If all goes well, Aliayha Carrasco-Garcia will have an operation next month that will change her life. She will shed many of the 240 pounds that now burden her 5-foot-2 frame. Like many who have bariatric surgery, she has tried diets and exercise to no avail. Surgery is her last best hope.

But there is a difference between Aliayha and almost everyone else who has had this operation: She is only 15.

While the number of adolescents who are overweight or obese has leveled off in recent years, the number who are severely obese — heavy enough to qualify for bariatric surgery — has nearly doubled from 1999 to 2014, according to national data, going from 5.2 to 10.2 percent of all adolescents aged 12 to 19. As a result, more and more doctors and parents are facing a difficult question: Should very heavy teenagers have bariatric surgery, a radical operation that is the only treatment proved to produce lasting weight loss in severely obese people?

The very idea fills many parents and doctors with trepidation, and with good reason, said Aaron Kelly, a physiologist and specialist in pediatric obesity at the University of Minnesota. “We’re at a point in this field where surgery is the only thing that works for these kids but we don’t know the long term outcomes.”

The best data are from two recently published small studies that so far have outcomes for just five years. Scientists say there’s an urgent need for more ambitious research.

The question for teens and their parents is: Which is worse — accepting uncertainty about the long term health risks from surgery or the likelihood of serious health risks from remaining obese?

An estimated three to four million adolescents are heavy enough to meet the criteria for bariatric surgery, Dr. Kelly said. But only about 1,000 teenagers a year have the operation. Many medical centers will not perform it on teenagers and many pediatricians never mention it to their heavy patients.

“It obviously is a controversial area,” said Dr. Marc P. Michalsky, a professor of clinical surgery and pediatrics at the Ohio State University College of Medicine.

Obesity carries serious health risks in teenagers — including type 2 diabetes, high blood pressure, sleep apnea, acid reflux, fatty liver and high cholesterol levels — that tend to be eased by surgery. Added to that are social problems, including isolation and depression.

Yet insurers routinely turn down teenagers on the first request, doctors say, leaving surgeons to appeal, sometimes multiple times, before they can operate on an adolescent. Dr. Kirk W. Reichard, the clinical director of pediatric surgery at the Nemours Alfred I. duPont Hospital for Children, said Delaware’s Medicaid program had denied coverage for all patients under the age of 18 with one exception.

When it comes to adult bariatric surgery, doctors say, insurers usually do not put up a fight. But doctors and patients often discount the option.

“We still struggle with acceptance in the adult population,” said Dr. John M. Morton, the chief of bariatric and minimally invasive surgery at Stanford. “Acceptance in the pediatric community is even worse.”

The two recently published studies on outcomes were encouraging, obesity experts say. One, involving 58 adolescents, was led by investigators at Children’s Hospital Medical Center in Cincinnati. The other, led by researchers at the University of Gothenburg in Sweden, involved 81 adolescents. Most of the participants in both studies lost at least a third of their weight and kept it off for at least five years. Diabetes often went into remission. Blood pressure readings that were high often fell to normal levels.

Despite their weight loss, 63 percent of the teenagers were still severely obese after the surgery — only one reached a normal weight — and nearly half had nutritional abnormalities, including iron deficiency anemia and low levels of vitamin D and vitamin B12. Nearly half had hyperparathyroidism, a serious condition that leads to the leaching of calcium from bone.

Many more could have reached a normal weight, said Dr. Thomas Inge, the director of adolescent metabolic and bariatric surgery at Children’s Hospital of Colorado in Aurora, who led the Cincinnati study, if they had had the operation when they were younger, before they got so obese. The participants’ average age was 17 and their average body mass index was 58.5. (That corresponds to person 5 feet 4 inches tall who weighs 341 pounds.)

Some researchers worry that five years may not be long enough to understand the effect of the surgery on people who undergo it as an adolescent.

The operations alter brain signals that control weight and appetite and change hundreds of nerve and hormonal signals to the brain. What, researchers ask, are the consequences for still-developing brains and bones and bodies? Could low levels of vitamin D cause osteoporosis?

The procedure for teenagers is the same as for adults — either a sleeve gastrectomy, in which much of the stomach is cut away to form a small pouch, or a gastric bypass in which the stomach is made smaller and part of the small intestine is rerouted. The operations are just as safe in teenagers as in adults, surgeons say: Mortality rates are around 0.1 percent, which makes them safer than gallbladder surgery or joint replacement.

Both operations require patients to follow detailed medical instructions, including taking supplements for the rest of their lives after surgery. Adolescents, though, are not always the most compliant patients. Dr. Inge said that when he first started offering the surgery in 2004, several of his patients developed beriberi, a serious condition that can affect the heart and nervous system. It is caused by a lack of thiamine.

