Talk about a body of work! Some actors bulk up, slim down, or pack on pounds all in the name of art - or a shot at winning Oscar gold. This year's Best Actress Natalie Portman shed 20 pounds from her already petite frame to be en pointe as a ballerina in Black Swan. Meanwhile, Christian Bale nabbed a Best Supporting Actor for his downright skeletal appearance in his portrayal of a drug addict in The Fighter. (This portrayal is only rivaled by his appearance in The Machinist in 2004).

These two actors are certainly not the first (nor the last) to undergo extreme body makeovers. Here, from our friends at Fitbie.com, is a look at some of Hollywood's most celebrated shape-shifters.

The actress packed on the pounds not once, but twice, to take on curvy Bridget Jones. An avid runner, Zellweger banned exercise and began eating high-fat foods to plump herself up from a size 6 to a size 14.

For Bridget Jones's Diary (2001), she ate lots of doughnuts and cheese pizza. For the sequel, she hired a nutritionist and gained nearly 30 pounds with a controlled, but high-calorie meal plan. Thanks to daily two-hour workouts, plus a diet of tuna, dressing-free salads, and raw veggies, the actress quickly slimmed down.

The teen idol wooed many women (of all ages) over to Team Jacob after he flaunted his newly ripped body in New Moon (2009) and (2010). In just a year after filming the first Twilight film, Lautner gained 30 pounds of lean muscle to transform from scrawny to brawny. His massive gain was possible because of his previously lean physique.

"The less muscle you have, the easier it is to gain muscle mass quickly," says Jordan Yuam, Lautner's trainer and the owner of Jordan's Virtual Fit Club. The martial arts trained actor lifted heavy weights and used resistance bands five to seven days a week.

The actress sculpted a million-dollar body for her Oscar-winning role as a boxer in Million Dollar Baby (2004). The naturally slim Swank gained 19 pounds of muscle by boxing two hours daily and lifting weights six days a week. Her trainer even had her push an SUV in neutral gear across the parking lot to train her legs.

"My back, my arms, my legs - everything changed," Swank revealed on the Today show. After the movie wrapped, she cut back on weightlifting and returned to her svelte shape with a combo of Pilates and running.

All eyes (and hearts) were taken by Reynolds' ripped physique in Blade: Trinity (2004). In five months, the actor gained 20 pounds of muscle and lost a third of his body fat.

"I attribute my results mostly to nutrition," Reynolds told Men's Health magazine. He ate every two to three hours to burn more fat.

"Your body doesn't need to store fat for energy if you're feeding it all the time," says Bobby Strom, Reynolds' Los Angeles-based trainer.

It's hard to imagine waify Demi as a hard-body, shaved-head Navy operative, but the actress massively bulked up for G.I. Jane (1997). Moore spent hours training rigorously under the direction of a real-life Navy SEAL and performed all her own stunts - who could forget those single-arm push-ups?
 
The ageless beauty now prefers a more Zen workout - she's into Bikram yoga.

The Scottish actor pumped up to perfection as a Spartan warrior in 300 (2007). Butler spent four months intensely training to build a fighter's chiseled bod. The actor's trainer created the "300-rep Spartan workout." It goes like this: Without resting between exercises, Butler performed 25 pull-ups, 50 dead-lifts with 135 pounds, 50 push-ups, 50 jumps on a 24-inch box, 50 floor-wipers, 50 single-arm clean-and-presses using a 36-pound kettle bell, and 25 more pull-ups.

After filming wrapped, an exhausted Butler stopped working out for eight months. He eventually returned to the gym for a more sane four-day-a-week workout schedule.

The Oscar winner cast off major weight for his shocking transformation into a survivor stuck on a deserted island in Cast Away (2006). After filming the first half of the movie, production shut down for a year so Hanks could scale down from 225 to 170 pounds. To lose the weight, Hanks went on an intense calorie-controlled diet. He claims the only thing he didn't give up was coffee, adding, "Nope, wasn't about to! Can't do it!" But this wasn't his only weight loss experiment: He dropped 30 pounds for his role in Philadelphia (1993), just a year after packing on pounds to play a baseball coach in A League of Their Own (1992).

Bale is the ultimate body transformer. He dropped a shocking 63 pounds to play an insomniac in The Machinist (2004). The 6'2" actor reportedly wanted to go as low as 99 pounds, but producers stopped him at 120.

Bale didn't seek out medical advice to achieve his skeletal frame. Instead, he opted for extreme starvation, existing on a can of tuna and an apple a day for several months. Last year he shrank down once again to play Dicky Eklund in The Fighter. "There's no easy way of doing it," Bale told Access Hollywood. "I'm starving the whole time, I'm hungry as hell."

The once-ripped gladiator packed on the pounds for his 2008 spy thriller Body of Lies. Russell gained a beefy 63 pounds by eating lots of junk food. The actor gorged on cheeseburgers for breakfast, and revealed in an interview with Access Hollywood, "If you want to put on weight, you just elect to live a sedentary life." But Crowe gained so much weight he had to "rock back and forth to get out of his car." Taking off the pounds wasn't easy - the actor hired a nutritionist and hit the gym daily.