“Are you ready to go to teenager college?” Rhoda Ross-Williams asked her 13-year-old daughter. “You really want to leave us?”  “Mm-hmm,” Rachelle said. She had pulled off her pajamas and was sitting on the toilet so her mother could bathe her. At 4-foot-7 and 278 pounds, she could no longer step over the side of the tub to take a shower.  Rhoda soaped a washcloth and began to scrub beneath the folds of her daughter’s skin. “They’ll have all kinds of stuff for you there,” she went on.  “Oh, yes, they have plenty of fruit.”  “Definitely, going to have the Jell-O.”  Rhoda helped her daughter towel off and pull on a pink T-shirt, black leggings and purple socks, which Rhoda worried aloud didn’t match. She combed Rachelle’s hair — freshly trimmed and blown out at a local salon — into a ponytail and added pink butterfly clips. She wanted her to feel stylish. “I need to lose weight more,” said Rachelle, who has cognitive disabilities. “I don’t want to eat too much food.”  When Rachelle was born, her limbs flopped, she couldn’t suckle or cry and her heart and lungs were weak. The diagnosis was a rare chromosomal abnormality called Prader-­Willi syndrome, which causes low muscle tone and impairs signaling between the brain and the stomach. For several years, Rachelle would show no interest in eating, doctors told Rhoda; then she would crave food intensely for the rest of her life. No matter how much Rachelle ate, she would never feel full. To make matters worse, she would also have an especially slow metabolism, predisposing her to morbid obesity. Though people with the syndrome now routinely live into their 50s and 60s, their average life expectancy in the United States is 30; most die of obesity-related causes. Frighteningly, because no sensation of satiety tells them to stop eating or alerts their body to throw up, they can accidentally consume enough in a single binge to fatally rupture their stomach.  In the South Georgia town of Valdosta, where Rachelle’s family lives, elaborate spreads of high-calorie fare are the centerpiece of every social activity. Rhoda, who grew up as the daughter of a Southern Baptist minister in Valdosta, is obese, as is most of her family. But Rachelle’s health problems are far more complicated. By the time she was 8, she weighed close to 200 pounds and had diabetes, pulmonary hypertension, asthma and sleep apnea. Rhoda knew that people blamed her for her daughter’s size. She consulted a nutritionist, tried to limit portion sizes and kept rigidly scheduled mealtimes; she took Rachelle and her older sister, Raquel, walking at a local track for exercise, and she got Rachelle physical therapy. But she couldn’t seem to halt Rachelle’s weight gain.  Desperate, Rhoda began to avoid family gatherings, where relatives, thinking her too strict, would sneak Rachelle treats. One Sunday, their church announced a “light snack” for the children of fried chicken, macaroni and cheese and cupcakes; after that, she stopped attending. Finally, through a Google search, Rhoda — who was briefly a biology major in college and has an M.B.A. — discovered an inpatient program at the Children’s Institute of Pittsburgh that promised to treat the most severe cases of her daughter’s disorder. When she applied to her insurance company to cover the cost, however, it declined, arguing that Rachelle’s medical problems were not “acute.”  Over the next four years, Rachelle gained 100 pounds. She fell and broke her ankle at school, putting her in a wheelchair, and often needed to wear a nasal cannula, a tube to deliver oxygen. In violent tantrums — a common feature of Prader-­Willi that stems from both the frustration of feeling hungry and a limited ability to control behavior — she hit, scratched, spit at and even hurled her oxygen tank at her teachers. Once, when her sister teased her, she grabbed her by the hair and threw her to the ground. At home, she started sneaking food scraps from the trash can. Then Rhoda lost her job on the support staff of the Valdosta city school system. With her income gone, she was able to get Rachelle on Medicaid, which approved two months of treatment at the institute’s Center for Prader-Willi Syndrome. Wanting Rachelle to get excited, she told her she was going to college just like her sister soon would.  On the last Saturday afternoon in May, after Rachelle’s bath, Rhoda piled suitcases into a rented minivan. Her mother, Edith Ross, took the wheel. Rain spattered the windshield, blurring the view of pecan groves and blackberry fields. At a convenience store, Rachelle’s sister helped her out and, walking backward and holding both of her hands, led her up the curb. To move forward without falling, Rachelle rocked from side to side, inching her feet ahead a little at a time. Inside, she maneuvered toward the restroom between shelves of rainbow-wrapped candy and glowing glass cases of soda. Her mother let her pick out a bag of pistachios. Back in the van, her breathing was still ragged from exertion. The next morning at a Waffle House, Rhoda ordered Rachelle a waffle with sugar-free syrup and a side of bacon. “Bacon good!” Rachelle remarked. “Not too much.”  