The Biggest Losers did what it took to be winners. They cut their grapefruits in half and separated their whites from their yolks. They spent more time at the gym than most of us spend at the office. Some of them shed an entire pound of fat each day, and on average, they lost 128 pounds each over the course of the 30-week reality show.

And then they gained much of it back. According to a devastating new study published in the journal Obesity and written up in a widely shared article by the New York Times last week, only one of the 14 Biggest Loser contestants studied weighs less today than when the competition wrapped. Four of them are now heavier than before they went on the show.

The culprit appears to be the contestants’ metabolisms, which slowed down dramatically as they lost weight. Now, they must eat hundreds of calories less each day than people of a comparable size, just to maintain their new svelte sizes. Failing that, they regress back to their “set points,” as some obesity researchers call the body’s preferred weight. The study suggests the bodies of obese people work against them, making it harder to get in shape even for those who really need to.

“For most people, the combination of incessant hunger and slowing metabolism is a recipe for weight regain,” David Ludwig, the director of the New Balance Foundation Obesity Prevention Center at Boston Children’s Hospital, told the paper.

We’ve known for decades that profound, sudden weight loss can throw the brakes on metabolism.

For a comparison stunt, one need only look at the Minnesota Starvation Study, which was conducted by the scientist Ancel Keys in the mid-1940s. Toward the end of World War II, millions across Europe and Asia had either starved or were at risk of starvation. Keys sought to understand the mechanisms of famine so that relief groups would know how best to help the hungry. Over the course of six months, he starved his 36 normal-weight subjects until they lost a quarter of their body weights. Their metabolic rates slowed just like those of the Biggest Loser contestants did. But in the Minnesota case, metabolism bounced back during the “refeeding” period. They all ended up at their pre-experiment weights—which, again, were healthy.

Lara Dugas, an epidemiologist who studies energy metabolism at Loyola University in Chicago, thinks the differences between the Minnesota Starvation study participants and the Biggest Loser contestants could be important. First, the Minnesotans were all of normal weight to start, compared to the Biggest Loser contestants, who are all morbidly obese. Second, the Minnesota study participants lost less of their body weights—25 percent compared to 40 percent with the Biggest Losers.

It all underscores how obesity has more to do with complex environmental and biological factors than a simple lack of willpower. But findings like these don’t necessarily mean it’s hopeless to try to lose weight. The key might be in how you do it.

Dugas thinks one element might be how quickly a person loses weight. The Biggest Loser contestants melted the fat away quickly in order to satisfy TV ratings, but most weight-loss programs recommend that people lose about one to two pounds per week.

As is usually the case with weight-loss research, the evidence here is conflicting: Some studies show that the rate of weight loss doesn’t matter, and others suggest rapid weight loss might even work best. But some papers do show that people who lose weight at their own pace, rather than through a very low-calorie diet, have more long-term success.

“The speed at which they lose weight may be a factor,” Dugas told me. “The key may be that when you have a dramatic physiological insult, such as a very low calorie diet coupled with extreme physical activity training, the resting metabolic rate might not return to normal.”

Finally, the method of weight loss might matter. Another study compared a group of gastric-bypass patients to Biggest Loser contestants and found that the bypass patients also saw their metabolisms slow. But surprisingly, their metabolic rates bounced back after a year, even though those subjects also lost weight. Other studies have similarly found no negative effects on metabolism after gastric bypass. That means until scientists find the elusive “reset” button for body weights, gastric bypass might be a better bet for obese people than reality-show-esque stunts.