When more than 40 former contestants from “The Biggest Loser” gather today for a reunion TV special, the winner of the program’s first season, Ryan Benson, who lost 122 pounds of his 330-pound starting weight, will be absent. Benson is now back above 300 pounds but says the reason he believes he has been shunned by the show is that he publicly admitted that he had dropped some of the weight by fasting and dehydrating himself to the point that he was urinating blood.

“The Biggest Loser” is one of NBC’s most-watched prime-time programs, drawing an estimated 10 million viewers each week.

It has clearly tapped into the American obsession with losing weight, as more than 200,000 people a year submit audition videotapes or attend open casting calls for the program. The series, in its eighth season, has spawned a licensed merchandise business that will generate an estimated $100 million this year.

It also highlights the difference between the pursuit of engaging television and the sometimes frenzied efforts of contestants to win, perhaps at the risk of their own health. Doctors, nutritionists and physiologists not affiliated with “The Biggest Loser” express doubt about the program’s regimen of severe caloric restriction and up to six hours a day of strenuous exercise, which cause contestants to sometimes lose more than 15 pounds a week.

At least one other contestant has confessed to using dangerous weight-loss techniques, including self-induced dehydration. On the first episode of the current season, two contestants were sent to the hospital, one by airlift after collapsing from heat stroke during a one-mile race.

Medical professionals generally advise against losing more than about 2 pounds a week. Rapid weight loss can cause many medical problems, including weakening of the heart muscle.

“I’m waiting for the first person to have a heart attack,” said Dr. Charles Burant, a professor of internal medicine at the University of Michigan Health System and director of the Michigan Metabolomics and Obesity Center. “I think the show is so exploitative. They are taking poor people who have severe weight problems whose real focus is trying to win the quarter-million dollars.”

Dr. Rob Huizenga, the medical consultant to “The Biggest Loser” and an associate clinical professor of medicine at UCLA, said the program is safe. “This is not only a major amount of weight loss, it is a totally different kind of weight loss compared with surgery or starvation diets,” he said.

In interviews, the show’s trainers and producers acknowledge that unsafe practices can occur.

“If we had it to do over, we wouldn’t do it,” Huizenga said of the recent one-mile race that resulted in hospitalizations. “It was an unexpected complication and we’re going to do better,” he said, adding that “that challenge has changed a lot of the way we do things,” including more closely monitoring contestants’ body temperatures during exercise.

JD Roth, an executive producer of the series who created its current format, said that while the show was extreme, “it needs to be extreme in my opinion.”

“For some of these people, this is their last chance,” he said. “And in a country right now that is wrestling with health care issues and the billions of dollars that are spent on obesity issues per year, in a way what a public service to have a show that inspires people to be healthier.”