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Last fall, Alexandra Duarte, who is now 16, went to see her endocrinologist at Texas Children’s Hospital, outside Houston. From age 10, she had been living with polycystic ovary syndrome and, more recently, prediabetes. After Alexandra described her recent quinceañera, the doctor brought up an operation that might benefit her, one that might help her lose weight and, as a result, improve these obesity-related problems.

Alexandra, who smiles shyly and speaks softly but confidently, says she was “a little skeptical at first because, like, it’s a surgery.” But her mother, Gabriela Velez, suggested that her daughter consider it. “Ever since I was a toddler, my mom knew that I was struggling with obesity,” Alexandra says.

Through the years, Alexandra seemed to have tried everything. She went through periods when she avoided all carbs and days when she drank only water. She turned to commercial products, from Herbalife to diet pills to meal-replacement shakes. Desperation — and hope — led her to try gimmicky lotions that promised to burn fat. Puberty seemed to make things worse. Her doctors dispensed “basically the same talk everyone else gives me,” she says: Follow the government’s MyPlate plan; eat lots of vegetables; have anything, but don’t overdo it.

Her mother diligently prepared her meals, cooking vegetables in ways that she liked. Alexandra herself worked on “getting fuller on less,” trying to limit each meal to a single plateful. Almost everyone in her immediate family carries extra weight — including her younger twin brothers, who put on more weight during the pandemic — but neither her father nor the twins nor another brother copied her efforts. Seeing them eat ice cream and cake made her vegetables less enticing. “I have a huge sweet tooth,” she says.

The teasing started in fifth grade. Alexandra couldn’t eat without her classmates staring at and judging her. Though she sought counseling for her sadness and anxiety, these troubles still caused her to leave school for a month. The bullying finally stopped after she switched schools in 10th grade, but Alexandra’s parents knew how deeply she continued to suffer. How much more could their daughter endure? After the doctor suggested bariatric surgery, an operation on the gastrointestinal tract that helps patients lose weight, they spoke to friends who had successfully been through the procedure as adults. They decided it was a smart option for her. Alexandra wasn’t sure, however.

When the family met the surgeon, Jose Ruben Rodriguez, he was quick to tell them that the operation was no “shortcut.” Indeed, Alexandra first had to complete the hospital’s strict, comprehensive behavior and lifestyle program, which lasts six to nine months; this effort would demonstrate her commitment to improving her health while also preparing her for surgery and life afterward. Many teenagers find this step too challenging: Rodriguez estimates that less than one-third of them end up receiving the surgery after their initial appointment. For Alexandra, the program’s tasks mimicked what she had tried already: keeping food journals, drinking more water, setting sleep schedules (which in turn often means restricting access to phones). She was also instructed to exercise; as someone who loves dancing to K-pop songs in her bedroom and going to the gym with friends, Alexandra didn’t find that difficult, either. “I guess for some people it could be intense,” she says, “but it honestly to me felt a little bit easy.”

Alexandra also saw a dietitian, a psychologist, a gynecologist and a social worker. She was tested too: blood draws, ultrasound, X-rays, sleep study. With each appointment, she felt increasingly comfortable with the prospect of an operation. It was especially helpful to hear Rodriguez explain the procedure, called a sleeve gastrectomy, and describe the experiences of other teenagers who had been through it. After several months of deliberating, she decided to go ahead. The other would-be fixes “never really solved the problem,” Alexandra says. “No matter how many pounds I lose, I gain it back. And it’s sometimes even more. It made me feel discouraged with my efforts, like maybe I wasn’t trying hard enough.”

Alexandra is one of the roughly 20 percent of children in the United States living with obesity, up from 5 percent in the 1970s. Another 16 percent or so are considered overweight. The prevalence of obesity increases as children get older: to more than 22 percent among adolescents from around 13 percent in 2-to-5-year-olds. At 5-foot-1 and 209 pounds, Alexandra had a body mass index that placed her well above the threshold for severe obesity, which is 120 percent of the 95th percentile, according to the C.D.C.’s Extended B.M.I.-for-Age Growth Chart. Without treatment, Alexandra would almost inevitably have obesity into adulthood. And the longer that continued, the more at risk she would be for chronic health issues, like joint pain and problems with breathing and sleeping. Worse, some children in her situation can develop what were once considered adult illnesses: hypertension, diabetes, fatty liver disease. Obesity can also magnify mental-health problems. Teenagers with obesity are more likely than those without obesity to contemplate suicide.

Severe obesity is the fastest-rising subcategory among youth — “an epidemic within an epidemic,” as the American Academy of Pediatrics calls it. During the pandemic, the rate of increase in children’s B.M.I. doubled; the greatest gains were in those who were already overweight. Worldwide, the rates of pediatric obesity, which is not solely an American affliction, could double by 2035.

