In the history of gluteal enhancement, Mexico City stands out. It protrudes. It was here, in 1979, that a plastic surgeon, Mario González-Ulloa, first installed a pair of silicone implants designed specifically for the buttocks. The textbook Body Sculpting with Silicone Implants calls González-Ulloa the “grandfather of buttock augmentation.” The early 2000s saw a new generation of Mexico City buttock transformation luminaries, notably Ramón Cuenca-Guerra. In his 2004 paper “What Makes Buttocks Beautiful?” Cuenca-Guerra laid out four characteristics that “determine attractive buttocks” as well as the five types of “defects,” with strategies for correcting each one. I, for instance, have defect type 5, the “senile buttock.” (González-Ulloa’s depiction of this took the form of charcoal nudes contrasting “the typical ‘happy buttock’”—high, rounded, dimpled—with its counterpart, the low-slung, drooping “sad buttock.”)

While I understand the value of standardizing procedures and setting guidelines for surgical practice, I tripped over Cuenca-Guerra’s methodology. How and by whom had the determinants been determined? Like this: 1,320 photographs of “nude women ages 20 to 35 years, as seen from behind” were presented to a panel of six plastic surgeons, who “pointed out which buttocks they considered attractive and harmonious, and features on which this attractiveness depended.” Oho!

I thought it would be interesting to talk to Cuenca-Guerra about the notion of a visually ideal female figure. As something that could or should be surgically created (or, in the case of the senile buttock, re-created). As something that even exists. I sent an email using the address on a more recent journal paper. There was no reply. Ramón Cuenca-Guerra’s buttocks are in worse shape than mine. He has been dead for some time. I was able to reach a colleague of his, José Luis Daza-Flores. Here was the third generation; just as Cuenca-Guerra had studied under González-Ulloa, Daza-Flores had studied under Cuenca-Guerra, extending the lineage and making Daza-Flores, I guess, “the son of buttock augmentation.”

Daza-Flores collaborated with Cuenca-Guerra on a paper called “Calf Implants,” in which the team did for the lower leg what Cuenca-Guerra had done for the butt: laid out “the anatomical characteristics that make calves look attractive” and the “defects” to be addressed. Here again, plastic surgeons were recruited to judge images—2,600 of them, a vast photographic millipede of female legs.

The paper took an unexpected turn. Referring to a marked-up photograph of a lower leg deemed attractive, the authors tried to show that its measurements conformed to what is known in mathematics as the divine proportion (or golden ratio)—1.6 (I’m rounding it off) to 1. When you divide a line into two parts such that the whole length divided by the long part is equal to the long part divided by the short part, both those ratios will be 1.6 to 1. I found an illustration of the divine proportion on a website called Math Is Fun (and convincing no one). The golden dividing line splits the length such that one chunk is roughly two-thirds and the other is around one-third. The ancient Greeks divided the “ideal” face into similarly proportioned thirds. This was the first time I’d seen the divine proportion applied to a leg.

The paper contained sentences like this: “Seventeen women had thin legs, in the shape of a tube, and a mere 1:1.618 ratio in the A-P and L-L projections.” Though I confess to not grasping the particulars of the discussion, I believe that to be a mathematically precise description of cankles.

The paper also cited the Fibonacci sequence, a progression of numbers wherein each successive number beginning with the second 1 is the sum of the previous two. It goes like this: 1, 1, 2, 3, 5, 8, 13, 21, 34, 55 . . . And, beginning with 5 and 3, if you divide two successive numbers instead of adding them—21 over 13, say, or 13 over 8—you will always get, ta-da, the divine proportion: 1.6.

The Fibonacci sequence turns up with some regularity in nature. You can find it in the turnings of the spiral traced by the inner chambers of the nautilus shell as it grows, or the growth path of sunflower seeds—two commonly cited examples. (Less commonly: the branching pattern of the sneezewort plant.) In humans, it’s apparently not just calves. Calculator-happy physicians and researchers have published papers purporting to show the golden ratio in the relative lengths of the bones of a finger, the proportions of incisors in relation to canine teeth, the structures of aortic valves and the branching of the coronary arteries, the dimensions of the uterus at peak fertility, “optimal nipple position,” and the helices of DNA.

It is an intriguing, if not entirely convincing, notion: that mathematics defines beauty.

