A new analysis of data from “The Biggest Loser” highlights the complex ways the body compensates when we drop pounds.

Many of us remember “The Biggest Loser,” the somewhat notorious reality television show that ran for more than a decade starting in 2004, in which contestants competed feverishly to drop massive amounts of weight over a short period of time. One of the biggest lessons of the show appeared to be that extreme exercise, along with draconian calorie restriction, would lead to enormous weight loss.

Media coverage of the contestants years later, though, seemed to tell a different story, of weight regain and slowed metabolisms and the futility of attempting long-term weight loss.

Now a new scientific analysis of the show and its aftermath, published last month in the journal Obesity, suggests many beliefs about “The Biggest Loser” may be misconceptions. The analysis tries to untangle what really happened to the contestants’ metabolisms and why some of them kept off weight better than others. It also looks into the complex role of exercise and whether staying physically active helped the contestants keep their weight under control for years, or not.

For those who may have forgotten, or tried to, “The Biggest Loser” ran on NBC to generally high ratings for more than a dozen seasons. Contestants competed to drop the most pounds using extreme calorie restriction and hours of daily strenuous exercise. “Winners” typically shed hundreds of pounds in a few months.

Such rapid and extreme weight loss caught the attention of Kevin Hall, a senior investigator at the National Institute of Diabetes and Digestive and Kidney Diseases, which is part of the National Institutes of Health. An expert on metabolism, Dr. Hall knew that when people drop lots of weight in a short period of time, they typically send their resting metabolic rates – the baseline calories we burn every day just by being alive – into free-fall. A lower resting metabolic rate can mean we burn fewer calories over all.

This effect was believed to be caused, in part, by the loss of muscle during dieting. A relatively active tissue, muscle burns more calories than fat, and more muscle typically means elevated metabolic rates. So, Dr. Hall wondered, would the maniacal levels of exercise during “The Biggest Loser” help dieters hold onto muscle and keep their resting metabolism high, even as they cut calories?

Starting more than a decade ago, Dr. Hall and his colleagues began the first of a series of experiments to find out. In a study from 2014, they compared 13 men and women who had lost massive amounts of weight by cutting calories, thanks to gastric bypass surgery, and 13 contestants from “The Biggest Loser,” whose extreme weight loss involved exercise as well as dieting. As expected, the bypass group shed muscle, as well as fat, while “The Biggest Loser” contestants kept most of their muscle and primarily dropped fat. But everyone’s resting metabolic rate dropped, and to about the same extent, whether they remained well-muscled or not.

Dr. Hall said he and his colleagues were surprised by the results. And their confusion intensified when, for a 2016 study, they rechecked 14 of the same contestants six years after their competition, expecting their metabolisms to have rebounded by then. Most dieters’ resting metabolisms rise somewhat after they stop actively losing weight, and especially if they regain pounds. Larger people burn more baseline calories than people who are slighter. By this time, most of the contestants had regained weight. But their resting metabolisms remained stubbornly slow, burning an average of about 500 fewer daily calories than before they joined the show.

The next year, a follow-up study concluded physical activity had helped some contestants stave off weight gain. If they moved around or formally exercised for about 80 minutes most days, they added back fewer pounds than if they rarely worked out. But their exercise did not boost their resting metabolisms. The exercisers, in fact, showed the greatest relative declines in their resting metabolic rates.

Perplexed, Dr. Hall recently began to reconsider the “Biggest Loser” studies in light of an emerging concept about how human metabolism fundamentally works. This idea grew out of an influential 2012 study showing that highly active hunter gatherers in Tanzania burn about the same relative number of calories every day as the rest of us, even though they move around far more.

The scientists involved in that research postulated the tribespeople’s bodies must automatically be compensating for some of the calories they burned while hunting for food by decreasing other physiological activities, such as growth. (The tribespeople tended to be short.) In that way, the researchers felt, the hunters’ bodies could keep the overall number of calories they burned each day in check, no matter how many miles they jogged in search of tubers and game. The scientists called this idea the constrained total energy expenditure theory.

Aware of this research, Dr. Hall began to see potential parallels in “The Biggest Loser” results. So, for the new analysis, he looked back at his group’s data for hints about whether contestants’ metabolisms had behaved, in effect, like the metabolisms of the hunter gatherers. And he found clues in their resting metabolic rates. That number plummeted early in their “Biggest Loser” filming, he noted, when they slashed how much they ate, and their bodies, understandably, reduced the calories they burned to avoid starving.

But in later years, when contestants typically returned to eating as they had before, their metabolisms stayed depressed because, he concluded – and this was key – most of them still exercised. Counter-intuitively, he wrote in the new analysis, frequent physical activity seems to have prompted their bodies to hold resting metabolic rates low, so total daily energy expenditure could be constrained.

“It’s still just a hypothesis,” Dr. Hall said, “but it seems like what we’re observing” in the “Biggest Loser” data “is an example of the constrained energy model.”

So, what could this rethinking of “The Biggest Loser” story mean for the rest of us, if we hope to keep our weight under control? First and most fundamentally, it suggests that abrupt and colossal weight loss generally will backfire, since that strategy seems to send resting metabolic rates plunging more than would be expected, given people’s smaller body sizes. When people drop pounds gradually in weight-loss experiments, he pointed out, their metabolic changes tend to be less drastic.

Second and more befuddling, if you have lost substantial weight, “Biggest Loser” style, exercise likely will be both ally and underminer in your efforts to keep those pounds at bay. In Dr. Hall’s new interpretation of contestants’ long-term weight control, frequent exercise kept contestants’ resting metabolic rates low but also helped them stave off fat regain. In essence, the contestants who worked out the most wound up adding back the least weight, even though they also sported the slowest relative resting metabolisms.

Exactly how, then, exercise aided with their weight maintenance is not yet clear, Dr. Hall said. He suspects that exercise affected people’s appetites in ways that may have made them less prone to overeating, while also burning some extra calories. He hopes to develop future experiments to elucidate how exercise influences metabolisms, for better and worse, he said.

For now, though, the most reverberant lesson of “The Biggest Loser” may be that long-term weight loss, although daunting, is not unfeasible. Yes, most “Biggest Loser” contestants regained weight, Dr. Hall said, but not necessarily every pound they dropped. After six years, most still weighed about 12 percent less than before joining the show, a meaningful difference, and the most successful of the former contestants were those who still worked out.


By Gretchen Reynolds for The New York Times