In response to so many grim facts, the A.A.P. in January released its first “clinical practice guideline” for those who care for children who have obesity. The academy now recommends that they immediately start “intensive health behavior and lifestyle treatment,” which it labels “the foundation” of obesity management; this approach supersedes the former strategy of “watchful waiting.” For older youth in certain circumstances — those with a higher B.M.I., say — drugs and, in cases of severe obesity, surgery should be made available as options. “The guideline supports what the evidence supports,” Sarah Hampl, its lead author, told me, when she described the yearslong review process. “We didn’t go beyond that.”

Semaglutide, one of the newest drugs being used for weight loss, was approved for adolescents 12 and older at the end of December, shortly before the A.A.P. guidelines were released. (Ozempic is the brand name of one semaglutide formulation that has dominated headlines and social media for its off-label use in helping adults swiftly and effortlessly shed pounds.) The drugs seem to offer a promising remedy, though it is too early to know if their use among adolescents will be as widespread. Also, it is unclear if the drugs will be sufficient to treat severe obesity; for now, surgery, a treatment of longer standing whose rates have increased substantially among teenagers over the last decade, remains the most effective intervention for them.

As soon as the recommendations were announced, they were met with fierce criticism for their screening and treatment methods, especially drugs like semaglutide and the surgery Alexandra opted for. Critics claimed that these approaches are far too aggressive and subject children’s bodies to unnecessary scrutiny and manipulation, potentially distorting how they view themselves, and that treatments could lead to eating disorders that cause more damage than obesity itself. Some critics challenged B.M.I. — a controversial measurement — and argued that doctors should not be diagnosing obesity in children at all. But, Hampl emphasizes, “the decision for treatment is really in the hands of the families,” rather than the doctors.

In June, grasping her white stuffed puppy with floppy ears and her red blanket printed with gray hedgehogs, Alexandra waited in the pre-op area. She was nervous, but “not as nervous as she is,” she said, pointing to her mother. Gabriela nodded and said, “I think I’m thinking too much.” Alexandra’s father sat quietly beside her as she tried to keep busy by drawing in her sketchbook, eventually burying her face in her stuffed animal.

The tenacity of body weight can be traced to our biology. Humans evolved to resist losing body fat so that we don’t become extinct, says Rudolph Leibel, chief of the pediatric molecular genetics division at Columbia University’s medical center. Scientists are still trying to unravel that evolutionary process. According to the “thrifty gene” hypothesis, which has been around for over a half-century, we gain weight (and keep it on) to help us prepare for and survive periodic famines. In 2008, John Speakman, an eminent British biologist, coined the “drifty gene” hypothesis: As human survival came to depend less on escaping predators, random gene mutations allowed our upper weight limits to drift higher. Today our brains may be regulating our body weight so that it stays within a range between upper and lower thresholds — while perhaps fighting harder to keep us above the bottom one. After all, starvation poses more immediate danger than obesity. “You cannot get a person to chronically defend a lower level of body fat by chronically restricting their calories,” Leibel says. But, he adds, that level may be pushed up “by virtue of environment.”

Our brains are “nonconsciously defending that higher weight,” Stephan Guyenet, a neurobiologist and the author of “The Hungry Brain,” told me — and they are even capable of slowing down our metabolism to that end. The hypothalamus, seated deep in our brains, is the master of this tightly regulated system. Cone-shaped and no larger than an almond, it’s involved in determining if we’re hungry and accordingly prompts us to increase our food intake, or if we’re satisfied, decrease it. It also helps control our metabolism. The hypothalamus responds to signals sent from parts of the body including our fat cells and guts — signals like leptin, an important hormone that Leibel helped discover, which increases when body fat rises, telling our brains to stop eating.

A small number of children with severe obesity are born with leptin deficiency, a gene mutation identified by Sadaf Farooqi, a professor at the University of Cambridge’s Institute of Metabolic Science. Their appetites seem to be bottomless. Though it’s rare, Farooqi cites the extreme effect of this mutation as a clear illustration of the “very strong” impact that biology has upon appetite. When Farooqi treated children with this deficiency by injecting them with leptin, they reduced their consumption; too much leptin made them stop eating entirely. “We can literally control how much they’re eating by the dose of leptin we’re giving them,” she says. In other words, appetite is not entirely within our conscious control or willpower. Ghrelin, a hunger hormone, increases when food intake is restricted, making us eat more. Insulin, another important hormone, helps turn the food we eat into energy and controls things like blood sugar that influence how much we eat.

“Hunger is absolutely instinctual,” Guyenet told me. “We don’t decide whether we’re going to be hungry or not, whether we’re going to have a craving or not.” And while we might be able to control our intake for a meal or a week, we probably can’t do so every time we eat, for a lifetime. Even our brain’s reward system, which interacts closely with the hypothalamus, motivates us to seek high-calorie food. “There’s just a lot of different ways your brain can kind of sneak calories,” he says. “These regulatory systems have their tendrils everywhere.”

