Study finds out why some 'Biggest Loser' contestants can't stay fit

NBC’s "The Biggest Loser" aims to help morbidly obese contestants reach a healthy weight. Over the years, a number of competitors have struggled to keep the weight off, and now scientists may know why.

A study in the journal Obesity tracked season 8 contestants after the show ended. More than six years later, 13 of the 14 people studied gained back some of the weight. Five were over or close to their starting weights from the beginning of the show. Researchers found this was because of their bodies' shifting metabolism.

When someone loses weight, his or her body starts burning fewer calories to try to compensate. Usually, the body has time to gradually adjust. But sudden, rapid weight loss, like on "The Biggest Loser," could be too much of a shock for the body to handle.

One of the study's authors said he was "blown away" when he studied the contestants and learned that their metabolisms kept getting slower as time went on, even as they packed on more pounds.

"Your body is trying to slow down and resist further weight loss and actually promote weight regain, and you’re fighting against that at the same time as you’re fighting against an increased appetite," Dr. Kevin Hall told The New York Times.

Season 8 winner Danny Cahill went from over 400 pounds at the beginning of the show to below 200. Now he’s back up to nearly 300 pounds. Researchers said that’s because Cahill burns 800 fewer calories per day than an average man of his size.

The show’s doctor said he was aware that it could happen but hoped that the metabolic drop off wouldn’t be so severe. Other experts said this should have been expected.

Obesity researcher Dr. Michael Schwartz told The New York Times: "You can’t get away from a basic biological reality. As long as you are below your initial weight, your body is going to try to get you back."

This video includes clips from NBC, People and The New York Times.

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