Rhoda is 39, an optimist with a self-effacing sense of humor and a bright, vulnerable smile. All her life, food has meant joy and, in times of trouble, comfort. Denying her daughter a treat when she was about to leave her for the first time, somewhere far from home, felt like denying her solace. She feared that she was failing Rachelle. Yet she was convinced that without intervention, the way she was breathing, she would die before Christmas.  Prader-­Willi syndrome was first identified by Andrea Prader, Heinrich Willi and Alexis Labhart, researchers at a Swiss children’s hospital, in 1956. But for 25 years, no one knew what caused the strange suite of symptoms that, in addition to low muscle tone and insatiability, characterize it. These include short stature, disproportionally short arms and legs and small hands and feet; a tendency to pick skin until it bleeds; a high pain tolerance; infertility; excellent spatial memory; and cognitive disabilities, including low I.Q. and difficulties processing emotions akin to those common in autism. People with Prader-­Willi are prone to troubles with empathizing, to obsessive thinking, to dismay over slight changes in plans and to problems regulating behavior. Many symptoms affect some people more than others — for instance, not everyone with the disorder is intellectually impaired — but all children with it show an early failure to thrive, followed by obesity, unless their diet is aggressively managed.  In 1981, researchers at the Baylor College of Medicine made a significant discovery: Errors on the chromosome 15 segment — which occur at conception and for which there are no clear risk factors, except in a fraction of cases — produce the syndrome, making it one of few known genetic causes of obesity. No other genetic disorder incites the same extreme lack of satiety or urge to obtain food. Researchers now know that the damaged stretch of chromosome affects at least a dozen genes, but which of those genes govern hunger and fullness — and how — is still a mystery. What is certain is that Prader-­Willi disrupts the functioning of the hypothalamus, a region of the brain that is involved in appetite control.  Prader-Willi’s specific genetic roots, while not expected to explain all forms of obesity, could offer universal truths about the biology of hunger and fullness.  One result is a heightened, permanent sensation of hunger. “They describe it as physical pain,” Jennifer Miller, an endocrinologist at the University of Florida who treats children with Prader-­Willi, told me. “They feel like they’re going to die if they don’t get food. They’re starving.” Parents must lock their pantries, refrigerators and trash cans, and their children frequently lie and steal to get something to eat. They have been known to memorize credit-card numbers and secretly phone for delivery, use a drill to remove the door from a locked refrigerator and break into a neighbor’s garage and eat, uncooked, an entire frozen pizza. Experts aren’t sure how many people are born with the syndrome; estimates range from one in 15,000 to one in 30,000. That means that as many as 20,000 children and adults in the United States could have it, yet doctors are often unfamiliar with the symptoms, and fewer than 8,000 cases have been diagnosed.  Researchers believe that hundreds of genes, combined with environmental factors, have the potential to influence whether a person in the general population overeats and how easily he or she gains weight. But in almost all cases of obesity, the body eventually fails to properly process leptin, a hormone produced by fat cells. Leptin and other hormones signal the hypothalamus to prompt you to eat or alert you that you’re full, but in obese people these systems break down, preventing them from feeling sated. In some cases, including Prader-­Willi, resistance to leptin appears to come first, leading to insatiability and weight gain. In others, the reverse happens: Weight gain causes resistance to leptin, which increases appetite and perpetuates the cycle. “As people gain weight, they inflict a chronic injury to the hypothalamus — to the cells in the brain that sense whether their body weight is appropriate,” says Rachel Wevrick, a professor of medical genetics at the University of Alberta who studies Prader-­Willi.  That lasting damage is a major reason weight loss is often so difficult to sustain. In Prader-­Willi, the cause of the hypothalamus dysfunction is entirely genetic, but the resulting problem — an inability to sense fullness — is similar. Figuring out how to restore proper signaling in the hypothalamus of people with Prader-­Willi, according to many researchers, could translate to a treatment for almost all cases of obesity. “I feel that Prader-­Willi holds the key,” Miller says. “If you could control their appetite, you could control the appetite of anyone.”  