On Tuesday night, contestants on “The Biggest Loser” stepped on the scale for the final time to weigh in for the competition’s end. Rachel Frederickson, 24, was crowned the season 15 winner, but many are left wondering: How did she lose 155 pounds and is it safe?

Frederickson, a former swimmer, lost nearly 60 percent of her body weight, dropping to 105 pounds from 260 pounds. The Los Angeles native won the $250,000 grand prize and is thrilled with her transformation.

"I started as a [size] 20 and now I’m a 0/2. I was a 6 during makeover week. It's all about maintaining now!" she told Us Weekly.

Frederickson says she gained so much weight at her young age after a breakup that ruined her self-confidence.

“I truly came on 'The Biggest Loser' to gain my life and get that second chance to take control and to be that confident girl again," she said. Now, she "can't wait to do everything.”

“[I] go out and make friends and take people up on offers to go for coffee or to the movies and just really embrace every moment of life and not hide anymore!"

But some fans of “The Biggest Loser” felt Frederickson looked nearly unrecognizable and questioned the safety of her shocking weight loss.

“In my opinion Rachel is too skinny. Going from obese to 105lbs on biggest loser CANT be healthy. I don't approve,” Twitter user @lilrabenn wrote.

“There needs to be a red line that disqualifies finalists for too much weight loss based on a minimum BMI threshold,” user @jaffejuice wrote.

At Frederickson's height of 5'4", Weight Watchers reports a healthy weight range of 117-146 pounds.

Some sites, like Hollywood Life, believe her massive weight loss is actually dangerous, since she now has a BMI of 18, just slightly under the normal range of 18.5 to 24.9.

“When a troubled person like Rachel learns to work out and diet excessively, it doesn’t necessarily mean that their problems are solved — because having a negative relationship with food and body image doesn’t only apply to heavy people,” the site reported. “If those issues aren’t dealt with at their source, it’s easy to see how a contestant could quickly switch from overeating to excessively under-eating.”

But Frederickson maintains that she is healthy and will continue to work on keeping up her new figure.

“I’m at the maintenance point now, so I need to find some balance and make sure I work out and I eat healthy and make good choices 90 percent of the time,” she told the "Today" show. “I’m not sure (I’ll maintain this weight), but I plan to try new exercises and just continue on this path and see where that takes me.”