Genetics may determine more than 70 percent of children’s body weight. Single-gene mutations are extremely rare (the most common one affects up to 5 percent of children with severe obesity); Farooqi says that nearly all children with obesity are living with the “cumulative effect of many different genetic variants, resulting in a bigger appetite.” So far, about 1,000 have been identified, most of which act on appetite and hunger, and there are probably many thousands more. Each variant might have a very slight impact — the strongest might be responsible for one person being five pounds heavier than the next — but, Farooqi says, “when you add them up together, they probably explain why some people are much more likely to get obese than others.”

But if our genes didn’t change significantly in the last century, why, then, are children getting bigger? No one knows for sure. One likely explanation, however, is the evolutionary mismatch between our genes and our surroundings. Children who end up with obesity were always at the highest genetic risk for that outcome, even if it wasn’t certain to develop, but now, Farooqi says, “the environment is likely unmasking their genetic susceptibility.” The most substantial transformation in their surroundings has been to the food they eat, which in the past was different in its composition and far more limited. Leibel refers to “a revolution in human environments” and notes that our genes haven’t changed “fast enough to accommodate something that’s really an invention of the past 75 years.” The amount of readily accessible food has expanded immensely, making it easier than ever to eat — open a phone app, say, or go to a drive-through. Plenty of Americans can consume as much as they want, whenever they want.

Today nearly 70 percent of what children eat is ultraprocessed food, which the NOVA classification system, a commonly used framework, defines as having been formulated from “ingredients mostly of exclusive industrial use, typically created by series of industrial techniques and processes” — which makes them extremely flavorful. These foodstuffs include things our great-grandparents would not have consumed: packaged chips, energy drinks, ready-to-heat-and-eat meals. They are thought to be an important driver of the childhood-obesity epidemic, in part because they seem to make us eat more. Kevin Hall, a researcher at the National Institutes of Health, found that even when meals are matched for calories, carbohydrates, protein, fat, sugar, salt and fiber, study participants who are instructed to eat freely will still, without realizing it, consume an average of 500 calories more a day if the food is ultraprocessed.

“Any kid is going to choose an ultraprocessed food,” says Marion Nestle, an emerita professor of nutrition, food studies and public health at N.Y.U. and the author of “Food Politics.” Nestle traces the deregulation of food marketing to the Reagan presidency and the shareholder-value movement. “After 1980, kids were fair game,” she told me. Corporations began aggressively marketing their products to children, potential lifetime customers who are easily influenced. Ultraprocessed foods appeal to parents too: They’re cheap, last for years in pantries and freezers and require little preparation. “All food companies are trying to sell products,” Nestle says. “That’s the system, and if the system makes kids fat, well, too bad. Collateral damage.”

In the United States, that damage is inflicted more harshly on some groups of children than others: The obesity epidemic disproportionately spares white and Asian children and those from socioeconomically advantaged families. Living in the highest-opportunity neighborhoods, especially at birth, could be associated with as much as an 80 percent lower risk of obesity. Among Black children, however, the protective effect of income against obesity seems to be weaker.

Over the past few decades, the variety of food items in some supermarkets has risen to more than 40,000 from 7,000. These “modern industrial products should not be recognized as foods at all,” says David Ludwig, a pediatrics professor at Harvard and co-director of Boston Children’s Hospital’s obesity-prevention center. “It’s up to parents and all of us to fight back and not to normalize these.” The A.A.P. urges doctors to “demand more of our government” to modify the food being sold to children. But Barry Popkin, a nutrition professor at the University of North Carolina who has worked with countries on their food policies, remains skeptical that similar regulations could be enacted soon in the United States, like Colombia’s tax on ultraprocessed foods or Chile’s restrictions on them in schools and on advertising. “We need our F.D.A. to be bold,” Popkin says. “We need a food czar who’s tough, not these namby-pamby bureaucrats that don’t really want to ruffle any feathers.”

At Texas Children’s Hospital, Alexandra lay asleep in the operating room, on a tilted bed, with her feet lower than her head. Gravity now worked to the surgeon’s advantage, pulling her intestines away from her stomach. Rodriguez, wearing a “Peanuts”-themed cap with his scrubs, made five half-inch incisions throughout her abdomen, while the music of Bruno Mars and Elton John softly played in the O.R. Rodriguez then inserted a camera and his tools through the cuts. Alexandra’s insides appeared on four surrounding screens.

