CHICAGO — A new study has found that many competitors on NBC’s “The Biggest Loser” leave the show with a slower metabolism, making it more difficult to keep off the pounds.

The National Institutes of Health study finds that participants come out of the weight-loss reality competition burning about 500 fewer calories a day than expected. What’s more, the contestants who drop the most weight see the greatest slowing of their metabolisms.

Researchers say many contestants experience substantial weight gain in the years after the show.

The results show “in the most extreme cases how strongly the body fights back,” said lead author Kevin Hall, a researcher with the National Institute of Diabetes and Digestive and Kidney Diseases.

Hall said 500 calories is the size of a big lunch and the results mean participants have to reduce their daily calorie intake by that much to avoid gaining weight. He said the study doesn’t mean dieting is a lost cause but that show participants must change their lifestyles to fight weight gain.

The news isn’t all bad for “Biggest Loser” competitors, however. The study notes that participants have been quite successful at long-term weight loss when compared to people in other intervention programs aimed at shedding weight.

The study was published this month in the journal Obesity. It involved 14 contestants from Season 8 who were evaluated six years after the competition ended in 2009.

Kai Hibbard, a Season 3 contestant who has criticized the show for what she calls drastic weight-loss methods, said the results came as no surprise.

“I really was dancing around my living room, screaming ‘vindication’ ” when a friend texted her about the study, Hibbard said Tuesday from her home in Spokane, Washington.

Hibbard lost 118 pounds on the show nearly 10 years ago and has gained some but not all of it back. She was not part of the study and declined to reveal her weight.

Dr. Samuel Klein, a Washington University obesity researcher who wasn’t involved in the study, said the results reflect limitations of the “Biggest Loser” dieting approach.

“Nothing is impossible, but it shows that it’s very, very difficult. One year of aggressive therapy is really not enough,” Klein said. “You really have to go into a lifelong plan.”

Producers of the show released a written statement responding to the study, saying they “routinely re-evaluate to ensure all contestants receive the best care possible.”