The beriberi problem was solved, Dr. Inge said, after he began warning patients and insisting they take a thiamine tablet for six months after surgery. “We never saw it again,” he said.

To qualify for bariatric surgery, teenage patients generally must have a body mass index of at least 40, with other medical problems like sleep apnea or diabetes, or a B.M.I. of at least 50 without related medical conditions. Bariatric surgery is the only effective weight loss option for most of these teenagers.

Diets, exercise and behavioral therapy almost never solve their weight problem, said Dr. Stephen R. Daniels, pediatrician in chief at Children’s Hospital in Colorado. Moreover, an obese adolescent is almost certain to become an obese adult. The grim prognosis, “debunks the wishful thinking that they will outgrow it,” Dr. Michalsky said.

Of course the best option would be to prevent kids from becoming obese in the first place, but that is not so easy. There seems to be a strong genetic component that is not easily overridden. Most of the teenagers who have the surgery have a parent who also is extremely obese, said Margaret H. Zeller, a professor of pediatrics and psychologist at Cincinnati Children’s Hospital whose research focuses on adolescents with severe obesity. Dr. Morton has operated on patients from three generations in some families.

But despite their qualms about possible medical problems with bones or other body systems in the future, many researchers and surgeons say those concerns are dwarfed by the consequences of being an obese teenager.

“I am less concerned about osteoporosis than that their lives will be completely destroyed if they don’t get some serious weight off,” said Dr. Lee M. Kaplan, the director of the weight center at Massachusetts General Hospital. By completely destroyed, he adds, he means “medically, socially and economically.”

“I’ve had many patients tell me they’d rather be dead,” than remain fat, Dr. Morton said.

In almost every aspect, life for very obese teenagers is “significantly impaired,” Dr. Zeller said. These difficulties, Dr. Zeller noted, are piled onto the normal angst of adolescence.

“It’s not just that people make them feel uncomfortable,” she said. The physical effort of carrying the weight around can make simple tasks arduous, like walking from one area of a high school to another or sitting at desks too small for them. Many have sleep apnea, making it difficult to stay awake in class. And, she said, “inside, they are feeling ashamed.”

Although not every obese teenager feels isolated and friendless, many, including Aliayha, do. She asked to be home-schooled, but her mother, Cristina Carrasco, refused.

Aliayha was not alone in wanting to leave school. Overwhelmed by their struggles, as many as 10 percent of obese students in grades eight to 12 leave the classroom and are home-schooled or take online classes instead, Dr. Zeller and her colleagues have found. That is nearly three times the rate of home schooling in the general population in those grades, according to the Department of Education.

But some teenagers expect too much from the surgery. They are told they will lose a significant amount of weight, feel better and be healthier but that they are unlikely to reach a normal weight. They are told the surgery will not solve all their problems. Some, though, hold out unrealistic expectations.

“They see shows about bariatric surgery or YouTube. They have this idea about the before and afters,” Dr. Zeller said. But, she added, “the average person having bariatric surgery isn’t doing a YouTube video.”

Dr. Morton worries about what he calls social re-entry. “When they lose the weight, they are not always prepared,” he said

Tiffany Hunter can attest to that. She had the surgery when she was 15 and weighed 350 pounds. Now, 27 and 5-foot-11, she weighs 190.

“Being overweight, you think being skinny will solve all your problems,” she said. “It doesn’t. It makes new problems that you don’t know how to deal with.”

She added, “You think everyone will immediately like you because of how you look on the outside.”

At 15, Tiffany was not only one of the first but also one of the youngest in the country to have the surgery at the time. But some bariatric surgeons wonder if adolescents and surgeons are waiting too long.

Dr. Daniels went from being shocked to hear at a meeting in 2003 that doctors were considering bariatric surgery for adolescents to wondering if a bar of a B.M.I. of 40 or 50 is too high. There was no real science behind those guidelines, he noted. He also worries about children who get fatter and fatter throughout childhood, reaching a level of obesity by adolescence that, even with surgery, will still leave them obese as they head into adulthood.

Dr. Michalsky shares those concerns.

“We can only expect so much out of these operations,” Dr. Michalsky said. The longer the teenagers wait for the surgery and the fatter they get, the less likely they are to reach a normal weight.

But how young is too young? Should it be done on children?

Dr. Morton said the only place he knows where surgeons have done bariatric surgery on children is Saudi Arabia. He and others think it is best to wait until puberty to avoid interfering with physical, mental and emotional development.

But, once again, doctors are acting on hunches, they say.

“We get asked very often what is the lower age limit, but we really don’t know,” Dr. Michalsky said.