Rodriguez pulled back her liver, its edges rounded from fat, and pushed aside mounds of squishy yellow fat — findings that he sees only in his bariatric patients — to reveal her stomach, pink and patterned with red vessels like fine tree branches. The anesthesiologist had snaked a tube through Alexandra’s mouth into her stomach. Rodriguez could now follow the contour of the tube with a surgical device that sequentially cut, stapled and sutured stomach tissue. Next came what Rodriguez regarded as the hardest step, wrestling the excised portion of the organ, about 80 percent of the whole, out through one of the tiny incisions. To make sure that what remained of her stomach had been fully stitched up, the anesthesiologist pumped air through the tube into the organ, which now resembled a slim shirtsleeve — hence the procedure’s name, sleeve gastrectomy — and Rodriguez flushed water into her abdomen: Bubbles would indicate a leak somewhere. The water was still. The operation was completed in under an hour. Most of Alexandra’s stomach now sat on a tray, cool and dusky.

In another room, with children’s artwork adorning the bright aqua walls, Alexandra’s parents sat on orange chairs and waited, until Rodriguez came in and knelt next to them. “There’s no blood loss at all, which is nice,” he told them in Spanish. In the postoperative area, Alexandra, still confused from the anesthesia, writhed around. With every movement, she hurt. “It’s so hard to see her like this,” Gabriela said, as she smoothed her hand over her daughter’s forehead and begged her to lie still. Alexandra let out a moan.

“When we first started, the hospital businesspeople told us there’s not enough surgeries to justify building a program,” Rodriguez told me. “Obviously, that’s not how things turned out. It’s the exact opposite.” The number of teenagers who underwent bariatric surgery more than doubled nationwide between 2010 and 2017 and continues to rise. According to Teen-LABS, which is funded by the National Institutes of Health and which has been conducting the most research on the procedure in the United States, adolescents who have had bariatric surgery — most of them white and female — experienced weight loss similar to what adults lost: around 25 percent of their B.M.I. And while nearly 90 percent of these teenagers needed diabetes medications before the operation, none did afterward; five years later, they were more likely to have their diabetes and hypertension in remission compared with people who had surgery as adults. “There’s a greater positive impact for operating on youth with these two conditions than waiting until adulthood,” says Thomas Inge, chair of surgery at Lurie Children’s Hospital, in Chicago, and the principal investigator of Teen-LABS. “You get more out of the same operation.” Even 10 years after the procedure, the data indicates that there are long-term benefits from surgery. Teenagers do not seem to experience more complications than adults.

“Patients lose weight, and they’re less hungry while they’re doing it,” says Randy Seeley, a professor at the University of Michigan who researches the surgery’s effects on a molecular level. Seeley emphasizes that weight loss doesn’t simply result from a newly tiny stomach that limits how much people can eat. Many chemical changes are also responsible, some of which resemble what happens when people are on Ozempic, although these similarities are not thought to be the main mechanism behind surgery’s effectiveness. The crucial roles instead seem to be played by bile acids and antimicrobial peptides in the gut, each of which increase markedly after surgery; Seeley is still sorting out the exact mechanisms of their influence. Also, a majority of the stomach that secretes ghrelin, the hunger hormone, is removed in sleeve gastrectomies. “Why bariatric surgery works so well is because you’re changing lots of things at the same time,” Seeley says.

Most important, bariatric surgery seems to reset, to a lower bound, the body weight that the brain tries to maintain. There is less of the persistent metabolic slowing that accompanies traditional weight loss. Based on fMRI scans, the response to food cues in the brain — around the hypothalamus and in the reward and cognitive-control regions — appears to be different in post-surgical patients than it is for those on low-calorie diets. But how altering the anatomy of the gut changes the brain is still a mysterious phenomenon — mostly “a black box,” Seeley says. And it still seems to surprise everyone, even those who perform the operation. “I’m just a basic gastrointestinal surgeon,” Inge says. “Never thought I’d get to claim to be a brain surgeon. But, hey, that’s where it seems like the impact is.”

Only a tiny fraction of the teenagers with severe obesity who qualify actually receive the operation. The obstacles in front of them are hard to get past: finding a pediatric bariatric surgeon, navigating the long, arduous process of insurance authorization — assuming the procedure is even covered by the insurer. “We’ve seen all sorts of shenanigans for trying to not approve kids,” Inge says.

Some parents, desperate to help their struggling children, feel they have no choice but to pursue treatment without insurance coverage. In 2021, after A’kaysha Studstill tried to take her own life by overdosing, she and her mother, Anastasia, traveled from Florida to Tijuana, Mexico, for bariatric surgery; the trip, including the sleeve gastrectomy, cost around $6,500. “People questioned me taking her out of the country,” Anastasia says. “But you don’t know what I’m dealing with. The bullying — she would shut down for days. She was going to kill herself.” This summer, A’kaysha told me she had lost a third of her weight. Wearing a crisscross black halter top, she beamed while packing up her room to start her freshman year at Florida State University. “There’s always more options than just giving up,” she said. “You don’t have to diet and starve yourself.”

