But that is what Manuel Uribe from Monterrey, Northern Mexico, has done.
Now the world's heaviest man is on track to become the planet's most successful slimmer.
Put another way, his weight loss in one year is the equivalent of shedding two fully grown adult males from his body.
Manuel is already in the latest edition of the Guinness World Records as the heaviest living person.
That's because, not long ago he weighed 560kg (88 stones), or half a tonne.
Supersized by nature, he has now downsized through diet and willpower.
And that will put him in the record books again.
"Look at my face," he says. "I have lost a lot."
Manuel puts it all down to something called the Zone Diet.
The diet, supervised by a team of scientists and nutritionists, consists of a strict formula of carbohydrates, proteins and fats.
It's about controlling hormone levels in the body, particularly insulin and glucagons.
Those behind the diet say that when these are at the correct levels through the right intake of food, anti inflammatory chemicals are released to keep the body's weight in check.
They say the body then uses its stored fat for energy, thereby causing weight loss.
"Life is good now because food is medicine," said Manuel. "If you have the right food your body gets what it needs. If I can lose weight, anyone can."
Manuel certainly doesn't starve himself to achieve his weight loss.
He eats roughly five times a day.
His lunch was a plate of chicken cooked in olive oil with broccoli, tomatoes and slices of raw red pepper.
He can eat fish, chicken, some meat, many types of fruit and pretty much any vegetables, but all in strictly controlled portions called 'blocks'.
He is even allowed one fizzy drink a day - sugar-free, of course.
"He likes his food," said his mother, Otilia. "But I am very proud for what he has achieved in the past year."
The Zone Diet is controversial.
The American Heart Association doesn't recommend diets high in proteins. It also says there is not enough evidence about the long-term effects of being on the diet.
The Zone Diet's backers say they have a lot of evidence to prove it is safe and that it is not 'high protein', as such.
They say that the amount of protein a person absorbs depends on their height and build. They say that goes for carbohydrate and fat intake as well.
Manuel's weight problems are partly genetic, partly down to overeating.
His scale of morbid obesity puts him in the top half of one percent of overweight people.
Dr Roberto Rumbaut, a surgeon in Mexico who specialises in obesity, puts Manuel's case in perspective.
"Manuel Uribe is an extreme case," he said. "Where the obesity crisis lies is in people who are 13 to 31kg (30 to 70lb) overweight."
Dr Rumbaut said there were 1.6 billion overweight people in the world, of which about 450 million are obese, according to figures from the World Health Organisation (WHO).
"It's these people who are putting pressure on health services everywhere," he said.
Dr Rumbaut says it's not just diet that will resolve what has been called the world's "globesity" problem.
"It's the old fashioned stuff like exercise and lifestyle changes," he said.
Back at the house, Manuel sits on the reinforced steel bed that he has not left in six years.
Next to it is a massage machine that he uses to draw the circulation along his limbs. His only movement is to use his hips to swing himself from the lying down position to sitting upright.
It is a dream of his to walk.
It's a dream shared by his new girlfriend, Claudia, who has helped to wash, feed and encourage him through this last year or so of dramatic weight loss.
"We are very happy for the effort he has been making recently," she said.
"Sometimes he is sad and cries because he cannot get off his bed. But he is an example for other obese people to move forward. As he says: 'If I can, you can'."
Alongside his copy of the Guinness World Records lies another text, The Bible.
"I have Claudia, my mother and God to thank," said Manuel. "I am happy."
Still larger than life, but now, the incredible, shrinking, Manuel Uribe
6 months ago