It is a question that plagues all who struggle with weight: Why do some of us manage to keep off lost kilos, while others regain them?
Now, a study of 14 participants from the Biggest Loser television show provides an answer: physical activity - and much more of it than public health guidelines suggest.
On average, those who managed to maintain a significant weight loss had 80 minutes a day of moderate activity, like walking, or 35 minutes a day of vigorous exercise, like running.
The researchers conducting the new study did not distinguish between purposeful exercise, like going to the gym and working out, and exercise done over the course of the day, like walking to work or taking the stairs.
Guidelines from the Centres for Disease Control and Prevention, by comparison, call for at least 150 minutes a week of moderate exercise, or 75 minutes a week of vigorous exercise for healthy adults.
The study was published in the journal Obesity. Lead author Kevin Hall, chief of the Integrative Physiology Section at the National Institute of Diabetes and Digestive and Kidney Diseases, and his colleagues also presented their work at the Obesity Society's annual meeting.
Although the study is very small and must be replicated, Dr Hall said, it is the first to assess obese people, years after they lost weight, with state-of-the-art methods to measure the calories they had consumed and the amount of exercise they had done.
The researchers did their measurements when the contestants were chosen, and again at six weeks, 30 weeks and six years after the contest began.
The amount of time and dedication it takes to manage one's food intake and prioritise exercise every day can be an untenable burden for many people. It's totally unfair to judge those who can't do it.
DR JENNIFER KERNS, a Biggest Loser contestant and now an obesity specialist at the Veterans Affairs Medical Centre in Washington, on the difficulty of maintaining one's weight.
"The findings here are important," said Dr Rena Wing, a psychiatry professor at Brown University and a founder of the National Weight Control Registry, which includes more than 10,000 people.
The food eaten "is the key determinant of initial weight loss. And physical activity is the key to maintenance", she said.
The study also helps explain why that might be.
One consequence of weight loss among the Biggest Loser participants was a greatly slowed metabolism.
The subjects were burning an average of 500 fewer calories a day than would be expected, Dr Hall and his colleagues found.
In essence, their bodies were fighting against weight loss.
Those who kept the weight off "are countering the drop in metabolism with physical activity", Dr Hall said.
During the initial weight loss, the equation was different.
Then, the difference in how much weight Biggest Loser contestants lost could be explained by the number of calories they cut from their diets. The amount of exercise did not distinguish those who lost more from those who lost less.
The contestants competed for six months to see who could lose the most weight.
Participants followed a gruelling diet and an exhausting exercise programme.
Contestants' average weight at the start of the show was 149kg. At the end, it was 91kg, a 58kg loss.
But six years later, their average weight rebounded to 131kg, just 18kg less than when they started.
That average, though, hid wide variations.
To learn more, Dr Hall and his colleagues divided the group of 14 into two.
There were the "regainers", the seven participants who ended up after six years weighing 2kg more on average than they had at the start.
And there were the "maintainers", the seven who maintained an average weight loss of 37kg.
To measure the amount of calories the contestants burned, the researchers asked the subjects to drink "doubly labelled water", in which hydrogen and oxygen atoms are at least partially replaced by stable isotopes, which have a different atomic mass.
The isotopes appear in carbon dioxide exhaled by subjects, which allowed the researchers to estimate the average amount exhaled each day. The more calories burned, the more carbon dioxide exhaled.
Some Biggest Loser contestants - including the study's first author, Dr Jennifer Kerns, now an obesity specialist at the Veterans Affairs Medical Centre in Washington - said the conclusions of the new study confirmed their own experiences.
Dr Kerns, a contestant in Season 3 of the show, says she has managed to keep off 45kg only by tracking everything she eats and by exercising on an elliptical cross-trainer for 35 minutes to 40 minutes a day. In addition, her job requires her to walk around the hospital seeing patients.
She has learnt that she cannot relax this regimen if she wants to maintain her weight. "My natural tendency is to regain," she said.
Ms Erinn Egbert was a candidate for Season 8 of the Biggest Loser but ultimately did not make the cut. So she went home "to figure it out on my own".
She hired two trainers and followed a diet-and-exercise programme while she finished her senior year at Ohio State University. She weighed 107kg when the show began and lost about 54kg.
She has maintained a weight that is just 4kg more.
She does it with rigid portion control and regular, intense exercise - 45 minutes to an hour a day, Monday through Saturday, doing the Beachbody programmes, a challenging combination of strength training and cardiovascular exercise.
Ms Egbert, who is 30 and lives in Lexington, Kentucky, says she learnt the importance of working consistently to stay thin, even with a slowed metabolism.
"You have got to keep at it every single day," she said.
Dr Kerns said that it is a difficult task for virtually anyone.
"The amount of time and dedication it takes to manage one's food intake and prioritise exercise every day can be an untenable burden for many people.
"It's totally unfair to judge those who can't do it," she added.
Dr Hall agreed. "The idea that people who regain lost weight are necessarily slothful and gluttonous is an unfortunate stigmatisation that is not based on fact," he said.
Mr Danny Cahill, who is 47 and lives in Tulsa, Oklahoma, is among those who found it increasingly difficult to keep up the sort of regimen he needed to avoid gaining weight.
He won the Biggest Loser competition in Season 8. He weighed 195kg when the show began, and lost 108kg.
For the four years after the show, he exercised more than 2 1/2 hours a day and gained back just 18kg.
Then the injuries began, forcing him to cut back his workouts to 1 1/2 hours a day. His weight crept up to 106kg.
The next year, "my body just started breaking down", he said.
"I had a foot injury, a wrist injury. I couldn't keep it up."
And he was exhausted.
His weight went up to 136kg. For the past two years, his weight has remained stable at about 154kg to 158kg, "but only because I am eating as very little as I can", he said.
"That's the disheartening part," Mr Cahill added. Losing the kilos is one thing. Keeping them off?
"I am still struggling with it," he said.