Several studies have shown an increased risk of suicide in adults after bariatric surgery. This association may not be a result of patients already at higher suicide risk, like A’kaysha, choosing to go through with the operation. Meg Zeller, a professor at Cincinnati Children’s Hospital who researches the relationship between mental health and pediatric bariatric surgery and has studied the Teen-LABS cohort, has not found this link, but she cautions that adolescent data is “just a different ballgame” compared with the abundance of long-term data generated by adult cases. Data from eight years out shows that alcohol-use disorder is higher after bariatric surgery, possibly because the operation causes alcohol to be metabolized more slowly. While feelings of stigma and body image can improve, Zeller told me that anxiety or depression may not subside to the extent that diabetes often does. All teenagers experience life changes and increased vulnerability, she explains. “We’re talking about intervening at a time in someone’s life when lots of challenges can exist that are unique to that age group.”

The morning after her operation, Alexandra, her hair in pigtails, shuffled slowly around her hospital room. She steadily took tiny sips of water. “There’s no going back now,” she said.

If our biology, ingrained by evolution, is no longer well engineered for our modern surroundings, can pharmaceuticals help redesign us? One medicine that Alexandra took, metformin, which reduces glucose, has a moderate benefit for teenagers like her who also have prediabetes and polycystic ovary syndrome. But the latest glucagon-like peptide-1 receptor agonists — as a group, commonly referred to as Ozempic — are the true game changers, a class of drugs that are making possible a degree of weight loss not seen before with medications. The pharmaceutical company Novo Nordisk manufactures GLP-1s for weight loss, one of which is semaglutide and sold under the brand name Wegovy. (Ozempic is the brand name for a lower-dose version of semaglutide that is prescribed to treat diabetes by increasing insulin but has become so popular in the last year for cosmetic weight loss that it has helped Novo Nordisk’s market value surpass the gross domestic product of Denmark, where the company is headquartered.) GLP-1s are the “new frontier of anti-obesity medications,” says Aaron Kelly, co-director of the University of Minnesota’s Center for Pediatric Obesity Medicine. In a 16-month study that appeared in December in The New England Journal of Medicine, co-authored by Kelly, adolescents lost 16 percent of their B.M.I. on semaglutide compared with the placebo group, which gained 0.6 percent. The treatment group also registered improved cholesterol and liver tests. (Both cohorts underwent lifestyle therapy.)

Semaglutide, currently administered through weekly injections at home, suppresses appetite primarily by acting on GLP-1 receptors in the brain. It may also reduce cravings by modifying the brain’s pleasure pathways. Outside the brain, it slows down the transit of food through the gut, so the feeling of fullness lasts longer. “When you put those three things together — the appetite, the satiety and the suppression of cravings — all of a sudden, you get a medication that has pretty profound effects on reducing weight,” Kelly explains. “I think we’re entering a new phase of pediatric obesity medicine, where we’re going to continue to see increasingly effective and durable treatments.”

For several years, Eriyanna Stovall, who just turned 14, has tried unsuccessfully to reduce her meal portions. Her attempts at more healthful eating were also thwarted by the temptation of junk food in the house that her younger siblings, who do not have obesity, love to eat. “I would crave junk food 24/7,” Eriyanna says. She snacked late at night on Doritos and strawberry shortcake in her bedroom. Her mother, who is a nurse, raised the A.A.P. guidelines with their pediatrician, but she doesn’t prescribe weight-loss medications. When I spoke with Eriyanna in May, she had been on Wegovy for a month, after the family found an obesity specialist. “At first, I was really scared because I don’t like shots,” she told me. “But it felt like a little instant pinch.”

For the first two weeks, Eriyanna didn’t notice a change. Then her mother observed that she was full after finishing half her plate, instead of asking for her usual seconds. Since the surgery, fruits and vegetables “are tasty to me now,” Eriyanna says. “I think the medicine did something to my taste buds.”

The major studies of children and these drugs have enrolled many fewer adolescent subjects than adults, but no new safety concerns have emerged. In addition to semaglutide’s principal side effects of nausea, vomiting and diarrhea — reported by two-thirds of study participants — more serious ones include gallstones and pancreatitis. Wegovy comes with a caution about possible thyroid cancer, and the F.D.A. mandates that it include a warning about the possibility of suicidal ideation, because it acts on the brain; further studies are being conducted in Europe. If patients discontinue the medicines, the weight returns. Compared with fen-phen, a popular weight-loss drug in the 1990s that was eventually linked to heart-valve problems, doctors are less concerned about semaglutide, because older drugs in its class have been used to treat diabetes for nearly two decades. But for any new medicine, the long-term risks remain uncertain.

