of those LC-PUFAs from our daily diet, these bodies start storing them from childhood. Other primates don’t seem to have this pattern.

Meanwhile, we found out just a few years ago—when someone finally asked the question—that a cis girl’s hip fat may be one of the best predictors for when she’ll get her first period. That’s how important this fat may be for reproduction. Our ovaries won’t even kick in until we’ve stored up enough of this fat to form a decent baseline. When we lose too much weight, our periods stop. We’ve also learned recently that while taking supplements can up a breast-feeding woman’s LC-PUFAs, the vast majority of what the baby’s getting is coming from her body’s fat stores. This was discovered by giving breast-feeding women a specially marked supplement that could be tracked via isotope. By sampling the mother’s breast milk, researchers were able to trace which of the fatty acids in the milk came from the supplements and which came from elsewhere. If all of that is right, it lines up pretty well with the state of knowledge about female biology in general: most women’s bodies begin preparing for pregnancy in childhood, not because it’s a woman’s destiny to be a mother, but because human pregnancy sucks, and our bodies have evolved ways to help us survive it.

But every year, nearly 190,000 women undergo liposuction in the U.S. alone. As reported in various medical journals since 2013, this violent disruption of women’s tissue during liposuction prevents fat from recovering at the surgery site. For the record, most people are happy with the result, and in a properly licensed clinic, it can be essentially safe. The issue here isn’t whether liposuction should occur; it’s whether we should be treating subcutaneous adipose tissue as fundamentally nonessential, particularly in women of reproductive age. More deeply, what’s at stake is whether the ways we think about what might “affect” the female body take into account the deep history of mammalian evolution—that what we are is made of how we got here.

More pressingly, I suspect that the new fat that accumulates elsewhere on women’s bodies after liposuction is not the same kind of fat that was sucked from their thighs. So, I have to ask: With a violently disrupted store of LC-PUFAs, which may or may not be able to do quite what it did before, what happens if that body becomes pregnant? I should not be the first person to ask this question. At some point during the many decades we’ve been “cosmetically” sucking out women’s fat as if it were as simple as getting a haircut, someone should have asked this question. Someone should have already run the study. But no one has, much as I did try to get something going after I read that Times article.

But back then, I was a PhD student at Columbia University in a department that didn’t have the right sorts of freezers for storing the breast milk I intended to analyze—milk I’d meant to gather from a bunch of women in Manhattan who’d had liposuction years earlier and were now breast-feeding their children. (There are important rules about how one stores and transports human tissue. Also, my little freezer on the Upper West Side didn’t exactly have good temperature control. And I had roommates.) So, I sent some emails to scientists at other labs. Everyone agreed that someone should do the study. Eventually someone will. Meanwhile, women keep undergoing liposuction, and no one has a clue if it matters which long-evolved depot of fat they destroy. As with huge swaths of modern medical science, female patients and their doctors are basically crossing their fingers. Will everything be fine? Maybe. The maternal body is surprisingly resilient. Human breast milk, as I’ve since come to learn, is also remarkably adaptive. Making babies the way we do is a messy, dangerous business, so the system has some fail-safes. 

Throughout my years of research, I’ve learned that the biological story of the bodies we live in is older and weirder and funnier than most of us could imagine. And knowing the true stories of our bodies has real-world impact. Unfortunately, because so few scientists take the time to read outside their disciplines (or remotely have the time to do so), there simply aren’t that many of us who hear the sirens when they ring.

When I read that Times article about liposuction—the one that made casual fun of women for their newly fat arms—I was pretty sure neither the writer, nor the authors of the research paper the writer was reporting on, nor the women who had undergone the procedure, knew that our adipose tissue and our livers and our immune systems all came from the same primordial organ, called the “fat body.” That’s probably why all three share so many properties: tissue regeneration, hormonal signaling, deep responsiveness to shifts in local environments. The ancient fat body is the reason you don’t need to transplant an entire liver into a patient who needs one: a little lobe and you’re good to go—the whole thing will regrow in situ. Adipose tissue famously regenerates, too. But unlike the liver, the separate fat depots in our bodies seem to be geared for different jobs, each intricately linked to the digestive, endocrine, and reproductive systems. This is why people who do research on adipose tissue have started calling it an organ system—that’s not a bit of fat under your chin but a small, barely visible part of your fat organ. Our subcutaneous fat does different things from the deep fatty deposits around our hearts and other vital organs. The fat on a woman’s butt might be more important for her possible offspring than the fat under her arms.

