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(CNN) -- Jillian Michaels just returned to "The Biggest Loser" in 2013, but she's already exiting the series once again.

Back in June, NBC confirmed that Michaels, a longtime celebrity trainer with the reality weight loss competition, would not be returning in the fall for season 16.

Michaels' participation with the series has been an on-and-off relationship, as she's left and rejoined the program twice since it started in 2004.

While prior departures either went unexplained or had to do with her family, the 40-year-old mom of two gave a different reason this time around.

Jillian Michaels is responsible for some of the fit bodies you see coming out of "The Biggest Loser." Here, we catch up with the winners then and now:

When "The Biggest Loser" contestant Rachel Frederickson showed just how much weight she lost on the NBC competition -- 155 pounds, to be exact -- not everyone was impressed. A number of viewers expressed concern that she had become "too skinny," although Frederickson said that she feels fine.

Dannielle "Danni" Allen won season 14 of "The Biggest Loser" in 2013. She entered the competition at 258 pounds, and by the end of it she was down to 137.

Season 13 champ Jeremy Britt lost an astonishing 199 pounds from a high of 389, effectively shedding more than half of his body weight during the 2012 competition. His weight loss was actually a family affair: his sister Conda competed with him and lost 115 pounds along the way, and his effort inspired his mom to lose 50 pounds as well.

John Rhode sealed the win in 2011 by ending the season 220 pounds lighter. Rhode entered the competition weighing 445 pounds, and after working with trainer Bob Harper he had lost nearly half of his body weight.

Olivia Ward lost 129 pounds after competing in the 11th season of "The Biggest Loser," which not only helped her feel healthier but also put her in first place. "It was the most life-changing experience I have ever had the privilege of going through," she said after her win.

Patrick House began season 10 of the competition at 400 pounds. By the time he'd sweated through to the end, he had lost 181 pounds and gained a book deal. "As Big as a House: How One Biggest Loser Took A Look at Himself and Made the Change of a Lifetime" was published in 2012.

Season 9's Mike Ventrella inspired viewers everywhere when he dropped from 526 pounds to 262. Not surprisingly, one of the first things Ventrella needed to do after he won the $250,000 prize was go shopping for new clothes.

Charles Daniel Cahill claimed the season 8 title by shedding 239 pounds. He began the competition at 430 and walked away from it weighing 191.

Season 7 star Helen Phillips worked her way from 257 pounds to a finale weight of 119. After she won, Phillips treated her husband to a Las Vegas vacation and relished feeling confident enough to wear "a cute bathing suit, strutting my stuff!"

Michelle Aguilar's 100-pound weight loss on season 6 of "The Biggest Loser" was more than just a physical change. "Not only did it show me that I was capable of more than I believed I was, but it also helped me to truly find myself," she said. "I learned to change from the inside out."

Season 5's Ali Vincent was the first woman to win the weight-loss competition. She slimmed down to 124 from her starting weight of 234, a journey she reflected on in her book, "Believe It, Be It: How Being The Biggest Loser Won Me Back My Life."

Bill Germanakos helped set the standard for what it means to be "The Biggest Loser" when he lost 164 pounds in season 4. Bill signed up for the show with his twin brother, Jim, who also lost a massive amount of weight thanks to the show. Although Jim was eliminated, he kept up at home and dropped 185 pounds.

Season 3 champion Erik Chopin walked away $250,000 richer thanks to the 214 pounds he lost. "Nothing could stop me from reaching my goal," Chopin said. "I felt like (winning) was my destiny."

Matt Hoover's 157-pound weight loss propelled him to "Biggest Loser" status during the show's second season. He walked away with more than just $250,000, though: Hoover wound up marrying his competitor, Suzy, in 2006.

The first "Biggest Loser," Ryan Benson, dropped down to 208 after entering the competition weighing 330 pounds. In his Twitter bio, he said that he's "gained some of the weight back, but have maintained my boyish good looks and cat like agility."

'The Biggest Losers': Before and after

'The Biggest Losers': Before and after

'The Biggest Losers': Before and after

'The Biggest Losers': Before and after

'The Biggest Losers': Before and after

'The Biggest Losers': Before and after

'The Biggest Losers': Before and after

'The Biggest Losers': Before and after

'The Biggest Losers': Before and after

'The Biggest Losers': Before and after

'The Biggest Losers': Before and after

'The Biggest Losers': Before and after

'The Biggest Losers': Before and after

'The Biggest Losers': Before and after

'The Biggest Losers': Before and after

'The Biggest Losers': Before and after

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'The Biggest Losers': Before and after

'Biggest Loser' winner: Too thin?

"I don't want fans to think I feel like I'm too big for the show or that I'm seeking greener pastures. I'm so grateful to the show and so sad to leave," she told People magazine. "(T)here were some fundamental differences (with the show's producers) that have existed for a while."

Many have wondered whether Michaels is leaving because of the controversy with the most recent "Biggest Loser" winner, 24-year-old Rachel Frederickson, whose 155-pound weight loss left Michaels and her celebrity trainer co-star, Bob Harper, visibly shocked during the final reveal.

But Michaels points to another issue she had: the way the show chose to portray her character.

"In the beginning of the show, it was tough love. You saw the tough, and you saw the love," she told People magazine. But lately, from her perspective, viewers saw "none of the relationships, (and) none of the bonds that I build with my clients."

The impact of her TV persona was starting to affect her family, and Michaels felt at the end of Frederickson's season, she needed to "take a hard look at my work," she told the magazine. "I came to the conclusion that moving forward, I need to be able to have an impact on the outcome of what I do."

So when "The Biggest Loser" returns on September 11, it'll be down one tough-loving trainer but will have added two new faces: NBC has announced that Jessie Pavelka and Jennifer Widerstrom will be the fitness buffs joining Harper and another returning trainer, Dolvett Quince.