For now, most adolescents who qualify for semaglutide probably won’t be able to get the drug at all. “I always say it’s an insurance game,” Treah Haggerty, a family-medicine obesity doctor at West Virginia University Medicine, told me. “What hoops can you jump through?” If families pay out of pocket, monthly costs could reach $1,400. “We’re creating haves and have-nots,” Haggerty says. Even with insurance, parents may still encounter drug shortages; Novo Nordisk is already restricting the U.S. supply of starter doses into 2024, effectively limiting new patients. Eriyanna was forced to stop Wegovy when the drug became unavailable to her this summer. She switched to an older, less effective GLP-1, but then that went on back-order too. “We are so devastated,” her mother texted me recently. “It seems like a very big mountain we are climbing.”

For every child given a diagnosis of being overweight or having obesity — even those who choose medication or surgery — the A.A.P. encourages immediately starting intensive health behavior and lifestyle treatment, like the one Alexandra undertook. For younger children, this may be the only therapy available to them. Amanda Staiano, a developmental psychologist who helped write the A.A.P. section on lifestyle programs, describes them as “trying to re-engineer the child’s environment and helping with positive parenting strategies.”

This spring, Micheal and Christi Pientowski took their 5-year-old daughter, Emmie, to one such program in Madison, Miss., a state with one of the highest childhood-obesity rates in the country. They were referred by Emmie’s pediatrician because she was gaining weight rapidly; her B.M.I. was pulling upward away from her growth curve on the C.D.C.’s B.M.I. chart.

At their first appointment, Emmie’s parents told the doctor, Whitney Herring, that they were worried about Emmie’s appetite. “She eats, then she’ll immediately be like, ‘Can I have a snack?’ or ‘When are we eating?’” Micheal said.

“If we didn’t stop her, she would keep asking for more and more,” Christi added. “She would get mad at us for having more than she did.”

“We wanted Emmie to see somebody before kindergarten,” Micheal says. “We know how kids are these days. We just don’t want her to be bullied.”

Herring told them not to expect a fast fix. “Even with the body mass index coming down, that is often not a quick process,” she said. If kids see any reduction in B.M.I. from lifestyle treatment, the average is a 1 to 3 percent change in percentile, though other health indicators may improve. Herring asked about Emmie’s meals.

“We’re working on it,” Christi said. “The only vegetable that she will eat is carrots.”

“What about at school?” Herring asked. Christi described a lunch with green beans and peas. The doctor turned to Emmie, who is blond with ruddy cheeks and dimples. “Do you like those at school?”

“No! I never eat those.”

“Do you ever have broccoli at school?”

“No!”

“We are very picky, too,” Christi said. She avoided most vegetables herself. (Some families have trouble affording vegetables or lack the time to prepare them.)

At school, children like Emmie may not touch their lunch, or they might snack from vending machines. But improving school meals involves more than just substituting asparagus for fries. Judith Siebart, a dietitian who worked with schools nationally, recalls being told, “We can serve all these healthy foods, but if the kids don’t eat it, what have we gained?”

Next, Rebecca Bagwell, the program’s dietitian, wheeled in a cart stacked with silicone foods. She pulled out a seven-inch plate sectioned into different proportions for various foods groups, following MyPlate suggestions. Emmie chose what she liked — chicken, mac and cheese — as Bagwell piled them on the plate in generous portions. “So do you feel like you would want more or less than this?” Bagwell asked.

Emmie looked at the plate. “More?”

Some of Herring’s patients have trouble getting exercise. One boy, who told me the farthest he has walked is down his block, signed up for track team. But the school never started practice. His experience is not uncommon among poorer youth, whose sports participation rates are lower. And while two-thirds of ninth graders attended physical education weekly in 1991, only one-third did so in 2019. Though exercise alone may not result in profound weight loss, it has extraordinary health benefits — and helps our brains regulate appetite.

Herring has expanded her program to cover three different parts of the state, but she still expects as much as 30 percent of her appointments to be no-shows — a problem for many lifestyle treatment programs. From parents missing work to children missing school, families have trouble going to them — if they can find one at all. Herring reckons that the A.A.P.’s recommendation that patients attend at least 26 hours in person over three to 12 months is not feasible for most: “I have yet to find even the most motivated person who’s willing to do two visits a month.”

Many primary-care doctors do not have a program like Herring’s to refer their patients to. “It puts more pressure on me to make sure that I’m not only identifying these kids,” says Sarah Hart, a pediatrician in Kentucky, “but in the short time that I do have with them, how can I really home in and find those areas that we can make a real sustainable change?” It can seem like an impossible ambition to achieve in 15-minute primary-care appointments.

How the medical community approaches patients’ weight — and whether it should even be diagnosing or treating obesity — has led to all sorts of criticism, which has been growing ever since the American Medical Association declared in 2013 that obesity is a disease. While some people felt relief in knowing that they had a medical disorder, many others felt injury in being considered diseased. As weight became a preoccupation in doctors’ offices and the size of people’s bodies became subject to medical treatment — even if they seemed otherwise healthy — patients felt increasingly more stigmatized and concerned that the care they received was biased. Remedies that were meant to help with obesity instead seemed to be hurting the people with the condition.

