America is obsessed with celebrity. But if there is one thing Americans are more obsessed with, it’s celebrity weight. Even Britney Spears referenced this form-fanatic phenomenon in her 2007 hit, Piece of Me. (I’m Mrs. she’s too big now she’s too thin.)

If you’re one of the millions of Americans who secretly relishes watching your favorite lanky leading ladies and lads balloon up with pregnancy, break-up issues, secret fast-food obsessions and the like, it can be downright fascinating to watch your obsession whale up—or slim down—for a role.

Indeed, a prestigious role is an excellent excuse for the queens and kings of Holllywood to fall off the Twiggy bandwagon—or in some cases, take it to an all-new extreme—all in the name of their art. It quiets media scrutiny about their mental function (it’s for the art!), still affords media attention (but look at that badonkadonk!), and draws millions of curious folk like you and me to the theatre to see the transformation first hand.

Despite the millions of dollars these bodily alterations bring to box offices and tabloids due to our never-ending curiosity about size, should stars really be undergoing such massive weight loss or gain? What are the long-term consequences?

If a star’s sudden change in weight is your new excuse to go on an all-liquid or all protein diet (depending on your desired outcome), maybe you should think twice before indulging (or not). Why? Because nothing in Hollywood is what it seems.

Let’s take a look at five Hollywood transformations that caught our attention, and how they affected the stars that transformed.

1. Mickey Rourke for The Wrestler

Rourke reportedly hefted 36 pounds of muscle onto his frame for his Golden Globe-winning portrayal of Randy “The Ram” Robinson.

Said Rourke in a recent interview with Rob Driscoll, “Before, when I was boxing, I used to walk around with 200lbs, but I used to fight at super-middle weight, which was 167lbs, so I used to take 12 weeks to train before each fight, and it was murder taking off the weight, so to do that in reverse, and have to gain the weight, I was so happy. I went, ‘Oh wow, this is gonna be fun, I can eat four steaks a day,’ because I can eat like a truck driver.”

Rourke hired a cage fighter and an “Israeli army guy” to help him pack on the muscle, which took 6 months of grueling training and a strict diet.

Worth it?

You betcha. The role solidified Rourke’s comeback from the depths of the has-been pit. And while it’d be hard for anyone with a day job to pack on that much muscle in 6 months, it’s not impossible (but tough for a man in his 50s!), or necessarily a health hazard.

However, as the New York Post said in December, “there’s less of Mickey Rourke to love these days.” He’s pretty much lost all the weight gained for the role—and is now making headlines for his slimmer size.

2. Renee Zellweger for Bridget Jones’s Diary

The usually diminutive star reportedly gained 20 lbs for both the original Bridget Jones film and the sequel, Edge of Reason.

Said the star about gaining weight for the first film: “It sounds like heaven. For two days it’s bliss and then you’re full, OK? And you can then indulge all your fantasies about over-eating. Fantasies about non-stop chocolate consumption or your fantasies about ordering the pizza and the spaghetti and the garlic bread. Then after a week your glucose levels are going crazy. You’re up and down and all over the place. It doesn’t feel good, and no one wants to hear that, but it’s the truth.”

If you think being paid to eat whatever you want sounds like the greatest job on the planet, Renee wants you to think again. She’s highly aware of the health implications of yo-yo dieting, saying, “…my body is whacked by the time we finish one of those [films]. It doesn’t know what has happened because it thinks there’s supposed to be a baby and there’s no christening.”

She added: “Did you see that movie about fast food, Supersize Me? I had a panic attack with all the specialists talking about how bad this is for you, long term, putting on that much weight in short periods of time and they’re all saying, ‘You must stop this now or you’re going to die’.”

Worth it?

Both Bridget Jones films brought in over a quarter of a billion dollars worldwide, and Renee reportedly made over $10 million for the sequel. It’s not the matter of gaining and loosing 20 pounds that’s harmful to the body, but more the matter of which 20.

Renee’s rapid weight loss following filming led many experts to believe she yo yo dieted her way into the Jones role. While yo yo dieting has a bad rap, technically the side effects of such a cycle for someone who is not overweight (Renee was super slim to begin with, at a reported 5’5”, and 115 lbs.) are minimal, with the worst effect being depression from constantly sabotaging one’s own goals.

Zellweger has, to this day, been notoriously coy about how she gained the weight. A self-professed exercise fanatic, binge eating was not at the top of Renee’s to-to list…until she received $10 million to do so.

Once a yo yo dieter upswings into the obese category, however, side effects like high blood pressure, heart disease, stroke, diabetes, and arthritis, become great concerns.

Having never been obese, Renee only put her sanity at risk for a big reward by becoming Bridget Jones. Totally worth it.

3. Christian Bale for The Machinist

A 63-pound weight loss is usually lauded in commercials for services like Jenny Craig and Nutrisystem. But if you’re one of Hollywood’s top hunks, you’d have to be clinically insane to loose your muscular good looks—or really, really dedicated to your art.

Enter Christian Bale—a consummate actor who just happens to be a pretty boy. Reportedly 6-feet tall exactly, and usually 185 pounds, Bale withered himself away to a weight of which only female runway models could be envious.

Surprisingly, the dramatic weight loss was not mandated by director, Brad Anderson. In fact, Bale took it upon himself to fully embody the character of Trevor Reznik—someone who Bale wanted to portray as having not slept for a year.

How’d he do it? Exercise. And starvation. Plain and simple, he didn’t eat much at all, and worked out—without weights, so as not to bulk up.

Worth it?

Bale reportedly gained back 100 lbs in the five months following wrap on production for The Machinist. Rapid weight gain like that can be a huge stress on all of the body’s organs—as is weight loss to the point of almost vanishing. It’s punishing on the heart and can cause arrhythmia.

Roger Ebert gave the movie three stars. Bringing in only a bit over $1 million worldwide, it was hardly a box-office success. But it was Bale’s chance to suffer for his art and create something he is proud of, so more power to him. As far as being physically worth it? Probably not. He’s lucky his heart didn’t explode during subsequent filming of the Batman movies.

4. Charlize Theron in Monster

Hollywood bombshell Charlize Theron took on a role that could prove to the world that she’s not just a pretty face—she’s got substance and acting chops to back it up. And that’s exactly what she did with her portrayal of female serial killer, Aileen Wuornos in 2003’s Monster.

Slinky Theron reportedly gained 30 lbs for the role, and many people said she still didn’t really look fat. Theron herself explained the weight gain wasn’t about becoming fat, but about accurately portraying the sense of physical strength and presence the real Aileen radiated.

With three months in between accepting the role, and starting to film, Theron didn’t have a ton of time to prepare. 30 lbs in three months is a lot of weight to gain in a short period of time. But like Renee Zellweger, Charlize’s weight gain probably put her at a normal weight for her height, not inducing any effects of being overweight.

Worth it?

Voted Best Actress by the Screen Actors Guild. Best Actress at the Berlin International Film Festival. Named Best Actress by the Broadcast Film Critics Association, the San Francisco Film Critics Circle, the Chicago Film Critics Association, the Las Vegas Film Critics Society, the Dallas-Fort Worth Film Critics Association, the National Society of Film Critics, and the Vancouver Film Critics Circle. Winner of the Golden Satellite Award and the Golden Globe.

Oh yes, and an Oscar! Who wouldn’t want to earn several million dollars to eat chocolate cake every day and win an Oscar? Totally worth it.