This article originally appeared on Time.com.
Like most people, Kevin Hall used to think the reason people get fat is simple.
“Why don’t they just eat less and exercise more?” he remembers thinking. Trained as a physicist, the calories-in-vs.-calories-burned equation for weight loss always made sense to him. But then his own research–and the contestants on a smash reality-TV show–proved him wrong.
Hall, a scientist at the National Institutes of Health (NIH), started watching The Biggest Loser a few years ago on the recommendation of a friend. “I saw these folks stepping on scales, and they lost 20 lb. in a week,” he says. On the one hand, it tracked with widespread beliefs about weight loss: the workouts were punishing and the diets restrictive, so it stood to reason the men and women on the show would slim down. Still, 20 lb. in a week was a lot. To understand how they were doing it, he decided to study 14 of the contestants for a scientific paper.
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Hall quickly learned that in reality-TV-land, a week doesn’t always translate into a precise seven days, but no matter: the weight being lost was real, speedy and huge. Over the course of the season, the contestants lost an average of 127 lb. each and about 64% of their body fat. If his study could uncover what was happening in their bodies on a physiological level, he thought, maybe he’d be able to help the staggering 71% of American adults who are overweight.
What he didn’t expect to learn was that even when the conditions for weight loss are TV-perfect–with a tough but motivating trainer, telegenic doctors, strict meal plans and killer workouts–the body will, in the long run, fight like hell to get that fat back. Over time, 13 of the 14 contestants Hall studied gained, on average, 66% of the weight they’d lost on the show, and four were heavier than they were before the competition.
That may be depressing enough to make even the most motivated dieter give up. “There’s this notion of why bother trying,” says Hall. But finding answers to the weight-loss puzzle has never been more critical. The vast majority of American adults are overweight; nearly 40% are clinically obese. And doctors now know that excess body fat dramatically increases the risk of serious health problems, including Type 2 diabetes, heart disease, depression, respiratory problems, major cancers and even fertility problems. A 2017 study found that obesity now drives more early preventable deaths in the U.S. than smoking. This has fueled a weight-loss industry worth $66.3 billion, selling everything from diet pills to meal plans to fancy gym memberships.
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It’s also fueled a rise in research. Last year the NIH provided an estimated $931 million in funding for obesity research, including Hall’s, and that research is giving scientists a new understanding of why dieting is so hard, why keeping the weight off over time is even harder and why the prevailing wisdom about weight loss seems to work only sometimes–for some people.
What scientists are uncovering should bring fresh hope to the 155 million Americans who are overweight, according to the U.S. Centers for Disease Control and Prevention. Leading researchers finally agree, for instance, that exercise, while critical to…