After successfully dropping pounds, dieters often see their weight bounce back. But they may not see the same rebound in their sluggish metabolisms.

Researchers followed 14 contestants from the TV weight-loss competition The Biggest Loser, and they found that the dramatic weight loss significantly slowed the rate at which the contestants burned calories while resting. Those metabolic slow-downs, which make it more difficult to keep off pounds, lingered six years after the competition—even after nearly all of the contestants regained much of the weight they lost.

The findings, published Monday in the journal Obesity, suggest that the body may purposefully slow down its metabolism to regain lost pounds and maintain a weight “set point.” If the finding holds true in larger studies of dieters, it may explain why it’s so difficult to keep off weight once it's lost.

“Long-term weight loss requires vigilant combat against persistent metabolic adaptation that acts to proportionally counter ongoing efforts to reduce body weight,” the authors concluded.

Weight loss is known to spark metabolic changes, but researchers had little data on how long those changes might last. The researchers tracked the resting metabolic rate of 14 contestants from the TV show’s eighth season, comparing their measurements before, directly after, and six years after the competition.

While some of the contestants lost hundreds of pounds during the 30-week show, the contestants on average lost about 128 pounds. At the start, their resting metabolic rates were 2,607 +/-649 kilocalories per day. By the end, their mean rate dropped to 1,996 +/- 358 kcal per day. And, the researchers noted, those that lost the most weight saw the biggest drops in their metabolic rate.

Six years later, only one of the contestants weighed less than they did after the competition; the other 13 contestants had gained back sizable amounts. Five contestants regained almost all of or more than the weight they lost. But, despite the weight gain, their metabolic rates stayed low, with a mean of 1,903 +/- 466 kcal/day.

Based on their individual weights, the researchers estimate that the contestants were burning a mean of ~500 fewer kilocalories a day than would be expected of people their size.

Moreover, the researchers found that the people who kept the most weight off during the six years also saw the biggest slowdowns of their metabolism. This, the authors speculate, suggests that these successful contestants didn’t have an easier time fighting their metabolisms. Instead, they may have been exceptionally determined to maintain their weight loss—perhaps due to the fact that they lost their weight in the public’s eye.

“Of course, the extreme and public nature of this weight loss intervention makes it difficult to translate our results to more typical weight loss programs,” the authors wrote.

Indeed, although most of the contestants weren’t able to keep the weight off, they still did better than most. After six years, 57 percent of the contestants had maintained at least 10 percent of their weight loss. In another study, only about 20 percent of participants managed to do that after just a year.

While the findings may discourage some dieters, the authors note that studies of gastric bypass surgery patients did not reveal similar losses in metabolic rates. This, the authors speculate, may suggest that surgical interventions can reset the body’s weight “set point," a hypothesis that should be followed up in future research.

Obesity, 2016. DOI: 10.1002/oby.21538 (About DOIs).