When I wrote to Daza-Flores, he confirmed that when he transfers fat to the hips and buttocks, he follows the Fibonacci sequence. I pictured him with a purple surgical marker and a ruler, drawing arcs and equations on an anesthetized torso. How was he coping with what those in his field refer to as “the Kardashian movement”: the surging demand for rear ends that fall well outside the Fibonacci ideal?

The plastic surgery clinic of José Luis Daza-Flores is on a residential side street in a pleasant Mexico City neighborhood called Noche Buena. The building also contains a day spa and a cosmetic dentist, but nothing about its exterior suggests the upscale beautifying that goes on inside. The waiting area seems designed to calm rather than to impress. The seating is comfy and the room is quiet, the clamor of a Mexico City weekday replaced by synthesized instrumental music and the occasional hushed spritz of a wall-mounted scent dispenser.

The lack of ostentation is in line with my impression of the surgeon himself. He is courteous, soft-spoken, deferential. This is a man who says “por favor” to Alexa when he wants to change the music in the operating room. When he speaks with patients in the waiting area or before surgery, I’ve seen him reach out to take their hand.

It’s 8:15 am now. Daza-Flores sits in the clinic’s kitchenette, dressed in scrubs, having an espresso while the morning’s patient is prepped for surgery down the hall. He looks young for his age, but not worked on. I don’t see any of what his ilk like to call “defects.”

From a folder, I slide out Cuenca-Guerra’s “Beautiful Buttocks” paper.

In the most respectful way possible, Daza-Flores distances himself from the work. Cuenca-Guerra, he says, concerned himself with the placement and size of buttock implants, but he overlooked the sides of the hips. It is this lateral projection that, coupled with a relatively smaller waist, creates the classic hourglass figure. You can make a pair of butt cheeks protrude from here to Puerto Vallarta, but it won’t change the patient’s figure viewed head-on. It won’t make an hourglass out of a grandfather clock.

In 1973, a few years before Dow Corning worked with González-Ulloa to design the first cosmetic buttock implants, Tennessee surgeon William Cocke pushed a pair of silicone implants under the skin on the flanks of a woman who was “quite concerned about her underdeveloped hips.” The results were described in one textbook as “less than optimal.” Likely this was because implants placed superficially—beneath the skin and fat, rather than under or between muscles—tend to shift, and their contours are often visible through the skin. (In his writeup of the results, Cocke mentions none of this, noting simply that “the patient has since married.” As though narrow hips had stood in the way of matrimonial bliss.)

“To reshape this lateral contour you need fat,” Daza-Flores says. He gets up and takes a bowl from the refrigerator. The timing makes me think for a moment that he’s retrieving a bowl of human fat. It’s not, of course. It’s papaya. He sets it on the table between us. “Fat transfer,” he continues. That is, suctioning fat from one area and injecting it someplace else. The technique is known generally as liposculpture. Specifically and casually, when applied to the nether area (with or without implants), it’s a Brazilian butt lift.

“So the surgery has changed,” he says, “but the concept of divine proportions has remained.”

I ask Daza-Flores if he can show me some examples of golden ratio divinity among his patients. He scrolls through photographs on a laptop. Lots of TikTok stars and beauty influencers. “These are pretty close to ideal proportions,” he says. Here’s the thing, though. Their befores look pretty ideal too. In 2021, a team of Norwegian and American plastic surgeons published a paper with the subtitle “Golden Proportions in Breast Aesthetics.” Here’s what stood out for me: Out of 37 subjects, only five had breasts that conformed to the golden ratio. (An additional four had one golden breast.) “Neither breast was considered optimal for 28 (76 percent [of the]) subjects.” The highest aesthetic ranking belonged to a “virtual subject”—a computer-generated torso. Plastic surgeons and their patients are chasing an ideal that rarely exists in undoctored human anatomy. The more work beauty influencers get done, either surgically or through TikTok filters, the shittier the rest of us feel about our utterly normal-looking bodies and faces.

And here’s the other thing: By now, the desired look has eclipsed even Fibonacci’s ideals and wandered into the realm of cartoon, of anime.

Before I left, I printed out a series of photographs from the London tabloid The Sun, purporting to show Kim Kardashian from behind in a thong bikini bottom. The woman’s buttocks and hips dwarf the rest of her legs, a “defect” referred to in an Aesthetic Surgery Journal paper as the “lollipop deformity,” or “marshmallow on a stick.”

