How Exercise Affects Metabolism and Weight Loss

Confused, Dr. Hall recently began revisiting the ‘Biggest Loser’ studies in light of the emerging concept of how fundamentally human metabolism works. This idea arose from an influential 2012 study showing that highly active hunters in Tanzania burn the same relative number of calories each day as the rest of us, even though they move much more.

Scientists involved in this research hypothesized that tribe members’ bodies should automatically compensate for some of the calories they burned while foraging by reducing other physiological activities, such as growth. (Tribal members tended to be shorter.) In this way, the researchers felt that the hunters’ bodies could keep the total number of calories they burn each day in check, no matter how many miles they ran in search of tubers and game. Scientists have called this idea the total constrained energy expenditure theory.

Recognizing this research, Dr. Hall began to see potential similarities in the findings for The Biggest Loser. So, for the new analysis, he looked at his group’s data for hints about whether the runners’ metabolisms were, in fact, behaving like those of the hunters. And he found evidence in resting metabolic rates. That number dropped early in the filming of “The Biggest Loser,” he noted, when they cut back on how much they ate and, understandably, lowered their bodies the calories they burned to avoid starvation.

But in later years, when runners usually went back to eating as before, their metabolisms remained depressed because he concluded – and this was key – that most of them were still exercising. Contrary to intuition, he wrote in the new analysis, repetitive physical activity appears to have prompted their bodies to keep their resting metabolic rates low, so their total daily energy expenditure could be restricted.

“It’s still just a hypothesis,” said Dr. Hall, “but what we observe in the ‘Biggest Loser’ data appears to be an example of a constrained energy model.”

So, what might rethinking the “Biggest Loser” story mean for the rest of us, if we hope to keep our weight in check? First, and most importantly, Dr. Hall said, he notes that sudden, massive weight loss will generally be counterproductive, as this strategy appears to send resting metabolic rates lower than expected, given people’s smaller body sizes. He noted that when people gradually lose pounds in weight-loss experiences, their metabolic changes tend to be less severe.

Second, and more confusingly, if you’ve lost significant weight, the “Biggest Loser” style, exercise can potentially be an ally and undermine your efforts to keep those kilos at bay. In Dr. Hall’s new interpretation of long-term weight control in runners, repetitive exercise kept runners’ resting metabolic rates low, but also helped them avoid fat recovery. In essence, the runners who worked out the most ended up adding the least weight again, even though they also exercised their relative slowest metabolism at rest.

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