For most of his life, David Smith was overweight, but by age 24 he tipped the scale at a whopping 630 pounds. He was morbidly obese and literally eating himself to death.  David Smith shares the story of how he dropped 400 pounds.  Smith's doctors told the Phoenix native to lose the weight or he would only have about four years to live. Smith felt so unworthy he had thought about ending his life even sooner than that. He believed he deserved to die, and in a very painful way.  "I just decided maybe me dousing myself with gasoline, maybe people could hear my screams and hear all the despair that consumed me for all these years," Smith said.  Watch the full story tonight on "20/20" at 10 p.m. ET  In those moments of despair something clicked for Smith. Fed up with the fat, he decided to get fit. Smith e-mailed Chris Powell, a local health and fitness expert at "Good Morning Arizona" and would not take no for an answer.  "To have someone say I really want to lose 400 pounds, I was like that's nice but are you really ready to make that commitment?" asked Powell, a personal trainer.  Smith's answer was simple. "I knew that if I didn't, I was going to die," he said.  When Powell met the overweight young man for the first time, he was taken aback by his massive girth.  "It was definitely shocking because I didn't know what 600 pounds looked like," Powell said. "When he opened the door it was doorframe to doorframe."  At first Powell wondered what he had gotten himself into, but he also saw something else in the painfully shy Smith.  "He was just so broken," Powell said. "You could see how weak he was. And I don't mean weak as in physically weak, but just that he had no social skills. He really didn't know what to say or what to do. He couldn't even look me in the eye."  The trainer knew he had to do something to help unlock the personality of this young man who had been imprisoned by shame and social anxiety.  "I had a very bad social phobia," Smith said. "I didn't leave the house and I didn't even feel comfortable in my own backyard until it was dark out."  CLICK HERE for tips from trainer Chris Powell to kick off your weight-loss journey.
Once morbidly obese, David Smith dropped 400 pounds without diet pills or surgery, a feat that saved his life. Now, his incredible battle with the bulge story has inspired others to lose weight.