In recent years, pharmaceutical companies have begun testing substances that they hope will curb the appetites of people with Prader-­Willi — and by extension those of others struggling with obesity. One drug, beloranib, seemed in a small pilot study to reduce weight, body fat and, for the first time, insatiability in Prader-­Willi subjects — results that its maker, Zafgen, is now trying to reproduce in larger trials. Ferring Pharmaceuticals recently completed Phase 2 clinical testing of a drug that it hopes will improve fullness-signaling in the hypothalamus. And Rhythm Pharmaceuticals is initiating the first trials of RM-493 in people with Prader-­Willi; that drug also targets the hypothalamus, and compounds similar to it have reduced appetite in mice whose condition approximates the syndrome.  What is control, and where does it come from? That is perhaps the most complicated, and controversial, question raised by Prader-­Willi. How does the drive to seek food, for people who have it, differ from the urge to overeat that anyone might face? Medicine has no definitive answer, but so far studies seem to suggest that people with Prader-­Willi are experiencing an amplified version of the cravings we all have when we’re hungry.  Our impulse to eat is linked, via the hypothalamus, to the reward centers of the brain that are also activated by drugs and alcohol. In brain scans, those centers light up if anyone who has been fasting sees pictures of food. But when subjects of normal weight are given glucose, to simulate eating, before they view images of food, their reward centers are quiet; in obese subjects, those regions become slightly more active, and in Prader-­Willi subjects much more so — in fact, they are particularly stimulated when people with the syndrome who should feel full see high-calorie foods. “I think Prader-­Willi makes you appreciate when we fault people for being obese,” says Janalee Heinemann, the research director of the Prader-­Willi Syndrome Association. “What’s to say other people don’t have different hunger-full messages? It’s certainly not just a matter of willpower.” But most families of children with Prader-­Willi find that they are the victims of stereotypes associated with obesity — schools, employers, medical professionals and even their own relatives simply can’t believe that the condition is just as intractable and potentially lethal as a peanut allergy.  In 2004, Peter and Gayle Girard held their annual Christmas Eve party for family members at their home in Orlando, Fla. Before dinner, they set out chips, vegetables and dip, shrimp, a bowl of punch and sodas. Their 17-year-old son, Jeremy, had Prader-­Willi, and they often hosted events at their home so he could join in while they kept an eye on him — as they believed they were doing that night. But the next morning, Jeremy’s belly was distended, and he complained of pain. At the emergency room, doctors pumped his stomach, but his condition worsened. A day passed before surgeons discovered that his stomach, which had been distended long enough to lose blood flow and become septic, had ruptured. Jeremy died that night. Only afterward did the Girards learn that other family members saw him eating more than he should have but didn’t alert them.  “I think the problem with most people’s perception of Prader-­Willi is: It’s just a fat kid,” Peter Girard told me. “They don’t understand that food is a death sentence to these kids. He was usually really good with us at parties — until the last one. You can’t take your eyes off these kids for a second. They’re good at what they do, gathering food, and before you know it, it’s too late.”  “Teenager college!” Edith announced, halting the van outside a squat brick building on a shady residential street. A case manager met the family in the lobby and led everyone to a long hallway lined with bedrooms. Rachelle’s had a hospital bed covered in a pink flannel blanket. There were plastic shelves bolted to the wall and a bathroom with a walk-in shower and a button to push for help cleaning up — her shorter arms meant that, like most people with Prader-­Willi, she couldn’t reach to wipe herself.  The family perched around the room, anxiously scanning Rachelle’s face. She sat on the bed while a staff member in scrubs named Nicole Smith unpacked her suitcase and logged each item, a subtle way of checking for contraband. The floor that housed the Prader-­Willi program was kept food-free and locked. Trays were delivered just before mealtime and removed as soon as everyone finished eating. Accommodations for heavy patients were prevalent but understated: In a central recreation room, extra-wide chairs were filled with sand to prevent them from tipping over or being thrown.  After a while, Nicole suggested that Rachelle follow her, so she could get her picture taken and meet some new friends. But about 30 feet down the hallway, Rachelle slowed, and Nicole pulled over a chair and had her sit down. Gregory Cherpes, the program director and a psychiatrist, appeared and slipped a monitor onto her finger to measure how much oxygen her tissues were getting. A healthy reading is 95; Rachelle’s was 70. She was close to fainting, and potentially even cardiac arrest. Nicole went to get a wheelchair while Cherpes waited to see if resting would bring the number back up. “That’s a pretty shirt,” he said gently. “Rachelle, have you ever been to camp before?”  