At the end of every season of The Biggest Loser, there is an unspoken, collective bracing by the contestants and the public. Sixteen contestants have made impressive progress in transforming their bodies, often losing hundreds of pounds and building new muscle. On the show’s ranch, diet and exercise are everything—but what would happen when they re-entered the real world, with personal and professional obligations and distractions? A new study published Monday in the journal Obesity indicates that these contestants face much more than mental challenges in their effort to maintain their weight loss.

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A group of researchers, led by Dr. Kevin Hall, a metabolism expert at the National Institute of Diabetes and Digestive and Kidney Diseases, studied what happened to the bodies of Season 8 contestants six years after the show.

The research found that after such an intense period of diet and exercise, the contestants’ bodies fought tirelessly to regain the lost weight.

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“It’s frightening and amazing,” said Hall in an interview with the New York Times. “I am just blown away.”

Two main factors contribute: the decrease of a hormone called leptin, which controls hunger, and a damaged resting metabolism. When contestants began the show, their resting metabolism (meaning the number of calories they burned daily) was normal. While any dieting regimen will result in a decreased metabolism, the contestants changes were so radical that it was virtually impossible to eat so little that the body could maintain its current weight. Even more surprising was that in the six years following, their metabolisms never recovered—in fact, they became even slower.

The New York Times reports:

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Six years after Season 8 ended, 14 of the 16 contestants went to the N.I.H. last fall for three days of testing. The researchers were concerned that the contestants might try to frantically lose weight before coming in, so they shipped equipment to them that would measure their physical activity and weight before their visit, and had the information sent remotely to the N.I.H. The contestants received their metabolic results last week. They were shocked, but on further reflection, decided the numbers explained a lot.

The season’s winner, Danny Cahill, started the show at 430 pounds and ended at 191 pounds—the biggest weight loss in the show’s history. He now weighs 295 pounds and burns 800 fewer calories per day than one would expect from a man his size. Dina Mercado started at 248 pounds and ended at 173 pounds. She now weights 205.9 pounds and burns 437.9 fewer calories per day than expected. Sean Algaier started at 444 pounds and ended at 289 pounds. He has regained all the weight and then some—he now weighs 450 pounds and burns 458 fewer calories per day than expected.

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“All my friends were drinking beer and not gaining massive amounts of weight,” Cahill told the Times. “The moment I started drinking beer, there goes another 20 pounds. I said, ‘This is not right. Something is wrong with my body.’”

Recently, Ali Vincent, who went from 234 pounds to 112 pounds to become the winner of season five and the series’ first female champion, posted on her own Facebook about nearly returning to her pre-show weight.

“I have had successes and I have had major losses. I have gone from feeling alone to having thousands of people reach out with support. I have experienced ultimate highs that I could have never dreamed of as well as nightmares I wouldn’t wish on an enemy. Quite frankly some of them have gotten the better of me and I have struggled,” she wrote, announcing that she had joined Weight Watchers.

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Dr. David Ludwig, the director of the New Balance Foundation Obesity Prevention Center at Boston Children’s Hospital, cautioned the Times against putting too much stock in the study’s results—the sample size is quite small and there was no control group of people who did not lose weight. But he was not surprised by its findings.

“This is a subset of the most successful [dieters,]” he said. “If they don’t show a return to normal in metabolism, what hope is there for the rest of us?”

He continued, “That shouldn’t be interpreted to mean we are doomed to battle our biology... It means we need to explore other approaches.”