The Biggest Loser, a reality TV show in which contestants compete to shed the most weight, has been a huge win for NBC — now airing all over the world in its 17th season.
It's also been a boon for obesity researchers, offering them an extremely rare opportunity to study the effects of intensive diet and exercise on a group of people trying to lose over 100 pounds on average.
The newest scientific study to use the contestants as subjects, published today in the journal Obesity, sheds light on why it can be so hard for people who've gained a lot of weight to keep it off once they've lost it. It also suggests the Biggest Loser approach, which involves extreme calorie restriction and several hours of exercise each day, may be particularly damaging to health.
For the paper, researchers at the National Institutes of Health followed up with Biggest Loser contestants from season eight. They took a number of measurements — body weight, fat, metabolism, hormones — at both the end of the 30-week competition in 2009, and again, six years later, in 2015.
Though all the contestants lost dozens of pounds through diet and exercise at the end of the show, by the six-year mark, their waistlines largely rebounded. Some 13 of the 14 contestants studied put a significant amount of weight back on, and four contestants are even heavier today compared to before they went on the show.
But the most remarkable finding was that the participants' metabolisms had vastly slowed down through the study period. The researchers think this at least partly explains why they had such a tough time keeping their weight off.
"Metabolic adaption": the body's way of fighting weight loss
For years, researchers have been documenting a phenomenon called "metabolic adaptation": As people lose weight, their basal metabolic rate — the energy used for basic functioning when the body is at rest — slows down. (The same thing happens when people add physical activity.)
For most people, the basal metabolic rate accounts for 60 to 80 percent of total energy expenditure. So if the body is burning fuel at a slower rate when at rest, a person needs to consume fewer calories just to maintain his or her body weight.
As I outlined recently in a story about why exercise isn't very helpful for weight loss, this effect has been documented in many contexts, though researchers don't quite understand why it happens, and it may not affect everyone the same way.
For one fascinating study, published in the journal Obesity Research in 1994, researchers subjected seven pairs of sedentary young identical twins to a 93-day period of intense exercise. For two hours a day, nearly every day, they'd hit a stationary bike.
The twins were also housed as in-patients in a research lab under 24-hour supervision and fed by watchful nutritionists who measured their every calorie to make sure their energy intake remained constant.
Despite going from being mostly sedentary to spending a couple of hours exercising almost every day, the participants only lost about 11 pounds on average. The participants also burned 22 percent fewer calories through exercise than the researchers calculated prior to the study starting.
One explanation for why adding an extreme amount of exercise didn't change the participants' body weights as much as expected: The subjects' basal metabolic rates slowed.
The new research on Biggest Loser found similar effects, but here, the researchers were able to show that contestants' metabolisms got even slower over time. "The magnitude of metabolic adaptation increased 6 years after the Biggest Loser competition," the researchers wrote.
Their bodies were essentially burning about 500 calories fewer (about a meal's worth) each day than would be expected given their weight.
And this effect persisted, despite the fact that most participants were slowly regaining the weight they lost.
The researchers also found interesting hormonal changes in Biggest Loser participants. They experienced significant reductions in the hormone leptin in their bloodstreams. Leptin is one of the key hormones that regulates hunger in the body and tells the body when it's full after eating.
By the end of the Biggest Loser competition, the researchers found that the contestants had almost entirely drained their leptin levels, leaving them hungry all the time. At the six-year mark, their leptin levels rebounded — but only to about 60 percent of their original levels before going on the show.
What does this mean for dieters?
The study may say more about the failures of the Biggest Loser approach to weight loss than whether we're all doomed if we try to lose weight.
In a 2014 study by the same group of researchers, also published in the journal Obesity, Biggest Loser participants were compared to people who underwent gastric bypass surgery for weight loss. This study found that the TV show contestants had five times less circulating leptin in their bodies and a greater degree of metabolic slow down compared with the surgical patients.
Yoni Freedhoff is an obesity specialist (and critic of the Biggest Loser approach) who was not involved with either study. In an interview, he observed that, "The bariatric surgery patients saw the metabolic adaption reverted after about a year. So it would appear that the Biggest Loser-style weight loss is devastating to a person's metabolism compared to surgery."
What isn't clear is whether a more gradual, non-surgical approach to weight loss would lead to the same outcome that the researchers found in the present study. "We don't have studies on people who slowly lost 40 percent of body weight and then tracked their metabolisms years later," Freedhoff said. "But we definitely know at this point that the Biggest Loser-style of weight loss is incredibly bad for a person's metabolism."
It's also not clear how generalizable the results about a very obese group of people — who were each trying to lose dozens of pounds — would apply to those who are overweight or have just a few pounds to lose.
For any would-be weight-loser, extreme diets and fitness regimens probably aren't usually sustainable. Diana Thomas, an obesity researcher at Montclair State University, noted that a number of studies of obese patients that involve intense levels of exercise — the kind seen on the reality show — have high numbers of drop outs.
"The fact research studies can’t [get obese people to stick to hours of daily exercise] tells you it’s probably not feasible for people to sustain this." In other words, while people can stick to it for a season of TV, it's less clear that it'll be feasible in daily living.
In order for people to keep weight off after the show, the show’s doctor, Robert Huizenga, told the New York Times that he prescribes nine hours of exercise per week. They'd also need to maintain an extremely restrictive diet. (The rate of metabolic slowdown would suggest they'd need to skip a meal a day to keep the weight off.)
"Clearly, the Biggest Loser dooms contestants to either a lifetime of superhuman weight loss efforts, or weight regain," Freedhoff said. "The take home message here is — if you're killing yourself to lose the weight in the gym and diet, you're killing your metabolism too."
Thomas says she tells patients who come into her obesity clinic to aim for just 5 percent weight loss initially.
"We know 5 percent is what you need to lose to see health benefits. We also know obesity is classified as a disease — so we need to treat is as a disease, not about getting into your skinny jeans." And certainly not as a reality TV spectator sport.