Cherpes is 49, tall and thin, with the muted voice and thoughtful expression of an experienced maître d’. He wears black-framed glasses and collared shirts but never neckties, because patients can grab them in a fit of temper. He and the staff were prepared for Rachelle to become aggressive, but she seemed remarkably content. Cherpes wondered if she was simply not feeling well enough to act out. The next day, a new patient hit him in the face; it took four staff members to restrain her.  As Rachelle left with Nicole, her family went to a conference room to meet with Cherpes and other staff members. They explained that Rachelle would be on a 600-calorie-per-day diet while at the institute and that she would need to eat 800 or fewer calories per day when she returned home if she was to continue to lose weight. To help make the meals look big without adding calories, they recommended offering a bowl of salad greens with lunch and dinner in addition to a medley of other vegetables, a small amount of protein, Jell-O and skim milk. Thinking about just the cost of the produce — and the gas required to restock vegetables that quickly go bad — made Rhoda’s stomach hurt. She said she would have to start budgeting. As the family prepared to leave, Rhoda warned Raquel and Edith that they had better not let Rachelle see them cry. “Don’t say ‘bye,' ” she said. “Say ‘see you later.' ”  Three weeks later, at 8 on a Monday morning, Rachelle started her breakfast: scrambled eggs, which she dressed with ketchup and pineapple. Sunshine poured through the windows of the common room. That day, there were eight patients on the Prader-­Willi floor. The youngest was 10, the oldest 35. Rachelle sat at a place near the door, her cheeks pink and her eyes bright. “Hi, beauty queen,” Luke Kanish, a 26-year-old Penguins fan, called to her as he came in. Alexandra Rudzitis, a sweet, bipolar 17-year-old with corn-silk hair — the patient who had hit Cherpes — wandered over to tell her a knock-knock joke. Rachelle had a deep, hiccupping laugh that everyone found contagious. Danielle DiCola, a bright 14-year-old homesick for her guinea pig, Lightning, called her “the giggle monster.” Surprisingly, even at mealtimes, the patients never talked about foods they missed, or asked for more to eat, or complained about feeling hungry. Some even left food, mostly vegetables, on their trays. That didn’t mean they were full, though. Whether they ate it or not, Cherpes explained, they wouldn’t feel any different.  All the patients had a green sheet of paper with the day’s agenda on it and boxes for points they could earn by completing each activity. Many refused, at least a few times, to participate. But that morning, as she had every day, Rachelle marched cheerfully through her schedule. In nutrition class, she filled in outlines of food with stoplight colors: green for near-­zero-­calorie products, yellow for more substantial items and red for things like ice cream or French fries, which she would be allowed to have once a month as a single 350-calorie serving. In physical therapy, she walked for 12 minutes, climbed up and down a flight of stairs and pedaled a seated exercise machine while singing along to Miley Cyrus’s “Party in the U.S.A.” “Tell Mama I’m doing good?” she said to her physical therapist.  The tight structure of Rachelle’s days, combined with the lack of food anywhere, led to a complete absence of her tantrums. Once, after an argument over cards, she put her head on the table and refused to participate in therapy. Another time, she was clipping ads from the paper, a favorite activity, and said she didn’t want to go to class. Cherpes sat down across from her and watched as Constance Casey, a staff member, encouraged her to go earn her points.  “I’ll keep your stuff here for you,” she told her. “I’ll make sure nobody bothers it.” A moment later, Rachelle got up and walked toward her classroom.  The Prader-­Willi program at the Children’s Institute of Pittsburgh began in 1982 with a single patient. Until 2012, when HealthBridge Children’s Hospital in Houston started a program, the institute was the only place in the country, and one of few in the world, to offer inpatient treatment for the syndrome. It now houses roughly 10 patients at a time — both adults and children — for stays of four to 10 weeks, depending on their medical needs and what an insurer will cover. The cost of care, per patient per day, averages about $1,500. Over the past few years, patients seem to be coming to the institute sicker than before, sometimes arriving in wheelchairs and on gurneys — a fact that the staff attributes to insurance companies’ increasing their requirements for inpatient rehabilitation while misunderstanding the disorder and what it takes to treat it. “As soon as they hear ‘obesity,' ” says Kwaj Overton, the nurse case manager, “they think, Well, they need to try Curves.”  