(CNN) -- When Rachel Frederickson, 24, stepped out onto the stage at NBC's "The Biggest Loser" finale Tuesday night, some wondered if she had gone too far.
While the show is known for its dramatic weight loss transformations -- most winners lose more than 50% of their body weight -- Frederickson appeared extremely thin. And the looks on trainers Bob Harper and Jillian Michaels' faces could be interpreted as shock -- or dismay.
Frederickson went from 260 pounds to 105 pounds, losing 59.62% of her body weight. At 5 feet, 5 inches tall, that puts her body mass index at 17.5. Anything under 18.5 is considered by the National Institutes of Health to be underweight.
NBC on Wednesday declined to comment on its $250,000 grand prize winner, a voice-over artist who lives in Los Angeles.
Social media, however, was buzzing.
"Are you kidding?" One woman wrote on the show's Facebook page. "Rachel looks anorexic! She has gone from one extreme to the other!"
Other posts called Frederickson "frail" and said she seemed "dizzy" and "disoriented" on stage.
"She obviously worked incredibly hard to achieve her weight loss goals, but I am wondering if the pressure of winning a large cash prize caused her to take it too far," one said.
Others posted on Frederickson's Facebook page, asking her to get help and expressing disappointment.
"I'm saddened that my 13-year-old daughter watched as you were rewarded for doing that to your body," one woman said.
Frederickson as of Wednesday had not responded to the posts and had not commented on the controversy on Twitter.
BMI applies less on an individual level and is more utilized for population norms -- many people, such as marathon runners or endurance athletes have low BMIs and are perfectly healthy. And some people who eat well and exercise normally are just naturally thin.
However, losing a significant amount of weight using unhealthy methods can be dangerous, said experts unfamiliar with Frederickson. It's the darker side of weight loss -- as obesity is unhealthy, so is being underweight.
"Can you be too thin? Yes. Can it be as dangerous as being too fat? Yes," said Dr. Steven Lamm, weight management expert and medical director of NYU's Preston Robert Tisch Center for Men's Health.
Severe weight reduction can result in hormone disruption or bone thinning (reduction in bone density) and can affect women's fertility, as they stop menstruating, said Lamm and Dr. Robert Kushner, clinical director of the Northwestern Comprehensive Center on Obesity in Chicago.
Kushner said most of his patients lose about 30% of their body weight over the course of a year. "On the other end, I have patients with a BMI of 30 that are in perfectly good health," he said
Lamm notes the television show is about extremes, "not what we as a profession are looking at."
But "if I were to see this in a patient, I would be concerned about their emotional stability," he said.
Frederickson is "just below that lower end" on the BMI scale, Kushner said. "Not watching the show ... I really couldn't say whether she's unhealthy, or she shouldn't weigh that much. Because she very well could have a healthy lifestyle."
Still, "there are some people who we see for weight management who develop disordered eating," he said. "It's like a pendulum and they swing to the other end ... I don't see a lot of those patients, but I do see them."
Such patients are afraid to gain any weight because of their fear of again becoming obese, he said.
The real message, said Lamm, "is that people should appreciate that when they are overweight they don't have to lose 100 pounds to improve their overall health, that small changes in body weight can have a very sigificant impact on blood sugar, cholesterol.
"They shouldn't think that if they can't lose 100 pounds they shouldn't try."