One of the most vehement responses to the A.A.P. guidelines came from the Collaborative of Eating Disorders Organizations. “This idea of weight loss for higher-weight children — I think all of us were just completely taken aback,” says Johanna Kandel, chief executive of the National Alliance for Eating Disorders, who helped spearhead an open letter from CEDO that responded to the guidelines. Calling them “harmful” and “irresponsible,” the letter’s foremost concern was that screening and treating children for obesity can lead to eating disorders, for which youth at higher weights are already at increased risk. Tracy Richmond, who directs the Boston Children’s eating-disorder program, explains that weight loss in children can trigger eating disorders by making them “hyperfixated on weight and body,” which can then “spiral out of control.” As children continue to restrict their intake, they can become “undernourished relative to where their body would like to be,” a type of anorexia, even if their B.M.I. may not have fallen into the low range. Other eating disorders — bulimia, binge eating — also may emerge.

CEDO’s open letter registered its unequivocal opposition to treating children with obesity with drugs or surgery on the grounds that the long-term effects of these procedures on pediatric patients haven’t been well studied and could also worsen disordered eating. But criticism was also directed at pushing children to make lifestyle changes — what the journalist Virginia Sole-Smith calls “textbook diet culture marketing” in her 2023 book, “Fat Talk” — because these can also eventually lead to eating disorders. (Sole-Smith has written for this magazine.) While research has shown that intensive treatments focusing on health and lifestyle behaviors decrease eating disorders, some experts have pointed out that those studies don’t track children long enough to know if they end up developing disorders; most of this research followed the children for only a year or less. Such concerns raised by CEDO are particularly worrisome, given that the rate of eating disorders doubled in teenage girls during the pandemic.

Focusing on weight in medical settings can also worsen the care children receive. Two-thirds of people with obesity report experiencing bias from doctors. In extreme cases, this can result in devastating consequences, including missed or delayed diagnoses — even death. The health care system may blame parents for their children’s weight and accuse them of overfeeding or not providing enough healthful food. A few states have even moved children with severe obesity into foster care.

The pushback against conceiving weight as medical problem has brought intense scrutiny to the act of measuring obesity. The backlash against B.M.I. — calculated as weight in kilograms divided by height in meters squared — has been especially forceful. B.M.I. doesn’t take into account whether the weight comes from muscle or fat or where in the body it’s distributed. In June, the A.M.A. began to move away from the measurement when it acknowledged that “B.M.I. cutoffs are based primarily on data collected from previous generations of non-Hispanic white populations and does not consider a person’s gender or ethnicity.” For now, the A.A.P. still endorses B.M.I., in part because alternative measurements can’t readily be performed in most pediatricians’ offices.

Diagnosing people whose B.M.I.s exceed a certain level with obesity — a disease — can increase weight stigma outside medical settings as well. “Our entire concept of ‘fat is bad’ is so rooted in anti-Blackness, white supremacy,” Sole-Smith writes. “When we strive for thinness, we’re reinforcing every other form of stigma at the same time.” People in larger bodies may be perceived as “lazy,” “stupid” or “sloppy” and are constantly vulnerable to insults, slights and jokes. Stigma can also be surprisingly insidious. “If parents say, ‘Fruits and veggies are awesome because it helps us lose weight,’ the weight stigma starts to creep in,” says A. Janet Tomiyama, a psychology professor at U.C.L.A. Experiencing stigma or discrimination because of body size can worsen or even cause health problems.

When it comes to individuals, it’s undeniable that people with obesity can be metabolically healthy, just as normal-weight individuals can be unhealthy. Some studies have even shown that normal-weight individuals may not live as long as those with higher B.M.I.s. Findings like these, along with a greater awareness of the drawbacks that can accompany the medicalizing of obesity, have fueled popular body-positivity movements like Health at Every Size, which seek to disentangle weight from health.

But despite the risks that can accompany obesity treatments — and despite the fact that the data doesn’t always present a clear picture — the prevailing attitude within the medical establishment is that, on balance, the potential negative consequences of obesity are too evident to ignore. When Andrew Stokes, a demographer and public-health professor at Boston University, used people’s weight histories and adequately accounted for important factors like smoking and illnesses, he found that obesity was associated with significantly shorter life spans. Studies can’t prophesy what will happen to any one person, he says, but “this is about probabilities and relative risks.” He adds that obesity is “a cumulative process.” Children with obesity may not be dying of heart attacks, but the Bogalusa Heart Study — one of the longest-running epidemiological studies, it recruited children in the 1970s and followed them through middle age — has nonetheless found that childhood obesity is already associated with cardiovascular and kidney changes that then developed into diseases in adulthood. These findings have been confirmed by autopsies of youth who died in accidents. During the pandemic, obesity was the most common medical condition associated with deaths of people under 21. Lila Monahan, a primary-care pediatrician who uses B.M.I. as a screening tool to help prevent diseases, asks why you would wait until someone started developing abnormalities. She says that’s “like saying someone has high blood pressure, but they’re not having heart attacks, so why are we even measuring their blood pressure?”

