And in between, they go through an intense fitness regimen that is, to put a good face on it, gruelling.
The hours-long, athlete-level routines take place from the start.
Some contestants have completed a quasi-mini-triathlon consisting of a 250m swim, a 5km bike ride and a climb up 42 flights of stairs.
Others have pulled aeroplanes down a runway or climbed up and down a hill as many times as they could from dawn to dusk - not just sweating copiously, but sometimes feeling dizzy, vomiting and crying.
With the show taping its seventh season and continuing to spawn an ever-increasing assortment of books, videos, online clubs and forums, The Biggest Loser has made training sessions seem a sure ticket to weight loss for sedentary, morbidly obese people.
And the success of its contestants suggests there's little risk - contrary to common advice - that such programmes should be undertaken only with a physician's seal of approval.
Mainstream physical health experts are appalled by such extreme work-outs.
"This is another example of taking a serious health condition and almost mocking it," says Jeffrey Potteiger, kinesiology professor and director of the Centre for Health Enhancement at Miami University in Ohio.
"I find it deplorable."
He points out that overweight people might have undiagnosed medical conditions such as high blood pressure, heart disease and diabetes.
"If you go out and do this type of work-out, you are going to dramatically increase your risk for some abnormal event and possibly exacerbate the condition."
"People could certainly have a heart attack, a stroke, or become hypoglycemic. People need to be aware of these kinds of things," Prof Potteiger says.
Also, the truly obese need moderate work-outs that help them build up their strength and stamina gradually, he says, not ones that send them sprinting out of the blocks, risking injury.
"This is not the way we deal with this kind of weight issue."
"At the end of the day, you're talking about behaviour change - nutritional, psychological - and that's hard to change. If it were easy, we'd be able to change all sorts of behaviours.
"The question in putting on a programme like this is that in having people watch, it is not a scenario that will help people change their behaviour and become healthy."
Nicki Anderson, named trainer of the year by IDEA Health and Fitness Association, criticises the show's portrayal of exercise as an almost herculean effort.
"All the show does is reinforce to those who are overweight and inactive 'See how hard [exercise] is?' ... For most people, exercise is going to be hard, but it doesn't have to be that hard."
Although some of her clients find the show motivating, Ms Anderson, owner of Reality Fitness, a Naperville, Illinois-based personal training studio, thinks they are being duped.
"It looks like in six weeks they lose 130lb. I have to struggle against what's reality and what's perceived reality. ... Our job is to help you develop steps that will develop a normal, healthy lifestyle. And nothing they're watching is about being normal and balanced."
Even if her clients do have the drive to hit the ground running - literally - the vast majority, she says, don't have the means, the time or the resources to accomplish it safely.
And then there's the matter of muscle strain that extreme exercise produces - and that can quickly crush idealistic work-out goals.
"They're going to be fatigued and sore and they're probably not going to be doing it the next day unless they're highly motivated," she says.
J. D. Roth, the co-creator and executive producer, says the show is simply redefining what is realistically possible.
Most people - including doctors and fitness professionals - still cling to the idea that standard recommendations of moderate exercise and moderate weight loss are right for almost everyone, including the morbidly obese, he says.
And some heavy folks have convinced themselves they can't do one push-up, let alone 10.
"Bob and Jillian had so much conviction about how much more these people could do," he says of the show's trainers, Bob Harper and Jillian Michaels.
And as for the contestants: "In a way, these guys are trained like special forces. They're tired, they're overworked, but they're changing their food and exercise habits."
The severe work-outs and stunts people do are the "extreme" part of the show, Mr Roth says, adding that viewers will use common sense in building their own weight-loss programmes.
"People are watching the show to be inspired and not to feel hopeless anymore.
Viewers are saying, 'If that guy who weighs 300lb can do it, so can I. I can go on that run tomorrow morning.' But they're not expecting to lose 30lb in a week."
Diet and exercise tips offered during commercial breaks reinforce more prudent ideas, he says.
If the show has a true believer about the power of abundant, intense exercise, it's Dr Rob Huizenga, associate professor of clinical medicine at the University of California, Los Angeles, and the show's medical consultant.
He knew from working with professional football players that serious work-outs lead to serious weight loss, and he thought that concept could be employed for severely overweight people.
"One of the big selling points of the show," he says, "is that people learn things no-one has taught them before, like how to exercise.
People have no idea what they're capable of and they don't understand that there are different exercise programmes for heart health, for weight maintenance and for weight loss."
He scoffs at the notion that minimal amounts of low to moderate exercise, even done every day, will make a serious dent in a large weight-loss goal and advocates longer, tougher work-outs - providing they are done with a doctor's approval and supervised if necessary.
About 50% of the contestants had stayed within 5lb to 10lb of their "finale" weight at a two-year follow-up, he says, a percentage far higher than in most clinical research that typically results in far less weight loss, usually 8% to 10% of total body weight.
