By Kathryn Doyle

(Reuters Health) - Six years after dramatic weight loss on the TV show "The Biggest Loser," most contestants in a recent study had regained the pounds - and on top of that, their metabolism had slowed and they were burning fewer calories every day than they did before their stint on the show.

Researchers studied 14 contestants who participated in the 30-week competition, which involves intensive diet and exercise training.

They started at an average weight of 328 pounds (about 149 kg) and ended at an average weight of 200 pounds (about 91 kg).

Six years later, when the six men and eight women went to the National Institutes of Health for follow-up measurements, their weight, on average, was back up to 290 pounds. Only one participant hadn't regained any weight.

Similarly, percent body fat started at an average of 49 percent, dipped to 28 percent and returned to 45 percent over time.

But resting metabolic rate did not follow the same pattern.

The group as a whole on average burned 2,607 calories per day at rest before the competition, which dropped to about 2,000 calories per day at the end.

Six years later, calorie burning had slowed further to 1,900 per day, as reported in the journal Obesity, May 2.

The slower the metabolism, the more a person has to cut back on calories in order to keep from gaining weight.

"There used to be a mythology that if you just exercised enough you could keep your metabolism up, but that clearly wasn't the case, these folks were exercising an enormous amount and their metabolism was slowing by several hundred calories per day," said senior author Kevin D. Hall of the National Institutes of Health (NIH) in Bethesda, Maryland.

Their metabolisms didn't speed up again when they regained the weight, he told Reuters Health by phone.

Perhaps counterintuitively, participants whose metabolisms had slowed the most at the six-year point tended to have regained less weight.

Metabolism appears to act like a spring, Hall said: the more effort you exert to lose weight, the more it stretches out, and the harder it will spring back, regaining and holding onto the fat that was lost.

"Your body is working to defend your energy stores - really your fat mass," said Dr. Michael Rosenbaum of Columbia University Medical Center in New York who was not part of the new study.

"When that fat mass is diminished (either by eating less or exercising more) most of us respond by changes in brain circuitry that increase our tendency to eat and changes in neural and endocrine systems, and especially muscle, that make us more metabolically efficient - it costs fewer calories to do the same amount of work," Rosenbaum told Reuters Health by email.

The Biggest Loser is an extreme weight loss program, and these results may not translate to other methods or approaches to weight loss, Hall said.

"The aim of The Biggest Loser program is entertainment, not sustained contestant weight loss," said professor John R. Speakman of the University of Aberdeen. "It is not a prescription for how to change your life."

The only weight loss method that seems to avoid metabolic pitfalls is gastric bypass surgery, Hall said. People who undergo Roux-en-Y gastric bypass surgery experience a similar dip in metabolic rate along with massive weight loss at the six month point, but after one year they have the expected metabolic rate for their size, rather than the much reduced rate of Biggest Loser contestants, he said.

"It may be that there is something special about bariatric surgery, resetting some set point in the body to not resist the weight loss," he said.

But The Biggest Loser results are not uniformly dire, Hall said. On average, the group regained much of their weight but did maintain about 12 percent weight loss even after six years, had better cholesterol profiles, and none had developed diabetes during follow-up.

"You don't need to lose that much weight to see benefits," he said. "A lot of people conflate the cosmetics of weight loss with the health benefits of weight loss."