Celebrity weight battles?

Fat to fit?! Lady Gaga slipped into a teeny floral bikini in Mexico on June 6, 2013, revealing her newly toned-up body. The quirky singer admitted back in Sept. 2012 to a 25-pound weight gain. "I'm dieting right now, because I gained, like, 25 pounds," Gaga admitted to radio host Elvis Duran. "I really don't feel bad about it, not even for a second." But whether or not Gaga felt any remorse, it seems she's worked a bit on her figure. Nearly a year later, Gaga is back in stage-ready shape, though she's been out of the limelight lately following a hip injury.

Does "One Direction" star Zayn Malik need a vacation? The 20-year-old singer stepped out in Miami on June 13, 2013 looking extremely gaunt and noticeably thinner while in town with his band members to shoot a music video. "He's always been the slimmest of the band members, naturally," a source told Us Weekly, adding, "but I think he's running around a lot, getting stressed, and not putting health first."

Christina Aguilera is slimming down. The sultry songstress took to Twitter to post a photo of herself flaunting her much smaller physique in nothing but a black bra and skirt. The 32-year-old singer, who was seen sporting a slightly fuller figure at the American Music Awards nominations in October (l.), was on the set of her new music video for the song "Hoy Tengo Ganas De Ti." Aguilera has been hitting up the gym and is said to be working with personal trainer Tee Sorge.

Mo'Nique's 80 pound weight loss is no laughing matter. The comedienne and Oscar-winning actress is getting serious about her health, taking to Twitter to document the grueling workouts that have helped her shed the pounds. "The weight right now, when I got on a scale two weeks ago, was 218," she told radio station Hot 97, explaining that she started at around 300 lbs. "My goal is between 190 and 200." So how does she do it? Squats, push-ups, dancing and yoga. "If you put the work in, baby, I promise you, it comes off," she said.

Khloe Kardashian is flaunting her famous curves -- and her 30 pound weight loss -- on the cover of Life & Style. The youngest Kardashian sister shows off her super slim new figure on the cover of the magazine's May issue, striking a confident pose in a bright and bold bikini.

Snooki has never looked skinnier! The pint-sized "Jersey Shore" star graced the cover of Us Weekly wearing only a high-waisted leopard print bikini, but it's her dramatic 42 pound weight loss that has everyone talking. Checking in at 102 pounds, the 4-foot-9 star shed serious pounds just six months after giving birth to her son Lorenzo. She cites herself as being her own biggest inspiration. "I lost [the weight] for me," she told the mag, though her soon-to-be husband certainly played a part. "I also wanted Jionni to know that I can be hot as a mom."

It looks like all that hard work has finally paid off! After six months of hitting the gym and watching her diet, Jessica Simpson proudly revealed her slim new figure while promoting her Jessica Simpson and Jessica Simpson Girls collections at Macy's South Coast Plaza in Costa Mesa, Calif., on Nov. 10, 2012. Simpson, 32, whose weight has been under public scrutiny since she gave birth to baby Maxwell Drew in May, signed a $4 million deal to become a celebrity spokesperson for Weight Watchers earlier this year after reportedly gaining 70 pounds during her pregnancy

Matthew McConaughey, is that you? The usually buff actor is taking weight loss to alarming extremes as he continues to drop the pounds for his upcoming movie, "The Dallas Buyer's Club." In the film, the former "Magic Mike" star plays Ron Woodroof, an electrician diagnosed with AIDS in 1986 who ends up saving his own life with a series of alternative medicines that he trundles into the country from Mexico. A gaunt looking McConaughey, who planned to shed a whopping 30 pounds for the role, looked shockingly thin as he made his way to the film's set carrying a large beverage on Nov. 12, 2012.

Kelly Clarkson has a whole new look. The former "American Idol" singer is a glimmer of her former self, having dropped nearly 30 pounds in six months. The newly blond singer was almost unrecognizable as she revealed her dramatic new look at the Much Music Video Awards in Toronto on June 17, 2012.