Actually, much of what the institute does — restricting patients’ caloric intake and getting them to exercise — would help any obese person lose weight. For people with Prader-­Willi, though, it is impossible to make those changes without also managing grave health complications and out-of-control behavior, which is hard to untangle from exposure to food. People with the syndrome can resist raiding an unlocked cabinet, especially in the company of others. But this effort at control often erupts later in aggression. The institute’s philosophy is that eliminating any exposure to food — except at rigidly scheduled mealtimes for which menus are posted weeks in advance — is the only way patients can forget their hunger and anxiety so they can enjoy other food-free activities: crafts, gardening, dancing. There are no punishments for acting out, only incentives for participating. “If the words are ‘You have to do this,’ maybe I do or maybe I don’t, but I’m going to test you and see,” Cherpes said. “In a battle of wills, the person with Prader-­Willi is always going to win.”  A major part of the program is teaching parents how to create a similar environment at home. The staff members also talk to schools about making accommodations, including securing classmates’ lunchboxes, finding a place for a child to eat alone outside the cafeteria and providing a one-on-one aide for transitions between classrooms — measures that administrators, especially those in strapped districts, often resist. Over all, the results appear to be positive and lasting. Patients discharged in 2012 lost an average of 27.5 pounds during their stays, and 96 percent continued or sustained their weight loss after 18 months. Nine months out, a vast majority had not needed emergency medical or psychiatric treatment.  For obesity researchers, the uniqueness of Prader-­Willi presents a paradox: The causes of obesity in the general population are far more complex and varied than those of the syndrome, but Prader-­Willi’s specific genetic roots, while not expected to explain all forms of obesity, could offer universal truths about the biology of hunger and fullness. Robert Nicholls, a geneticist at the Children’s Hospital of Pittsburgh who studies Prader-­Willi, believes it is “very unlikely” that our species evolved multiple, separate systems to govern eating. Understanding why people with Prader-­Willi switch in early childhood from an extreme lack of interest in food to insatiability could offer clues about the nature of appetite — which might eventually help scientists minister to a person’s specific type of overeating or prevent it altogether. It’s possible that it could do the same for conditions of undereating and malnutrition, like anorexia. Whether damage to the hypothalamus can be undone at all, regardless of its cause, is still an open question, but one that a successful treatment for Prader-­Willi could answer. “Taking that genetic condition — if that is curable, there could be hope there,” says Joan Han, an endocrinologist at Le Bonheur Children’s Hospital in Memphis, Tenn., who conducts research at the National Institutes of Health. “It may not tell us everything about obesity in general, but it’s an essential part of the big picture.”  One promising avenue of exploration illuminated by Prader-­Willi involves a gene disrupted by the syndrome, Magel2. During the last 10 years, Rachel Wevrick, the University of Alberta geneticist, and colleagues discovered that mice lacking the gene develop many of the same symptoms that people with Prader-­Willi have, including behavioral abnormalities, low muscle tone and obesity. And though they are less active than normal mice, they do not eat less, so they gain excessive amounts of fat. By analyzing samples of their brain tissue, Wevrick found that the ability of their hypothalamus to sense the leptin produced by the body’s fat cells was impaired, disabling a neural pathway that tells the animals whether they need to conserve energy and consume more calories. Within the past year, researchers have also identified more than a dozen children in whom only the Magel2 gene on the chromosome 15 segment is mutated. At least several of the children have abnormally large appetites — all have low muscle tone — reinforcing the idea that the gene may play a central role in producing those same Prader-­Willi symptoms. The Rhythm drug, RM-493, uses a synthetic protein to bypass the neural pathway (in humans) that should be governed by leptin but does not work in the mice that lack the gene. When a similar compound is given to those mice, they eat less food.  And yet the drug that has progressed furthest in human clinical trials, beloranib, highlights what an enigma Prader-­Willi still is. Originally designed to treat cancer, beloranib had the unexpected side effect of decreasing hunger and increasing weight loss. Though it has shown preliminary success in curbing the appetites of people with Prader-­Willi, no one is sure exactly how it might be doing so.  