Amazing Celebrity Slim-downs

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She is a part of the fitness nation! The 46-year-old singer once tipped the scales at 180 lbs., but thanks to daily workouts and Nutrisystem (she's a spokesperson), Jackson has kissed her weight-obsessed days goodbye. "I don't know how much weight I've lost, only because I stay away from the scale," Jackson told PEOPLE. "You're setting yourself up to fail."

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The 30 Rock star lost 30 lbs. in four months by cutting out sweets and rounding out his fitness routine with Pilates, spin, and a little yoga – which his bride-to-be Hilaria Thomas teaches for a living. "When we're shooting, and I can't work out, I just have to eat less," the actor, 54, said in January. "But sugar was the real killer for me – that was the problem."

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The Duets has spoken out against Hollywood's thin ideal for years, but boyfriend Brandon Blackstock inspired her to shape up. "No one likes to be not toned when you are dating someone," said Clarkson, who flaunts a slimmer shape thanks to diet, cardio and strength training. "I'm eating better and working out, but I've always fluctuated within 20 lbs." Still, the singer stands by her curves: "I have no desire to be super-skinny."

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The soul singer, 68, shed an impressive 60 lbs. during her two-month stint on Dancing with the Stars. "You've got to eat healthy and you've got to put a little movement into it," Knight said of her weight-loss secret.

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When she won American Idol five years ago, Sparks was a size 14. Thirty pounds later, the 22-year-old is now happily a size 8. "My body craves the gym," says the singer, who dropped the weight off her 5'11" frame with a combination of cardio, Zumba and diet. "I watch what I eat a little more strictly."

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The once-overweight funnyman got serious about his health when he was hospitalized for chest pains in 2001 and diagnosed with type-2 diabetes. A strict no-carbs diet helped him lose 80 lbs., and go from a 46-inch waistline to a 34. "I like being skinny," Carey, 52, said. "I was sick of being fat on the camera. Really, I just got sick of it.

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The Cosby kid-turned-Disney star blamed the stress of having a TV show for her weight gain, but lost a reported 35 lbs. by kickboxing and eating more frequent, smaller meals. "It feels good to put something on and not have a girdle underneath!" the 26-year-old star said.

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The Hot in Cleveland star lost 40 lbs. in 2009 – which prompted her to wear a bikini for the first time in 30 years. Bertinelli, 52, who credited her weight loss to her gig as a Jenny Craig ambassador, wants to lose five more pounds before putting on her green bikini again. "It's calling my name!" she told PEOPLE.

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The Queen of Funk, 59, showed off her slender figure at the American Idol finale in May, but what viewers didn't see was the singer's year of hard work. Khan lost 60 lbs. thanks to a four-month liquid cleanse of protein and veggie smoothies. "I did nothing but liquids; me and my VitaMix were best friends," she told Access Hollywood.

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When his marriage to Britney Spears ended in 2006, the former dancer, 34, gained 85 lbs. Even he admitted that he "looked pregnant." Nowadays, K-Fed's 35-lbs. lighter, thanks to a few stints on weight-loss reality shows.

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The diva mom gained her fair share of maternity weight carrying dembabies in 2011, but she earned back her svelte figure – and made 70 lbs. vanish! – with a strict diet and plenty of aquatic workouts. "I had, like, no bones for a while," the Jenny Craig spokesperson said. "It's important to me to feel my bones!"