Regular exercise after weight loss may slow metabolism, study suggests

Many are familiar with the American sitcom “The Biggest Loser”, a somewhat notorious reality show that ran for more than a decade on American TV, starting in 2004, in which contestants feverishly competed to lose massive weight in a short time.

One of the biggest lessons the program seems to have taught is that intense exercise coupled with draconian calorie restriction would lead to massive weight loss.

But media coverage of the participants years later seems to have revealed a different reality involving weight regained, metabolisms that have slowed, and the futility of trying to lose weight in the long term.

A new scientific analysis of the show and its aftermath, published in May in the journal Obesity, suggests that many of the show’s common ideas may be misguided. The analysis seeks to unravel what actually happened to the participants’ metabolism and why some of them managed to maintain their weight loss better than others.

The study also looks at the complex role exercise plays and whether or not staying physically active helped participants keep their weight under control for years.

For those who don’t know, “The Biggest Loser” was broadcast on the American network NBC, with a generally large audience, for more than 12 seasons. Participants competed to lose the most pounds using extreme calorie restriction and hours of intensive physical exercise. The “winners” usually shed a lot of pounds in a few months.

This rapid and extreme weight loss caught the attention of Kevin Hall, a senior researcher at the National Institute of Diabetes and Digestive and Kidney Diseases (USA), which is part of the US National Institutes of Health.

A metabolism expert, Hall knew that when people lose a lot of weight in a very short period of time, it usually causes a sharp drop in their basal metabolic rate — the calories we burn daily just to stay alive. A lower basal metabolic rate can mean we burn fewer calories overall.

This effect was thought to be caused in part by the loss of muscle mass produced when people dieted. A relatively active tissue, muscle burns more calories than fat, and more muscle mass generally means higher metabolic rates.

So Hall wondered: Would the extreme levels of exercise practiced during “The Biggest Loser” help the participants conserve their muscle mass and keep their basal metabolism high while cutting calories?

Starting more than a decade ago, Hall and his colleagues began the first in a series of experiments to find the answer.

In a 2014 study, they compared 13 men and women who had lost massive amounts of weight through reduced calories thanks to gastric bypass surgery with 13 participants in “The Biggest Loser,” whose extreme weight loss involved exercise and diet. .

As predicted, the bariatric surgery group lost muscle mass in addition to fat, while the “The Biggest Loser” participants retained most of their muscle mass, losing mostly fat. But everyone’s basal metabolic rate dropped, and at about the same rate, whether people kept muscle or not.

Hall said he and his colleagues were surprised by the results. And their confusion increased when, for a study done in 2016, they re-examined 14 of the same participants six years after the competition, in the expectation that their metabolism would already have picked up again.

In most cases, the basal metabolism of dieters goes up quite a bit after people stop actively losing weight, and especially if they regain the weight. Heavier people burn more basal calories than lighter people.

By 2016, most of the reality show contestants had already regained their weight. But their basal metabolism remained stubbornly low, burning an average of 500 fewer calories a day than had been the case before they participated in the program.

A follow-up study the following year concluded that physical activity had helped some competitors avoid weight gain.

Those who moved or exercised formally for about 80 minutes most days gained fewer pounds than those who rarely exercised. But the exercise they were doing didn’t raise their basal metabolism. In fact, those who exercised showed the greatest relative drops in their basal metabolic rates.

Puzzled, Hall has recently begun to re-evaluate the studies on “The Biggest Loser,” looking at them in light of an emerging concept of how human metabolism actually works. This idea was born out of an influential 2012 study that showed that highly active people in Tanzania who live by hunting and gathering burn the same relative number of calories daily as the rest of us, despite moving around much more.

Scientists involved in the study postulated that the bodies of Tanzanian hunters must be automatically compensating for some of the calories they burned while hunting for food, reducing other physiological activities such as growth (hunters tended to be short stature).

In this way, the researchers thought, the hunters’ bodies could limit the total number of calories they burned daily, regardless of how many kilometers they ran in search of roots and animals to hunt. Scientists have called this idea the theory of constrained total energy expenditure.

Aware of this research, Hall began to see potential parallels in the results of “The Biggest Loser.” So for the new analysis, he re-examined his group’s data for signs that the participants’ metabolism would have, in practice, behaved like the metabolism of hunter-gatherers. And he found clues in his basal metabolic rates.

Those rates dropped sharply early on in the filming of “The Biggest Loser,” he noted, as participants reduced their food intake and their bodies, understandably, reduced the number of calories they burned, to keep them from starving.

But in later years, when contestants typically reverted to their earlier eating patterns, their metabolisms remained sluggish because, Hall concluded — and this was the key factor — most of them were still exercising.

Counterintuitively, he wrote in the new analysis, frequent physical activity appears to have caused his body to keep its basal metabolic rate low, so that total daily energy expenditure could be limited.

“This is still hypothetical,” Hall said, “but it looks like what we’re seeing” in the data on participants in “The Biggest Loser” “is an example of the constrained energy model.”

Translation by Clara Allain