In fact, the only drug available so far for managing the weight of people with Prader-­Willi — human growth hormone — replaces a deficiency in the pituitary gland that is not directly related to general obesity. Approved for use 15 years ago, the hormone improves muscle tone and increases metabolism, helping people with Prader-­Willi stay thinner. When administered early in life, it also appears to improve cognitive ability. This could partly be why more young adults with the syndrome are graduating from high school. Afterward, though, they’re often stuck. They can’t go away to college, because food is everywhere there. Likewise, it is almost impossible to find a job at which there isn’t, at the very least, access to an employee fridge — and they can’t be allowed to manage money, because they can use it to buy food. There are a few adult group homes around the country equipped to manage the disorder, but not enough to meet demand. Noah Thorner, who is 23, graduated from high school with honors and now lives with his parents. In lieu of a steady job, he makes wooden toys that he sells on Etsy. Finding friends has been difficult. “Everywhere you go, you see people eating,” he told me. “And I realize that I can’t participate, and that’s really hard.”  Another young high-school graduate, Rachel Crowley, attends community-­college classes while her mother, Debra, waits outside to prevent her from straying to the vending machines or campus cafe. “She’s a great candidate for someone who could have her own apartment,” Debra says, “but she can’t prepare her own food — that’s the main stumbling block. We’re hoping some of these new medications they’re researching could open that door for her.” But questions remain about whether even a miracle drug would be enough to safeguard against a fatal binge: What if it doesn’t work perfectly? What if, one day, a person forgets to take it? “It gets exhausting trying to keep your daughter in the world, but a world that just doesn’t get it,” Duane DiCola, the father of Danielle, one of the girls in ­Rachelle’s­ program, told me. “We really struggled with ‘No, my child is different.' ”  Thirty-four days after she dropped off her daughter, Rhoda flew to Pittsburgh for a visit. She would stay for 10 days — in housing offered by the institute — and attend meetings to help prepare for Rachelle’s return home. When she arrived, her daughter was waiting for her almost shyly by the door. She had lost 18 pounds and was wearing an olive outfit her grandmother had sent her. A nurse had cornrowed her hair. Rhoda kissed her and petted her cheeks. “Do I know you?” she said. “Who is this pretty girl?”  It was lunchtime on a Sunday. Rachelle led her mother to the rec room, and Rhoda sat near her table to watch her eat. On her tray was sliced turkey breast, steamed cauliflower, a salad, sweet potatoes and cucumbers; a carton of skim milk; a cup of decaffeinated coffee; a cup of Crystal Light; a packet of Splenda, ketchup, salad dressing and Mrs. Dash seasoning; and a cup of sugar-­free Jell-O. “I didn’t realize they needed all this other stuff to go with it,” Rhoda said.  ‘Everywhere you go, you see people eating. And I realize that I can’t participate, and that’s really hard.’  In thinking about how she had managed Rachelle’s illness so far, Rhoda told me, she graded herself 65 out of 100. But she believed she recognized where she went wrong. “She’ll have a big bag of nuts,” she said. “I knew nuts were high in calories, but I was looking at the sugar. Now I’m like, that was a horrible choice. Planning would be the way to avoid that. Those slip-ups add up. If I had not been slipping up, she would not have gained that weight.” Rhoda had never met anyone else with Prader-­Willi before, and as Rachelle ate, she surveyed the room. Some of the patients were as heavy as Rachelle, but others were barely overweight. Nearby, Danielle was chatting about Disney movies. “You wouldn’t even know anything was wrong,” Rhoda said.  The next morning, Cherpes, Heather Roach, the program psychologist, and Amy McTighe, the program teacher, took part in a conference call with the director of special education for the Lowndes County school system, where Rachelle is a student. The administrator seemed mostly agreeable to the recommendations they offered, but when Roach suggested celebrating holidays and other events without food, the administrator balked.  “Realize this is South Georgia,” she said, before quickly adding, “But if we could teach all of us not to celebrate events with food, we’d be better off.”  Still, she suggested that Rhoda simply keep her daughter at home on the half days before school breaks, when cake and candy were rampant. McTighe and Roach shook their heads at the phone. “We’re thinking it’s best that the accommodation happen at school, rather than her having to stay home,” McTighe said.  