“That to me is not beautiful,” Daza-Flores allows. With his fork, he indicates the fold at the bottom of the woman’s butt cheeks. “It’s too long, and too creased.” The intragluteal fold, as it is professionally known, should, he says, be more of a gentle ravine and extend no more than a third the width of the buttock. The folds of Alleged Kim’s traverse the entire cheek. “It looks like a heavy buttock.”

A woman with double-D breast implants is hefting 4 pounds. When women complain of sagging breasts, Daza-Flores will suggest a lift rather than implants. “If you use an implant—for about six months the breast will look nice, but eventually it will fall down worse than before.”

Daza-Flores slides the printout back to me. “She has everything you can get, all at once. Implants, fat transfer, injections of Sculptra,” he says, referring to a filler. “She’s probably getting something every six months. And girls ask for that. They say, ‘I want the Kardashian surgery.’”

And who wins? Fibonacci or Kardashian?

“I try to suggest not going that far,” Daza-Flores says. He tries to get inside their heads, to see why they’re asking for this. Is it something their partner wants? He counsels patients against getting implants to please someone else. Because, as he puts it, the surgeries often outlast the relationships. He has had patients who’ve “changed out their breast implants every time they change boyfriends.”

He also reminds patients that the look is a trend, and like all trends, it will fade. (Indeed, two years later, when I return to this chapter before going to print, the American Society of Plastic Surgeons is reporting an Ozempic-fueled trend for a leaner “ballet body.”)

Daza-Flores drains his cup. “If I don’t do it, they’ll do it anyway. They’ll go with another MD.” Often, that MD is not a board-certified plastic surgeon. It may be an aesthetician or a general practitioner who got a certificate through an online course. Daza-Flores calls them “nomads,” because once the lawsuits start to accumulate, they move to another state. One way to judge practitioners, he says, is to see how long they’ve been practicing in the same location.

Daza-Flores won’t be using buttock implants of any size on this morning’s patient, only fat transfer. Lately he’s been using implants less often. The Brazilian plant that manufactures his preferred brand burned to the ground, and other companies have been having trouble obtaining or renewing their import licenses. Daza-Flores suspects corruption: government officials demanding exorbitant fees and bribes.

But the main reason is that José Luis Daza-Flores loves to work with fat.

Medical equipment catalogs fascinate me. It never seems to occur to the people who name the equipment and write the copy that anyone but a doctor will be browsing the offerings. Would you otherwise name your liposuction cannula The Fat Disruptor? The Bayonet Infiltrator? Clearing the path for the cannula is the “punch,” which, in the words of the Black & Black Surgical catalog writer, “make[s] the initial stab wound.”

That’s done now, and the cannula is in, nosing around the patient’s lower back. The to-and-fro of liposuction looks very much like the motion used to wield that other wanded suction appliance. But quicker and more vigorous. More like mopping than vacuuming.

Daza-Flores waits for me to finish nattering. “Grating cheese. That’s what it feels like,” he says.

Because 20 percent of the suctioned fat is blood, the stuff moving through the tubing is dark pink rather than yellow. This, combined with the fat collection canister’s resemblance to a blender, makes it appear that Daza-Flores is extracting raspberry smoothie from his patient. His wife, also an MD, had worried that I might faint or feel sick at the sight of fat being sucked from a body. If anything, it’s making me hungry.

Once the raspberry treat begins to fill the canister, blood cells and serum settle out, leaving a layer of fat above. Daza-Flores uses the verb decant, which adds a touch of class to the proceedings. Some surgeons use a centrifuge to separate out the fat, but Daza-Flores feels that the forceful spinning can damage or destroy the cells. Even with the gentler treatment, around half the injected fat cells will die and be absorbed by the body.

The fat Daza-Flores loves most is the fat most of us hate. Love handles. The blub at the waistband. “This is the fantastic fat,” he is saying. He loves it for the reason we hate it: it is stubbornly persistent. If a person diets, it’s often the last fat to disappear. If they start to regain weight, that’s where the fat first appears. In liposculpture, Daza-Flores moves the recalcitrant back blub cells away from where they’re unwanted and installs them where they’re wanted. “The genetic information remains,” he says. Now if a patient puts on weight, it doesn’t show up as back blub but as curvaceous hips and happy buttocks. Donor dominance!

For this same reason, Daza-Flores never uses back blub to fill out parts of the face. “If you take it from down here, and then they gain weight, the cheeks get fat.”

