By last Christmas, Carmen had regained all the weight. She pleaded with her parents to send her back to Wellspring, full-time.

"I know they had to add to their mortgage to send me here, so I feel some pressure," she says as she heads off to her next class.

The Wellspring program was designed by Daniel Kirschenbaum, a professor of psychiatry and behavioural sciences at Northwestern University in Chicago. It grew out of research he had done with elite athletes - figure skaters in particular. He found those who monitored their daily efforts, their strengths and weaknesses, improved the most over time.

The 57-year-old author of The Healthy Obsession says children have to become a bit obsessive about their food, controlling their weight and remaining active. This demands that they be separated from their old lives, their families and homes.

"It has to be immersion. If they keep one foot in their old way of living and their former lives, it's not going to work."

Only with progress through an elaborate level system are students bumped up from 10-minute to 20-minute calls home, followed by 60 minutes and then, in the final stages, unlimited calls.

"Students are more likely to be homesick than hungry," Mr. Obbard says. "But we get the calls from parents every day who hear from their kids that they want to come home."

Chris Grayson, the student from Connecticut, gripes that dealing with the immaturity of the younger kids was his one major complaint. At 12 or 13, they are away from home for the first time, he says, living in a dorm and unschooled in the matters of basic hygiene.

"Who wants to live with that smell?"

When Toronto parents Audrey and Fred Guth enrolled their 16-year-old, 321-pound son, Jesse, at Wellspring in 2006, they understood that the school sought to limit family contact because "parents might be part of the problem." But Ms. Guth did not realize the strict policies would make them feel like "parents were just getting in the way."

She says, "It was like a drug rehab. ... We were totally cut off from him."

At one point, Jesse developed a respiratory infection at the tail end of a visit they had in San Francisco. She flew back to the school to be with him, but officials told her that she should not be there.

"I live in another country 5,000 miles away and this is my kid," she says.

"He had a really bad chest infection. ... We weren't even allowed to call." He ended up in the hospital.

Still, Ms. Guth says, Jesse shed 84 pounds during his seven-month stay at Wellspring.

"I can recommend it because there's nothing else out there for kids. What else is there? What are you going to do? He was missing life, he was so depressed," she says. "I thank God there was a place like this."

The school runs a weekend workshop every three months for parents to learn the program and help their kids make the transition back home. One of the first things Dr. Kirschenbaum tells parents, and their children, is that they are not to blame for their bulky bodies.

"Obesity is primarily and foremost a biological condition," he says. "They have to know the enemy."

He estimates that genes account for 75 per cent of the reason people become overweight and that roughly half the parents of Wellspring students are heavy themselves.

Genetics makes them slow metabolizers who have a propensity to pack on pounds. One person might be born with 30 billion fat cells, he says, another with 200 billion.

"It's not just them being pathetic like the rest of the world might think."

And unfortunately they and their genes have been born into a lousy environment - an "obesogenic culture," Dr. Kirschenbaum calls it. "This is a disease of affluence," he says, and it's a cultural phenomenon spreading worldwide.

Fifteen per cent of Wellspring students come from outside the U.S. - Canada, Mexico, Europe and the Middle East. This year, for example, three sisters arrived from Kuwait, which has one of the world's highest childhood obesity rates at more than 30 per cent.

"In Kuwait, there's not a lot for us to do. It's about shopping and food," said Muneera, 19, the eldest. "It's also too hot to walk anywhere and we all have drivers." McDonald's, Burger King and Pizza Hut all opened two minutes from their high school.