Later, Roach explained to Rhoda that if her daughter veered off her dietary schedule, even once, not only would she be ingesting calories she couldn’t afford, but the exception would become an expectation. If a family member slipped her a treat, for instance, she would probably throw tantrums wanting to see that person again. Spontaneity would be impossible. If Rhoda wanted to take the family to a restaurant, she would need to tell Rachelle several days in advance where they were going and exactly what she would be having there; she would need to speak to the servers and ask that they not put breadsticks on the table or offer specials or sodas or desserts; and if Rachelle acted out, Rhoda would need to be prepared to leave and try again another time.  Rhoda nodded and jotted notes on a sheet of loose-leaf paper. “Regarding the movies,” she said, “what is the action to get past the concession stand?”  Almost a month later, on Rachelle’s second-to-last day at the Children’s Institute, Cherpes found her in the rec room. Rhoda was coming to take her home, and she was too distracted to cut ads or color. “Where’s my mom?” she kept asking.  “She’s going to get here, don’t worry,” said her tablemate, Lana Aliff, a motherly 19-year-old from West Virginia.  Rachelle put her head down on her arm. “Airplane — you hear it?” she said. “I think I hear it in the sky.”  Cherpes asked her if she would come chat with him. As she followed him to a private meeting room, her gait was noticeably quicker. She had lost just over 30 pounds and was off insulin. She had also begun to say more, probably in part because her brain was getting more oxygen. They sat at a table, and Cherpes asked Rachelle if she was excited about going home. She nodded. He started to explain that her mom would be doing some things differently.  “I don’t like boys,” Rachelle said, changing the subject. “Boys at my school laugh at me.”  “That doesn’t feel very good, does it?” Cherpes said.  “Boys tell me I’m fat. Fat and ugly.”  “Do you feel fat and ugly?”  Cherpes suggested ignoring the boys and telling her teacher. Rachelle just kept repeating the problem: “They don’t stop. Call me names.”  “It gets kind of hard not to cry sometimes,” Cherpes said after she left.  If Rachelle had said she did feel fat and ugly, he said, he would have pointed out how much she had changed — something she could control: “What would you like to do that you can’t do? Well, just a couple weeks ago you couldn’t walk down the hall.” Now she could walk for 30 minutes on the treadmill.  That evening, at dinnertime, Rachelle heard the doorbell and speed-walked — she still couldn’t run — toward the door. “Mama, guess what?” she said when Rhoda greeted her. “I lost some weight!”  “I know you did,” Rhoda said, cupping her face. “You did teenager college like a champ.”  Rhoda debriefed with Cherpes and the rest of the staff the next morning. She and Raquel had cooked meals for the week. Rhoda had found an inexpensive gym membership for her and her daughters and discovered cheap frozen vegetables at Walmart. She had tried, via Facebook, to implore her relatives again not to eat in front of Rachelle or sneak her even seemingly healthful snacks.  As she went to collect Rachelle for the airport, though, Rhoda just hoped she could make it home without her daughter melting down over all the available food she would see. In the rec room, Rachelle was starting lunch. Once the trays were passed out, Roach, the psychologist, waved a certificate. “It’s Rachelle’s last day today, and I want to give her her physical-­therapy award,” she said. “She has done 65 days of P.T.! Congratulations, Rachelle!”  Everyone clapped. Rachelle, sandwich in hand, froze. Then the corners of her mouth dropped, and tears ran down her cheeks. As if unsure what to do, she took a bite. Rhoda leaned over her with a napkin and dabbed her face. “Don’t you want to go home?” she whispered. Rachelle shook her head from side to side. Soon, Rhoda’s eyeliner was running, and she was using the napkins herself.  Alexandra gave Rachelle a high-five. “Have a good time, O.K.?” she said. Lana pressed a goodbye card that all the patients had signed into her hand. Rachelle clutched the pink envelope, unopened, all the way to the airport.  Later, I asked Rhoda why she thought Rachelle had become tearful. “I think she was emotional because she was like, ‘I’m in a place where there’s other people like me.’ They have this understanding among themselves,” she said. “I think she felt anxiety about going back into the real world. That’s why I started crying, because that’s what I thought she might be thinking. It was just her own little world there, and she was happy.”  Kim Tingley is a contributing writer. She last wrote for the magazine about mitochondrial DNA and reproductive medicine.  A version of this article appears in print on January 25, 2015, on page MM36 of the Sunday Magazine with the headline: The Weight. Today's Paper|Subscribe
Day by day, meal by meal, training children to cope with Prader-Willi syndrome, a rare genetic form of obesity.