Almost done. The cannula has widened the wounds made by the punch. They look like small slots, as though the woman has USB ports. An assistant switches out the cannulas, from aspiration to injection. In all, about 6 quarts of fat have been sucked from the patient, 2 of which will now be injected in her buttocks and the sides of the hips. This is where Fibonacci steps into the room. Daza-Flores starts at the outside flank, spirals down and around, and ends up at the center of the buttock. He likens the path he follows to the spiral of a snail shell, yet another example of Fibonacci numbers in nature. I had imagined him working from calculations, plotting points of the curve on the patient’s skin. By now, he says, he can eyeball it.

The injection cannula has a plunger, which Daza-Flores squeezes as he guides the instrument backward. If you could see beneath the skin, it would look like someone decorating a cake. With human fat. Happy birthday! He pauses at the summit of the patient’s left buttock.

“Mira. This is a dangerous area.” Underneath the gluteal muscles are several large blood vessels. Even small amounts of fat in the circulatory system can spell trouble, in the form of a “uniformly fatal fat embolism.” I’m quoting a 2018 Australasian Society of Aesthetic Plastic Surgeons press release entitled “The Brazilian Butt Lift Named as the Most Dangerous Cosmetic Procedure.” An embolism is a clot that breaks free and gets stuck in a narrow blood vessel, blocking the flow of blood. If it lodges in the heart or brain or lungs, that’s serious trouble.

“There were a lot of deaths,” Daza-Flores says. “A lot of girls dying from fat transfer. We didn’t understand what was happening.” Daza-Flores was part of a task force that reviewed the autopsy reports. In every case, fat was found in the muscle. Rather than keeping the injection cannula more or less parallel to the muscle, some practitioners were aiming down into it. Even if the angle was just 10 or 15 degrees off, Daza-Flores says, it was enough for the fat to enter the danger zone.

“They keep dying, to tell you the truth. Probably once a month here in Mexico City. Even now, people are still doing it, injecting into the muscle. There are clandestine clinics with poorly trained or untrained aestheticians.” It happens in the United States as well. Women were dying in Miami so often that in 2019, the Florida Board of Medicine issued an emergency ruling. Florida Administrative Code R 64B8–9.009(2)(f) prohibits the injection of fat into or beneath the gluteal muscles.

I have to say, the patient looks great. Though in my estimation, she looked great when she walked into the waiting room. I support everyone’s right to do to their body whatever makes them feel good about themselves, but it would be lovely if feeling good about oneself didn’t depend on altering the normal effects of aging. Particularly when the alterations involve the risks of major surgery. Interestingly, a majority of the surgeons in the aforementioned study of surgeons and patients and breasts seemed to feel the same. The surgeons, 45 in all, were asked to imagine that a female partner or acquaintance had come to them expressing dissatisfaction with the size of their breasts. Would they recommend getting implants? Sixty-one percent said no.

Let’s see about this one. “Dr. Daza-Flores, what would you say to your mom if she wanted you to do a full-torso liposculpture on her?”

He laughs. “I already operated on my mom.” He extracts the cannula and hands off the empty syringe. “My mom loves surgeries!” He turns to the assistant. “Remember? We did the tummy tuck? But we didn’t do fat transfer.”

I recall what he said about embolisms. “Because of the risks?” “Because she didn’t want it. She doesn’t need it!” An assistant hands him a new syringe of fat. “She’s got a very big butt.”

“Mira! I’m making a fossette.” Fossette is the medical word for dimple. A set of supragluteal (above the buttocks) fossettes is one of Cuenca-Guerra’s four “determinants of gluteal beauty.” Daza-Flores presses his forefinger into the woman’s flesh to keep the fat from filling in that spot, first one side, then the other.

He steps back to assess his work. Removing the fat above the buttocks has given the patient a more defined waist and a longer-looking back. Bringing the torso closer to the golden ratio: two-thirds back and one-third ass. He makes a final sweep. The cannula moves around under the skin like a leg under a bedsheet. There are sounds, recognizable ones—sounds of sloppy eating, of things pulling up out of mud. I’m both disgusted and piteously envious.

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Excerpted from Replaceable You: Adventures in Human Anatomy. Copyright 2025 by Mary Roach. Used with permission of the publisher, W. W. Norton & Company, Inc. All rights reserved.