(CNN) After competing in "The Biggest Loser," many of the reality-show contestants have reluctantly regained the weight they sought to lose through extreme dieting and exercise.

"The truth is, I kind of spiraled," Ali Vincent, the first woman to win the competition, told Oprah last year. "I got home and I was like, 'What do I do?'

"I'm supposed to be strong. I'm supposed to know how to do this."

But some contestants discovered for themselves the key to keeping their weight down long after the cameras turned off, according to a study published Monday.

Six years after appearing on the show, a group of 14 contestants had kept off a median 13% of their original body weight. The researchers split the group in two -- the "maintainers," who had kept a substantial amount of the weight off, and the "regainers," who did not -- and looked at what set them apart.

"Did they cut more calories from their diet ... or did they become more physically active?" asked study author Kevin Hall , a senior investigator with the National Institute of Diabetes and Digestive and Kidney Diseases.

Hall had been "puzzled by how quickly the contestants were losing weight." So he contacted the show's physician and began studying the season eight contestants.

Early on, Hall observed that weight loss was better predicted by food intake, not physical activity alone. But in the long term, the opposite was true.

"You have to be very physically active in order to keep the weight off," Hall said.

The "maintainers," who kept off about 25% of their preshow weight on average, increased their physical activity by roughly 160% from before the competition. Those who regained the weight increased their activity only 34%. They were 1.1% heavier on average than their original weight.

The researchers couldn't find any major differences between the food intake of both groups; both cut their calories by about the same amount.

"While we know that the most important aspect of helping one lose weight is the dietary intake, that is pretty much reversed when it comes to weight loss maintenance," said Dr. Jennifer Kraschnewski , associate professor of medicine and public health sciences at the Penn State College of Medicine. She was not involved in the study.

Hall's findings echo previous studies that promote more exercise for long-term weight loss maintenance. However, experts caution that doesn't mean people can neglect their diets in the long run, either.

To maintain the weight loss, Hall's team estimated that contestants would need to engage in 80 minutes of moderate activity or 35 minutes of vigorous activity each day beyond what they were doing at baseline.

The researchers didn't observe the contestants working out directly. They used tools like traceable water molecules to figure out how much energy was expended -- the "gold standard" measurements in this type of research, Kraschnewski said. Many larger studies have instead asked people to self-report how much they exercised, which can be riddled with errors.

This method, however, was not able to tell how much of the physical activity was due to normal daily activities -- like cleaning the house or walking the dog -- versus following a workout plan, Hall said.

Part of the reason the contestants' weight rebounded was because of "metabolic slowing," Hall said. As contestants lost weight during the competition, their bodies burned fewer calories than expected.

"This can make it more difficult to lose weight and is another hurdle to overcome," Kraschnewski said. After going on a very low-calorie diet, "the body's mechanisms kick in to preserve body mass at all costs."

Six years later, however, the contestants' resting metabolisms still hadn't sped back up, Hall showed in a study last year.

This is a stark difference from people who have had gastric bypass surgery for weight loss, whose metabolisms may speed back up in a year's time, Kraschnewski said. Researchers are trying to figure out why this is -- and it may have something to do with the intricate connections between the gut, brain and hormones that regulate hunger, Kraschnewski added.

For Hall, metabolic changes are just one part of the puzzle. Genetics may also play a role, as well as social factors that can make it hard for some people to incorporate so much exercise in their daily lives.

"To do that requires a huge investment of time and energy," he said. "The bar is set very high for those folks to be able to maintain the weight loss without surgery or drugs."

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Hall added that it's not necessarily those who were most physically active during the competition who kept it up six years later.

"You can't really reproduce the expectation that those individuals must have had on the show to lose weight -- the pressure," Kraschnewski said.

"The take-home point here is that obesity really is a disease," she said. "It's not something that you can treat once and will cure it. It's something that has to be continually addressed in one's life."