"The Biggest Loser" trainer Bob Harper was "stunned" by season 15 winner Rachel Frederickson's 155-pound weight loss.
Harper made the comments in an interview for the "Rachael Ray Show" set to air Feb. 13.
Bob, who has been a trainer on "The Biggest Loser" since 2004, said the TV weight-loss competition has never had a winner who weighed so little. "I was stunned," he said. "We've never had a contestant at 105 pounds."
Fans of "The Biggest Loser" noted that Harper and co-star Jillian Michaels looked visibly shocked when they saw Frederickson at the finale, with Jillian gasping, "Oh my God. Oh my God" at Frederickson's gaunt face and emaciated body.
Bob explained that the trainers don't see the contestants after they go home from the Biggest Loser ranch, which is why Rachel's extreme weight loss drew such a dramatic reaction.
"What people don't understand is, when the contestants leave to go home, they're in charge of themselves," said Harper, author of Jumpstart to Skinny: The Simple 3-Week Plan for Supercharged Weight Loss.
"I had not seen her until that night, and so when she walked out, I was just kind of like, 'Whoa.' And I've been on the show since the beginning."
The 5-foot-4 Frederickson lost 155 pounds in just four months, slimming down from 260 pounds to a scrawny 105 pounds. Her extreme weight loss immediately fueled rumors that she's suffering from the eating disorders anorexia or bulimia.
Meanwhile, Rachel insisted she lost weight naturally — through a strict diet and lots of exercise. She said she ate five small meals a day consisting of protein, carbs and fat totaling 1,600 calories.
Frederickson, a former competitive swimmer, also said she worked out "a ton," including walking for hours every day at a treadmill desk, squeezing in daily gym workouts, and attending up to four exercise classes a day, including Zumba and spinning.
Rachel says she has never felt better, and has cut back her workouts since she's in maintenance mode. "I've never felt this great, and it's very exciting!" she said.
"Now I'm really at maintenance mode, so now it's gonna be about doing a yoga class this day, a spin class this day, and I think I'm going to try dance classes because I don't have a lot of rhythm."
Frederick's trainer, Dolvett Quince, author of the 3-1-2-1 Diet, defended her extreme weight loss on Facebook: "Please try not to look at one slice of Rachel's journey and come to broad conclusions."