Stars Get Fit!

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Christina Aguilera

"I feel sexiest in the nude," the 29 year-old told British Cosmopolitan magazine. Of her post-baby body, the mother to son Max (he turns two in January 2010) says "it's important to keep a strong sense of yourself when you're a mum... be confident in your sexuality."

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Heidi Klum

Five weeks after giving birth to daughter Lou -- her fourth child -- the model was thrilled she got to show off her post-baby body on the runway at the Victoria's Secret Fashion Show. "I'm keeping it all together," she told Us.

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Kelly Osbourne

The 25-year-old dropped 20 pounds during her stint on Dancing With the Stars, but not without sacrifice! "I'm f---ing starving right now!" she told Us. Although the size-2/4 star didn't win, she says she "already proved myself."

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Kevin Federline

After his dramatic weight gain over the summer, the 31 year-old dropped the pounds for the next season of VH1's Celebrity Fit Club, which premieres in January. "Everybody will be very, very shocked to see the end results," Shar Jackson (his ex and fellow Fit Club participant) told Us.

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Khloe Kardashian

Thanks to QuickTrim, the reality star dropped 25 lbs. before wedding Lamar Odom in September.  "It's hard work, but it's worth it, and once you start seeing results, it keeps you motivated," she has said.

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Lady Gaga

The breakout pop superstar of 2009 -- known for her avant-garde and often revealing outfits -- has been sporting a thinner, more muscular frame as her celebrity continues to grow.

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Nadya Suleman

Nine months after giving birth to octuplets, the Octo-mom stepped back into her pre-baby jeans. Without a trainer or plastic surgery, she lost a staggering 145 pounds! "I topped 270 pounds while pregnant," she told Us. "Even my Ob-Gyn is astounded at how I've snapped back!"

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Seth Rogen

Despite shedding serious pounds for his title role in The Green Hornet, "I personally do not care how I look or physically feel," he has said. His secret to getting svelte? "The lamest answer ever: I exercised and I dieted."

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Taylor Lautner

To make sure he kept the part of Jacob Black in Twilight sequel New Moon, Lautner, 17, packed on 26 pounds of muscle for his beefed up role. His trainer Jordan Yuam told Men's Health: "Inexperience works to your advantage. The less muscle you have the easier it is to gain muscle mass more quickly."

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Nicole Richie

After giving birth to her son Sparrow in September, the star has debuted an impressively svelte post-baby physique. Her secret? Jay-Z! "I've lost 10 lbs. from dancing!" she boasted on Twitter. "H to the izzo. V to the izza!"