It’s very hard to lose weight, but it might be even harder to keep it off. Researchers studying contestants who appeared on The Biggest Loser say our bodies make it nearly impossible to maintain extreme weight loss, The New York Times reports. They published their findings today in the journal Obesity.

“Just eat less and exercise more.” How many times have we heard that one? And for a long time, doctors believed that willpower was the key to weight loss. And yes, eating right and exercising is a good start. But as scientists dug deeper, they found that there’s a whole lot more to it. We’re learning now that countless factors affect the composition of our bodies, from the bacteria in our guts to the chemicals in our water.

More than one-third of American adults are obese. It’s no surprise, really, that weight and shedding it have become a national obsession, one that reached new levels in the form of extreme-weight-loss reality show The Biggest Loser. Each week, viewers tuned in to watch contestants endure grueling weight-loss regimens, dangerous water and calorie restriction, and verbal abuse from trainers.

You could say that these practices “worked”: The contestants certainly lost weight. Danny Cahill, who won the show’s eighth season, dropped a record 239 pounds. Yet since the show ended, he’s regained 100 pounds. Most of his fellow contestants are in the same boat. Some are even heavier than they were when they started, despite strenuous efforts to keep the weight off. So what gives?

That’s what Kevin Hall wanted to know. Hall is a metabolism expert at the National Institutes of Health (NIH), as well as a fan of reality TV. He decided to track Biggest Loser success stories for six years after the show to see how their bodies responded to the drastic change. Cahill and other Season 8 contestants became participants in a long-term experiment, in which their weight, calorie intake, and metabolic rates were measured.

Cahill didn’t need a laboratory to see that something was wrong. “All my friends were drinking beer and not gaining massive amounts of weight,” he told The New York Times. “The moment I started drinking beer, there goes another 20 pounds. I said, ‘This is not right. Something is wrong with my body.’”

The data agreed. Analysis of the contestants’ information revealed that their metabolisms were completely out of whack. At 295 pounds, Cahill’s body burned 800 fewer calories a day than another man his size. In other words, he’d have to eat even less than someone else to stay the same weight. His body was working hard to regain lost pounds. The same was true for his fellow contestants.

Normal, small-scale weight loss typically causes some metabolic changes. But the contestants’ metabolisms continued to slow even after they’d lost the weight. As time went on, to keep the same amount of weight off, they’d have to eat less and less.

“It is frightening and amazing,” Hall told the Times. “I am just blown away.”

Diabetes researcher Michael Schwartz was not a part of the study, but he found the results disturbing. “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,” he said. "As long as you are below your initial weight, your body is going to try to get you back.”

Obesity prevention expert David Ludwig, also not involved in the study, was equally concerned. “[These contestants are] a subset of the most successful [dieters],” he told the Times. “If they don’t show a return to normal in metabolism, what hope is there for the rest of us?”

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

[h/t New York Times]