“The key point is that you can be on TV, you can lose enormous amounts of weight, you can go on for six years, but you can’t get away from a basic biological reality,” said Dr. Schwartz, who was not involved in the study. “As long as you are below your initial weight, your body is going to try to get you back.”

The show’s doctor, Robert Huizenga, says he expected the contestants’ metabolic rates to fall just after the show, but was hoping for a smaller drop. He questioned, though, whether the measurements six years later were accurate. But maintaining weight loss is difficult, he said, which is why he tells contestants that they should exercise at least nine hours a week and monitor their diets to keep the weight off.

“Unfortunately, many contestants are unable to find or afford adequate ongoing support with exercise doctors, psychologists, sleep specialists, and trainers — and that’s something we all need to work hard to change,” he said in an email.

The study’s findings, to be published on Monday in the journal Obesity, are part of a scientific push to answer some of the most fundamental questions about obesity. Researchers are figuring out why being fat makes so many people develop diabetes and other medical conditions, and they are searching for new ways to block the poison in fat. They are starting to unravel the reasons bariatric surgery allows most people to lose significant amounts of weight when dieting so often fails. And they are looking afresh at medical care for obese people.

The hope is that this work will eventually lead to new therapies that treat obesity as a chronic disease and can help keep weight under control for life.

Most people who have tried to lose weight know how hard it is to keep the weight off, but many blame themselves when the pounds come back. But what obesity research has consistently shown is that dieters are at the mercy of their own bodies, which muster hormones and an altered metabolic rate to pull them back to their old weights, whether that is hundreds of pounds more or that extra 10 or 15 that many people are trying to keep off.