An operation to reduce the stomach’s size is the last resort in the fight against weight gain.

Eric Decker knew at age five that he had a weight problem. “I was so much bigger than anyone else,” he recalls. “I could never wear children’s clothes as a kid.” By age ten, Decker had seen dozens of doctors and nutritionists about his condition and was attending meetings of Weight Watchers.

He did lose weight, but usually put it back on. By age 17, Decker stood 5ft 9in and weighed more than 27 stone. He had a hormonal disorder, back and knee strain, and was at high risk of developing heart disease and diabetes, according to his doctors.

That’s when the American teenager turned to bariatric surgery, an operation to reduce the size of his stomach and severely limit the amount of food he could ingest.

The surgery took place 20 months ago. Today, Decker weighs just 13 stone.

“At first my [mum] was really apprehensive about my having this surgery,” says Decker, 19, who became interested in the procedure after he saw Carnie Wilson - a singer with the 1990s band Wilson Phillips - talk about her experience with bariatric surgery on television. “But this was a last resort. I had tried everything. Doctors told me I had to lose weight because it would eventually kill me.”

Bariatric surgery, once considered a drastic alternative for obese adults, is gaining acceptance among doctors who treat severely obese children, though it remains controversial in some medical circles. Although no-one tracks the precise number of children and teenagers who have undergone the surgery, the number is thought to be small. Many surgeons who perform the procedure on adults now have patients who are children, and several children’s hospitals across the US are launching bariatric programs.

An estimated 140,000 American adults will undergo weight-loss surgery this year - up from 75,000 last year. Ten years ago, the procedure was fairly uncommon in adults, and its growing popularity reflects a sobering reality: more people than ever before are severely - or morbidly - overweight. (Morbid obesity is usually defined as having a body mass index greater than 40; a score greater than 30 is considered obese.)

Morbid obesity has risen rapidly in children and teenagers too. An estimated 127,000 to 255,000 US teenagers have a body mass index of 40 or greater.

Bariatric surgery reduces the stomach from about the size of a football to the size of an egg. If patients don’t adhere to permanent changes in eating habits they will suffer from pain, diarrhoea and vomiting.

For example, sugar, high-fat foods and carbonated beverages should be avoided. Total calories are typically limited to 1,000 to 1,200 per day - about a cup and a half of high-protein food.

Vitamin and mineral supplements have to be taken every day to replace nutrients that can no longer be absorbed in the intestine.

These changes must persist for the patient’s lifetime to maintain the weight loss. It’s a tall order for someone who hasn’t yet left secondary school. And bariatric surgery in children is certainly controversial.

“Children’s nutritional needs are different than adults,” says Jeannie Moloo, a spokeswoman for the American Dietetic Association, who works with adult bariatric surgery patients.

“The surgery itself isn’t just shrinking the stomach size. Some of it is to bypass some of the absorption area of the intestine, and there is the potential for malnutrition. I strongly urge that parents try to pursue every other option available prior to surgery. But for some severely overweight children it may be the best option they have.”

According to Dr Thomas Harris Inge, a paediatrician who has performed the bariatric procedure on 42 adolescents in the last two years, some obese children develop such serious health problems that surgery is considered life-saving.

Decker and his parents sought Inge’s advice after an adult bariatric surgery practice turned him down because of his age.

He underwent counselling for a year before the surgery. Even with the risk of complications and death, Decker believed surgery was his only hope: “I knew surgery was a risk. But I was going to die from being obese. I used to sleep on my stomach because I was so afraid the fat would choke me during the night. I thought that if I died in surgery, at least I died trying to get better.”

After two surgeries - the second to repair a leak in his stomach, a serious complication that delayed his recovery - Decker began a new life that included a highly restricted diet.

“It was hard to cope in the beginning,” he says. But after losing five stones in the first month, he was exhilarated. He is a student who is studying theatre and hopes one day to perform on Broadway.

“I never want to go back to the way I was,” he says.

Despite the progress made by Decker, many questions about bariatric surgery for children remain, a fact which even Inge readily acknowledges.