Jonah Hill: From chunk to hunk -- and then back to chunk again. The Hollywood star was spotted in Soho looking as though he'd fallen off the weight loss wagon. The 28 year-old actor appeared as though he'd gained back much of the 40 pounds he'd lost to get in shape for the 2012 film '21 Jump Street.' In 2011, Hill made headlines for dropping the weight and for embracing a healthier lifestyle.

Janet Jackson didn't sing a song called 'Discipline' for nothing. The 45 year-old pop diva is showing off a svelte figure after losing weight on Nutritystem. The diet brand released a photo of the singer in a tight red dress and sky-high heels. 'This is what success looks like,' she says in the ads for the preprepared diet food company.

There's skinny and then there's scary skinny. Newlyweds LeAnn Rimes and Eddie Cibrian finally cleared their schedules and jetted off to Cabo San Lucas for their honeymoon. Rimes stripped down under the hot Mexican sun to reveal her skinny frame. Luckily, Rimes, who has been criticized for what appears to be a shrinking frame, showed off a far healthier figure while vacationing in Maui on Jan. 5.

The premiere of season four of the hit MTV show 'Jersey Shore' left many viewers wondering Ö what happened to JWOWW's body?! The buxom brunette shocked viewers with her dramatic weight loss and gaunt face. Her new appearance didn't garner rave reviews from viewers, who mostly tweeted negative reviews about the reality star's new look. 'Does anyone else think J-Woww looks really weird and skinny?' one viewer tweeted.

There's a whole lot less of Moore. At the Sept. 26 premiere of 'Five,' Demi Moore, 48, debuted something more than just her short film. Stepping out in support of the Lifetime project -- which features five short films all dealing with breast cancer and directed by celebrities including Jennifer Aniston and Alicia Keys -- Moore looked exceptionally thin. 'It looks more like overtraining with restricted calories, than an eating disorder,' a nutritionist who has not treated Moore told RadarOnline.com.

Jennifer Hudson first debuted her drastic weight loss in 2010 when she announced she'd become the spokesperson for Weight Watchers. But in 2011, she proved that she wan't stopping there. Back in February, Hudson said, 'I didn't expect to go as far as I did.' Apparently the singer and actress still can't quite believe how much she's shrunk.

It's been a year of ups and downs for Channing Tatum. Tatum made a promotional appearance in February (c.) for his upcoming film, 'The Eagle' looking less than super-fit. The 30-year-old actor, normally known for his extremely fit physique, looked noticeably heavier than the version of himself on the movie poster displayed behind his seat. But spotted again in Sept. 2011 (r.), it appears that Tatum has shed any extra pounds.

He's not even close to '300' these days. Actor Gerard Butler, 41, who was famously muscled when he appeared in '300,' director Zack Snyder's 2006 action epic, showed off a new and very slim figure Aug. 4 at a Hollywood Foreign Press luncheon in Beverly Hills. 'Butler looked noticeably trimmer around his torso area as well as his open shirt collar drawing attention to his thinner face and neck,' said the Daily Mail. 'His trousers looked baggy around his legs and Butler's single-breasted jacket also highlighted his slimmer physique.'

Heidi Montag was rocking a hot new bikini bod at June 19's Wet Republic party in Las Vegas, proof that spending 14 hours a day at the gym really does pay off. 'I've been working out from, like, 5 a.m. to 7 p.m. for two months now,' she told UsMagazine.com. 'I've been working out really hard because I had this pool party and I was like, I have to be in shape.' 'And I was actually a lot overweight,' added the 5'2" reality star, who says she weighed 130 pounds at her heaviest.

Tara Reid was spotted looking gaunt while out in Los Angeles with friends on May 26. In comparison to this 2003 photo (l.) it appears Reid has slimmed down tremendously. Look at those legs!

Claudia Schiffer's appearance on a May 2011 Cannes red carpet has heads turning and jaws dropping – and isn't due to her sheer lace dress. The 40-year-old German supermodel was little more than skin-and-bones when she walked the red carpet at the French film festival. Schiffer, who gave birth to her third child in 2010 was barely recognizable, save for her trademark long blond hair. She bragged to Vogue that breastfeeding helped her drop most of her pregnancy weight and Pilates took off the rest.