We don’t know when that started, exactly—most mammals have special fatty deposits near their ovaries and hindquarters. But we do have a rough guess as to when our ancient ancestors split off from the line that made fruit flies—which, by the way, still have the ancient “fat body”—600 million years ago. Thinking about that timescale for too long will give you vertigo, but at least it’s useful. It gives you a reason why it’s hard to “get rid” of one’s fat. If adipose tissue is a body-wide organ system with regenerative properties that go back 600 million years, maybe lopping off a piece of it in one spot naturally triggers a self-protective response that effectively “regrows” it elsewhere. And like anything that terribly old, there are bound to be younger, newer features laid on top: specialized regions that don’t grow back. Functionality that gets lost.

Bodies are basically units of time. What we call an individual “body” is a way of bounding a series of cascading events that follow self-replicating patterns until finally entropy sets in and enough goes wrong that the forces that keep you from flying apart at the seams finally let go. Species, in a way, are also units of time. But what’s unusual about the body, when you start to think about it this way, is that your basic digestive system is radically old. Your brain is not. Your bladder is a workhorse, doing essentially the same job it’s been doing for hundreds of millions of years—keeping the waste products of your many millions of cells’ ongoing metabolism from poisoning you to death. It’s not your bladder’s fault that the mammalian uterus evolved to squat on top of it like Quasimodo. That only happened about 40 million years ago. Actually, if we’re talking about the gravity problem, that was only four million years ago. Before then, our ancestors had the good sense not to walk on two legs, smooshing all our long-evolved organs on top of one another in our trunks (not to mention generally screwing up the spine). This impacted both sexes of our species, and all eventual genders. But it shaped the biologically female body in unique ways—ways we’re only just finally starting to figure out in labs and clinics around the world. It’s about time. Better understanding these differences and how they evolved is going to give all of us better medicine, less pain, and a higher quality of life. But just as importantly, it’s changing how we understand ourselves—the very story of what we are.

So, what do you see when you look in the mirror? Who stands there? What? First, you see your anxieties. If you’re femme folk, that’s especially true: a loose amalgam of lifelong fears and sexist crap that has a vague outline of a human form, crammed into whatever clothes you’ve managed, and depending on how many years of therapy you have under your belt, it might take a while to see through all that. But then. After. What do you see? 

I can tell you the longer you learn about evolution, the more you’ll be able to see what’s actually standing there: your body isn’t so much a thing that is as a thing that’s happening. You’re more like a point in a stream of light, cascading backwards behind you through time and pushing out inexorably forward. It’s funny—science isn’t very good at describing this feeling, these brief moments when you can see it: the shining gearworks of the universe that comprise you. Maybe religious traditions are a bit better at this. Because what you really are is glory.

So go on. Pour your glory into a pair of Spanx. Take your glory out for a walk on any given Tuesday. Take it to a coffee shop, talk to a friend, and try to pretend you don’t know what you’ve learned is true, because you simply can’t spend the entirety of your life in existential awe. It makes you weird. It’s not practical. You have to care about the coffee, and the friend, and the little wonders of a little life. But it’s true, though. But you just can’t. But it’s true.

Adapted and reprinted from EVE: How the Female Body Drove 200 Million Years of Human Evolution by Cat Bohannon. Reprinted by permission of Alfred A. Knopf, an imprint of the Knopf Doubleday Publishing Group, a division of Penguin Random House LLC. Copyright © 2023 by Cat Bohannon.

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