In “The Fighter,” Christianmv/ccr Bale is as thin and wan as the smoke from his character’s crack pipe. But radical weight loss is nothing new for the Oscar nominee — he shrank his 6-foot frame to an emaciated 121 pounds to star in the 2004 film “The Machinist.”

Going gaunt isn’t only for hard-core method actors. To attain a ballerina’s boniness in “Black Swan,” the petite Natalie Portman reportedly dropped 20 pounds — and also earned an Oscar nom.

Stars who starve always get attention in movie acting and are also a good bet for Oscar notice. To play concentration-camp victims, both Adrien Brody (in “The Pianist”) and Meryl Streep (in “Sophie’s Choice”) drastically lost weight — and gained Oscars. But if starvation has become its own acting technique, so has fattening up. Think of Robert De Niro’s Oscar-winning bloat in “Raging Bull,” Charlize Theron’s Oscar-winning stoutness in “Monster” and Renee Zellweger’s Oscar-nominated curves in “Bridget Jones’s Diary.”

On the face of it, extreme physical transformation seems to underscore an actor’s seriousness, and the backstory of cheeseburger-overload or puritanical self-deprivation helps sell the movie. When Portman was doing the publicity circuit for “Black Swan,” interviews centered on the severity of her weight-loss regime and workouts. “You don’t drink. You don’t go out with your friends. You don’t have much food,” Portman told Australia’s Sunday Telegraph magazine. “You’re constantly putting your body through extreme pain and you really understand the self-flagellation of a ballet dancer.”

But does slimming down or beefing up have artistic merit? Does it contribute to great acting?

Some observers decry the practice and see the Academy Award attention that an actor’s appearance gets as part of a misguided benediction of movie stars who de-glamorize in pursuit of their art.

“Oscars have often gone to people who do something to themselves,” says film historian and Wesleyan University professor Jeanine Basinger. “But the idea that gaining or losing weight has anything to do with acting is, to me, utterly insane.”

Author and film critic Molly Haskell agrees. “Since time immemorial, the Oscars worship this kind of self-transforming, where you become someone almost unrecognizable,” she says. “Acting is supposed to be about using the imagination, creating a character with your imagination, but you get stuck in the visual of it, and how brave it is for this beautiful woman to make herself ugly. It’s a kind of vulgar idea of what acting is.”

Haskell equates Bale’s seesawing weight (after “The Machinist,” he packed on 100 pounds for “Batman Begins”) to a stunt. “He must just like to do this,” she says. “I like him in ‘The Fighter.’ I think it’s an interesting performance — over the top, but interesting. But I don’t think he needed to do the weight-loss to do it.”

Yet slimming down to play drug-addicted ex-boxer Dicky Eklund must have seemed like second nature to Bale. His sinewy look in “The Machinist” gained considerable attention; the Los Angeles Times praised his “assuredly disturbing effect.” But it came as a surprise to Brad Anderson, who directed the dark psychological thriller about a factory worker consumed by guilt. The script described the title figure as “a walking skeleton,” says Anderson in a recent interview, but when the frail Bale showed up on the set, his appearance stunned the cast and crew.

“I wouldn’t have had the guts to say, ‘You gotta lose 60 pounds,’ ” says Anderson. “But once we saw how he looked, we said, ‘Let’s make it worth his while. We don’t want him to have suffered through this for nothing.’ ”

Anderson worked in more shots than he had planned of Bale undressed, looking more dead than alive and offering corporal evidence to the madness gnawing at his character. Those on the set were careful not to eat in front of him, though Bale mostly kept to himself to save his energy. (They were filming in Barcelona, which had the added benefit of tapas bars and their tiny portions, says Anderson.) But one of Bale’s scenes especially worried the director: It was shot in the city’s underground sewer system.

“It was the middle of August, and he’s so exhausted ’cause of how he’s not been eating. And now he’s in this toxic environment, running through tunnels,” says Anderson. “We had to cut a few times, let him go up to the surface to get fresh air, ’cause he looked like he was about to pass out.”

Anderson said that the physical deprivation “allowed [Bale] to get into the head of that character. . . . Some might say, ‘Well, you can act it.’ But Christian is one of those actors who likes to become the role.”

Becoming the role in that way wasn’t always possible. One reason actors can undertake serious weight changes now is that the more fortunate among them can be choosy about their movies. In the old days of the Hollywood studio system, it wasn’t possible to spend months gorging on cheesecake for just one part. Stars were tied to multiyear contracts, and typically churned out whatever films their studio put before them — relying purely on acting skills to bring their characters to life.

“I think with great acting you can convey a lot,” says Haskell, author of “Frankly, My Dear: ‘Gone With the Wind’ Revisited.” “A beautiful woman can convey being plain; Olivia de Havilland did that all the time.” She points to her convincing portrayal of a homely loner in “The Heiress,” for which the famed beauty won an Oscar.

And the reverse is also true, says Haskell. “Bette Davis can make you think she was beautiful even when she wasn’t.”

Women attract special attention for going to great lengths for movie roles, especially if they fatten up, such as Theron and Zellweger. This seems more horrible to our diet-obsessed culture than being paid to do the opposite. But some roles defy any sort of bodily re-sculpting, no matter how drastic. An actor who downs protein shakes and works hard in the gym can emerge with a plausible boxer’s physique (see De Niro, “Rocky’s” Sylvester Stallone and Mark Wahlberg of “The Fighter”). But no amount of weight loss, nor anything less than years of nonstop training, can turn an actress into a ballerina. This is a chief drawback to Portman’s performance, though undoubtedly her impressive scrawniness distracted fans from that fact.

“The key word here, especially where women are concerned, is ‘brave,’ ” says Haskell. As in, making themselves ugly by some punishing physical alteration: gasp-worthy skinniness or fleshiness, or wearing a prosthetic. (Take Nicole Kidman’s much-debated false nose in “The Hours,” for the role that, by the way, won her an Oscar.)

But instead of taking great pains to subvert our notions of glamour, what about actresses who simply allow themselves to be seen as they are — especially if they are middle-aged, bespectacled and wrinkly, as Annette Bening looked in “The Kids Are All Right”?

“I did think Annette Bening was brave in being so unadorned; she seemed very real and authentic, and it was an organic part of the part,” says Haskell. “But that’s not the kind of bravery the Academy rewards.”

Tonight, Bening’s nuanced and believable lesbian mother is up against Portman’s much-hyped wraith for the Best Actress prize. Going by precedent, Bening would surely have the Oscar sewn up if only she had played a fat lesbian mother . . .