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CBS "The Biggest Loser" winner Rachel Fredrickson's 155-pound weight loss is turning heads -- but not all for the right reason.

Fredrickson, 24, lost 59.62 percent of her body weight during the course of the show and won the $250,000 prize. She started the program at an unhealthy 260 pounds and ended up a svelte 105 pounds at the final reveal just seven and a half months later.

While audiences applauded the contestants' efforts, some critics said her weight loss journey may have gone too far.

At her new, post-diet weight, Fredrickson (who is 5'5" according to NBC) would have a body mass index (BMI) of 17.5, which would classify her as underweight. For comparison, Milan and Madrid have banned fashion models from walking down the runway with a BMI of under 18.5.

"She's actually pretty significantly underweight," Kelly Hogan, a clinical dietician at The Mount Sinai Hospital in New York, told CBS News.

Hogan explained that it was also concerning that Fredrickson lost all the weight in a relatively short period of time. Healthy weight loss is deemed to be a loss of 1 to 2 pounds of a week.

"Anything that severe or that quick is risky... Especially in this case of 5 to 6 hour workouts and severely restricted caloric intake, there can also be electrolyte disturbances, cardiovascular problems and issues cognitively in the brain. If you aren't getting enough energy in the form of carbohydrates, it's hard to think clearly," Hogan said.

But Kylene Guerra, a nutritionist who does weight management at the Cleveland Clinic, explained to CBS News that it's hard to tell if Fredrickson is actually unhealthy or not. Even though BMI may indicate a person is underweight, genetics and her level of physical activity also play a role in determining a healthy weight level.

In order to confirm if Fredrickson is at a healthy size, you would need to do a body composition test. For example, a person could have a BMI of 18 but a healthy fat composition of 20 percent. In that case, losing that much weight, even in a relatively short period of time, could be okay.

Rapid weight loss could also be affected by a person's previous diet and exercise levels. Guerra said that if Fredrickson was eating 3,000 to 4,000 calories a day and not exercising, it would make sense she lost more weight than the recommended rate after being put on a healthy, calorie restricted diet and an extreme exercise routine. The fact that she was a high school swimmer could also have "put her mindset (to succeed) in a completely different place than other contestants."
"We really don't know what she was doing before she came on the show," Guerra said. "If it's done in the correct way with supervision, it could be okay."

Both experts said that with any rapid weight loss, there is a concern that the dieter would end up gaining the weight back. Hogan pointed out that many of the previous contestants on "The Biggest Loser" put on pounds after the show ended.

They emphasized the need to make permanent lifestyle changes so you can keep the weight off long term. Hogan suggested that making small changes to your diet and lifestyle is lot easier to keep up than drastic ones.

To help with limiting calories, Hogan said a food diary can make it clear what items in your diet are bad for you and what times of the day you are overeating. Then, you can adjust your diet to avoid the problem areas.

Guerra pointed out that many dieters try to restrict themselves to too few calories. Most people need at least 1,200 calories a day, so anything less would cause the body to store fat to make up for the deficit. She suggested talking to a doctor or nutritionist to determine your appropriate calorie range before starting a diet.

As for exercise, people are more likely to stick to something that they actually like doing.

"More than anything, find something you enjoy before you worry about duration or intensity or finding a personal trainer. Then work your way up from that -- even if it's just walking," Guerra said.

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