SILVER SPRING, Md. (WWAY) — Weight loss remains a big priority for millions of Americans but permanently dropping pounds has proved difficult in spite of a huge weight-loss industry. But a new study shows how dropping large amounts of weight can still lead to long-term issues for patients as their metabolism may slow dramatically.

The study, published today in the medical journal Obesity Biology and Integrated Physiology, followed 14 contestants of the “The Biggest Loser” TV show six years after they competed.

All except one of 14 contestants had slowed metabolic rates after losing weight, the study found. The researchers followed the contestants six years after their time on the show and found that all except one regained significant amounts of weight.

Out of the 14 contestants, 13 regained weight within 6 years and four are even heavier than they were before the competition began. Only one contestant, Erinn Egbert, sustained weight loss despite having a slower metabolism. She burned 552 less calories than what would be expected for another woman her size, the study found. There was only one contestant, Rudy Pauls, who had an improved metabolism but he underwent weight-loss surgery after the show to reduce the size of his stomach.

While the authors said further research is needed, the study points out how difficult it can be to achieve long-term weight loss.

“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.

Producers of “The Biggest Loser” told ABC News they are evaluating the study findings.

“We have comprehensive procedures and support systems in place which we routinely re-evaluate to ensure all contestants receive the best care possible,” the producers said in a statement. “The lead medical doctor on the show, who has worked with the National Institutes of Health on initiatives in the past relating to ‘The Biggest Loser,’ has been made aware of this most recent study and is in the process of evaluating its findings.”

Experts said the study could be important by showing those struggling with losing weight how much of weight loss is physiological and not just a matter of “willpower.”