Dr. Jeanine Albu of the New York Obesity Research Center at St. Luke's-Roosevelt Hospital has been Hebranko's doctor for the last 20 years.
"There's signals in his brain. The signals are telling him to eat, and they are never telling him when to stop," she said.
Scientists say the morbidly obese are radically different from the rest of us. A genetic mutation may be driving their appetites.
In one study, they narrowed the difference between fat mice and thin mice to a hormone in the brain called leptin. The mice without it were always hungry, always enormously fat. It's the "off switch" for eating. The fat mice didn't have it.
A new study in The New England Journal of Medicine shows that only 3 percent of the obese people in the study have the disturbed leptin gene.
Says Albu: "Could any one of us gain, to be 1,000 pounds? I don't believe so. I think most people would probably get sick, but there would have to have been something with Michael or people of this size that allows this to happen."
Some experts also call morbid obesity an addiction.
Marc Burd, Hebranko's therapist, has worked with addicts in prison.
"To me, the overeating is an addiction, and if you ask most of the patients. … The words or the symptoms that they describe when they get the cravings to eat is exactly the same thing you'll hear from any drug or alcohol [addict]," he said.
In 2003, Hebranko, weighing 769 pounds, entered the Brookhaven Rehabilitation and Health Care Center in Far Rockaway, N.Y., a home that treats morbidly obese patients.
He's currently on a 1,200 calorie-a-day diet to lose weight. He uses a ventilator at night because the pressure of his weight could stop his breathing.
When asked why he couldn't stop gaining weight, Hebranko told ABC's John Quinones: "It is a mystery. There's something inside of you, because not everybody can be 500 pounds. … I don't know how to stop it. I know that it needs to be controlled."
"I don't know if there is a way to stop it. … I'm pleading for the professionals out there to figure out. While me, the little guy, tries to fight it every day," he said.
In Mexico, Uribe deals with his weight loss. He doesn't want a gastric bypass. He's on the Zone diet -- a moderate program in carbs, protein and fat. No more rice and beans! Always by Uribe's side, his mother, Ofelia Uribe, prepares five meals a day.
"What do you want for your son?" Quinones asked.
"That he can get up and walk," a hopeful Ofelia Uribe said.
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