Though the A.A.P. guidelines include advice on how to minimize weight bias and avoid stigmatizing patients, and even though they recommend screening children with obesity for eating disorders — and were even vetted by eating-disorder experts, Hampl told me — such safeguards may not be enough. The potential harm that the CEDO letter warns about became a reality a few years ago for a fifth-grader whose middle name is Esther. Her school in New Jersey sent home a letter to be signed, a “B.M.I. report card,” that said her B.M.I. was greater than the 95th percentile for her age. The signature that was required was not her parents’ but her doctor’s. The pediatrician told her mother, Lynn (also a middle name), that the weight gain “was not a big deal” and mentioned an academic medical center’s program that “you guys may want to look into.” The first session taught Esther to assign traffic lights to foods: vegetables, green; candy, red. “They weren’t like, ‘Oh you’re too heavy,’” Lynn says, “It was more like, ‘This is a way to be healthier and change your lifestyle so you can keep this as part of your routine for the rest of your life.’” Esther seemed uncomfortable, though, and cried in the car afterward. “The whole family can do better to eat healthy,” Lynn reassured her.

Esther refused to go back. But that summer, she told her parents she was trying to be “healthier.” She started eating less and walking to school. Then her weight began to drop. The following spring, Esther was diagnosed with anorexia. Over all, she needed four hospitalizations and five months in a residential treatment facility across the country during the pandemic. There, she kicked so hard to resist a feeding tube that she nearly broke Lynn’s ribs. Later, Esther’s heart rate plunged dangerously. “We almost lost her,” Lynn says, her voice cracking. Esther is better now, but her family remains unsettled. The pandemic’s social isolation was central to her daughter’s eating disorder, Lynn says, but the lifestyle program “was probably the catalyst for it.”

“I’m feeling more confident in myself — like, I’m no longer wanting to hide away in a jacket or hoodie,” Alexandra told me in October. Four months after her surgery, she has lost more than 50 pounds and is thrilled to feel good wearing a dress for the first time in a long time, for homecoming. “I’m so proud of her,” Alexandra’s mother says. “She’s doing so great.”

Alexandra’s improved health means she has been able to stop taking her diabetes pills. She will, like all patients who receive bariatric surgery, have to take vitamins for the rest of her life, in part because her altered gastrointestinal tract does not absorb nutrients as well. A decision she made at 16 will continue to affect her entire life.

For someone like Fatima Cody Stanford, a specialist at Harvard who cares for both children and adults with obesity, that’s a positive thing: Redirecting Alexandra’s health in this way is how clinicians should approach childhood obesity. Stanford, who has been using many of the therapies in the A.A.P. guidelines for over a decade, sometimes sees her pediatric patients or their families defer more aggressive treatment, then grow up into adults with obesity, which may pose even greater dangers to their health and be harder to treat — a progression Stanford would like to interrupt when people are still young. “Many people still have this really strong intrinsic belief that if it’s not diet and exercise, then you must have just failed as a human being, particularly in the pediatric population,” she says. As increasingly more effective remedies become available to children like Alexandra, more of them might be set up to lead longer, healthier lives — the reason the A.A.P. put out such comprehensive and urgent guidelines. Yet they are also the same children that others fear will be deeply harmed the most by these very recommendations, and in ways that they may not grasp at such a young age.

Alexandra still exercises most days, but it’s tough to balance a routine with a demanding junior year — two A.P. classes, SAT prep, competing for a summer language program in South Korea. After her operation, dining out with her family has become less enjoyable. She feels full so easily now; even when meals taste delicious and she wants to eat more, she can’t, because she is no longer hungry. Sugar also makes her feel nauseated, so she avoids sweets even though she really wants them sometimes. But, Alexandra says, she was so disheartened before the surgery, putting so much effort into losing weight and feeling like she was “accomplishing nothing.” Now, she says, “it’s refreshing to see the hard work I’m putting in is finally showing results.”

Helen Ouyang is a physician and associate professor at Columbia University and a contributing writer for the magazine. Her writing has been a finalist for the National Magazine Award and anthologized in “The Best American Science and Nature Writing.” Kholood Eid is a Palestinian American photographer, filmmaker and educator based in New York. She was part of the team at The Times that won a 2020 Robert F. Kennedy Journalism Award for a series investigating online child sexual abuse.