Bewildered, Dr. Hall recently reconsidered 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 as the rest of us every day, even though they exercise a lot more.
The scientists involved in that study hypothesized that the tribesmen’s bodies had to automatically compensate for some of the calories they burned while hunting for food by reducing other physiological activities, such as growth. (The tribesmen were usually short.) That way, the researchers believed, the hunters’ bodies could control the total number of calories they burned each day, regardless of how many miles they jogged in search of tubers and game. The scientists called this idea the theory of limited total energy consumption.
Aware of this research, Dr. Hall can see possible parallels in the results of “The Biggest Loser”. So for the new analysis, he looked back to his group’s data for hints as to whether the participants’ metabolisms had, in fact, behaved like the hunter-gatherers’ metabolisms. And he found clues in their resting metabolism. That number plummeted early in their “Biggest Loser” recordings, he noted, as they reduced how much they ate, and their bodies, understandably, reduced the calories they burned to avoid starvation.
But in later years, when the participants habitually returned to eating as before, their metabolisms remained depressed because, he concluded — and this was the key — most of them still exercised. Counterintuitively, he wrote in the new analysis, frequent physical activity appears to have prompted their bodies to keep resting metabolic rates low so that overall daily energy expenditure can be reduced.
“It’s still just a hypothesis,” said Dr. Hall, “but it seems that what we observe” 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’re hoping to keep our weight in check? First and most fundamentally, said Dr. Hall, suggests that abrupt and colossal weight loss in general will backfire, as that strategy appears to lower the resting metabolic rate more than would be expected given people’s smaller body sizes. When people gradually shed pounds in weight loss experiments, he pointed out, their metabolic changes tend to be less drastic.
Second, and more confusingly, if you’ve lost significant weight, the “Biggest Loser” style, exercise is likely to both ally and undermine your efforts to keep those pounds at bay. in dr. Hall’s new interpretation of participants’ long-term weight management, frequent exercise kept participants’ resting metabolic rates low, but also helped them avoid fat regain. Essentially, the participants who worked the most ended up adding the least weight, although they also had the slowest relative resting metabolic rate.