What’s the best way to lose weight and keep it off?

Contestants on the reality TV show "The Biggest Loser" put in a gargantuan effort to lose weight: They diet rigorously, exercise for hours a day, and compete with each other to completely change their bodies. And many do change their bodies, quickly losing 100 pounds, 200 pounds — or more.

But what happens after? It turns out that, like many people who lose weight, maintaining that smaller frame is hard, even for those whose bodies changed so rapidly and completely.

A new study of 14 contestants from Season 8 of the show found that the majority had gained weight in the six years since, and some of them had gained back most or even all of what they lost. What surprised the researchers was this: While a drop in metabolism after the initial weight loss was expected, every participant’s metabolism remained much slower than it should be for someone their size — despite the passage of time.

“We were very surprised by the fact that despite regaining a substantial quantity of the weight that they had lost, their resting metabolic rate — that’s the number of calories their body is burning in order to just basically maintain life — [it] was still at this very low level that we observed at the end of the weight loss period on average,” says study author Kevin Hall, a researcher at the National Institutes of Diabetes and Digestive and Kidney Diseases in Bethesda, Maryland. “And [the resting metabolic weight] was much less — about 500 calories a day less — than would be expected for their newer, slightly higher body size. So this means that they're at some sort of disadvantage in terms of trying to keep the weight off.” 

Hall and his team aren’t completely sure why “The Biggest Loser” contestants have such a long-lasting decrease in their metabolism, but he wonders if it isn’t a result of the extreme amount of weight they shed over such a short period of time. 

“We don't know if it was something that was a result of this rapid weight loss that they experienced early on as part of the competition,” Hall says, “What we do know is that everybody experiences this sort of metabolic slowing to some extent when they try to lose weight, especially in the early time period. And what we think that we're able to show here is just how strong that can be when you look at extreme weight loss like these folks have experienced. I don't think that everyday folks will experience either this extreme amount of weight loss or this metabolic slowdown that these folks experienced in a quantitative sense, but they will experience it to a smaller degree.”

Hall compares the body to a spring, saying that it seems like the body might have a ‘set weight’ that it always ‘wants’ to return to. In other words? Weight loss is incredibly difficult, and takes a lot of work to both initially lose pounds, and then maintain that new weight.

“One of the perhaps counterintuitive findings that we obtained was that the people who had actually had the most success at maintaining the weight loss were the folks who actually were experiencing the greatest metabolic swelling,” Hall says. “So they were burning the fewest number of calories and the way we like to think about it is that the body is kind of behaving like a spring. And you can pull on the spring and intervene by dieting and exercising and you're able to stretch the spring out and analogously lose weight. But the harder the spring is [then] going to pull back on you. And so the folks who are most successful are pulling the hardest on the spring and stretching it out the most, but they're also the ones that are experiencing the greatest pull back. … The body has a sort of preferred weight that it would like to maintain and your efforts to change from that weight are met with some resistance.”

As part of his study, Hall compared “The Biggest Loser” contestants to people who undergo bariatric surgery to lose weight, and his results were surprising. Those who lost weight as a result of surgery also experienced a metabolic slowing, but that slowing didn’t last as long as those who took part in “The Biggest Loser.”

“Bariatric surgery subjects … also experienced the metabolic slowing within the first year, but after that, very interestingly, that greater than expected change in metabolic rate went away. In other words, their metabolism normalized a year after bariatric surgery. But that was obviously not the case in ‘The Biggest Loser’ contestants,” Hall says. 

In other words? Bariatric surgery seems to lead to longer-lasting weight loss than extreme lifestyle change weight loss.

“What we know is that, right now, those are the only persistent long-term reproducible kinds of interventions that we can do to lead to long term weight loss in people,” Hall says. “We're really trying to understand the biology of what's responsible for the relative success of bariatric surgery compared to the lifestyle interventions that we're all familiar with.”

For now, though, Hall’s advice for people who want to lose weight and keep it off is to not go for extreme weight loss, but just focus on the benefits of losing even just a little bit of weight with gradual and sustainable lifestyle changes. 

“I think a lot of people conflate the cosmetics of weight loss with the health benefits,” Hall says. “You don't actually have to lose the sort of ‘Biggest Loser’ style amounts of weight in order to improve your health. … If you could change your diet and eat healthier foods and add some exercise in your life — not crazy amounts like the biggest loser does on the program — you're going to see independent health benefits regardless of your weight. And if you make those changes and make them in a sustainable way that you could actually keep them and integrate them within your life over the long term, you know, your weight will settle at a different value or it might not settle at a different value but regardless your health will be improved.”

This article is based on an interview that aired on PRI's Science Friday.