How much weight you should lose each week if you want to keep it off, according to a nutritionist

When JD Roth first got involved with "The Biggest Loser," the reality show in which people compete to lose the most weight in five months, he assumed he could get contestants to shed about 100 pounds each.

The doctors he spoke with, however, had a much different idea. They told him people should only lose about one to two pounds per week, or around 30 pounds in one season of the show.

When Roth took that number to the network, they turned him down. So season after season, the show's contestants lost one to two pounds per day - essentially seven times what doctors had said was healthy.

Today, many former "Biggest Loser" winners have regained some, all, or more weight than they lost. A 2016 study from the National Institutes of Health found that of the 14 former "Biggest Losers" they followed, 13 regained a significant portion of the weight they lost on the show. Four were heavier in 2016 than they were before they arrived on the set.

Experts say there are many reasons for this, but one glaring cause is that the contestants were simply losing too much weight too fast. That approach doesn't give people the time they need to create new lifestyle patterns like cooking with new ingredients, substituting fast food for meals and snacks that are packed at home, and incorporating more vegetables and whole grains into their diets.

"You've got to give yourself two, three, four years of consistent behavioral changes. That is hard work. You're building new habits. And that takes time," Andy Bellatti, a registered dietitian and the cofounder of Dietitians for Professional Integrity, told Business Insider.

Bellatti and other experts agree that losing one to two pounds per week is a good ballpark number for safe, sustainable weight loss.

Philip Stanforth, a professor of exercise science at the University of Texas and the executive director of the Fitness Institute of Texas, says losing "a few pounds a week" is "a lot more sustainable than losing a whole bunch at once."

Losing weight gradually gives people enough time to create new healthy eating and exercise patterns that they can maintain for life.

"I'd say nine times out of 10 the people who change slowly and do manageable goals are the people who three years out still have success," said Bellatti. "I know many people who've gone on some kind of crash diet for a week and lose a bunch of weight and a few months later they're back to square one."