In her transformation from aspiring actress to Posh Spice in the mid-1990s, Victoria Beckham was a head-turner, known for her curvaceous good looks and designer frocks. But at a London Fashion Week party thrown by jewelry designer Jade Jagger (daughter of Mick) Sept. 20, eyes turned to Beckham for a different reason. Dressed in a silver chain-mail halter top and tight black trousers, Beckham's jutting ribs and collarbones shocked her fellow guests. "She definitely looks like she's had a dramatic weight loss," says Carmel Allen, health and beauty director at British Vogue, "and that is rather worrying."
     It wasn't the first time Beckham's weight had been a source of concern. During her pregnancy with son Brooklyn, born in March, Beckham, now 25, reportedly gained a mere 14 lbs. Then reports circulated that she dieted to fit into an 18-inch corset for her lavish July 4 wedding to the baby's father, British soccer star David Beckham, 24. Victoria says that her waiflike appearance is actually the result of nursing her baby, a low-fat diet and "200 sit-up before bed." But some observers think other factors may be at work. "The more famous you become, the more you're seen, and one becomes terribly self-critical," says Vogue's Allen. "You try to mold yourself into something that you're not."
     Beckham is hardly alone. From backlot soundstages to sitcom sets, many of Hollywood's most celebrated actresses have become sisters in pencil-slim arms, battling to stay competitive in an industry where thin is always in. Although Ally McBeal's Calista Flockhart has drawn the most attention for her swizzle-stick figures, others - from The Practice's Lara Flynn Boyle and All My Children's Kelly Ripa to Friend's Courteney Cox Arquette and Party of Five's Paule Devicq - are also toeing the super-skinny line.
     But have some of these stars gone too far, choosing frailty and fame rather than risk being criticized for having curves? (Witness the drubbing that Alicia Silverstone received for adding a few pounds before filming 1997's Batman & Robin). "They look too thin and unhealthy," says Dr. David Herzog, director of Harvard University's eating-disorders center. With poor nutrition, "they may run the risk of infertility or of osteoporosis, which could put them at risk for fractures. And if you look at the activities that these women are involved in," he adds, referring to the stars' long work days and arduous visits to the gym, "it's unbelievable what they're taking on. Even at a healthy weight it's extraordinary. Underweight, it scares the hell out of you."
     Stuntwoman Sophia Crawford, 31, who doubles for Buffy the Vampire Slayer star Sarah Michelle Gellar, 22, says that, with her weight ranging from 105 to 110 lbs., she's barely a match for the 5'3" Gellar, who, aided by an on-set nutritionist, has eliminated fried foods, sugar and dairy products from her diet and now tips the scales at 98 lbs. "If I was 98 lbs., I wouldn't have the energy to do what I do," Crawford says. "[Sarah] is really, really thin." Courtney Thorne-Smith, 31 (and size 4), has said that if she were not on Ally McBeal, she'd be 5 lbs. heavier but won't risk it for fear she'll look big next to her size-2 castmates. Even Jennifer Aniston, 30, feels the pressure. Despite already having one of the most enviable bodies in Hollywood, the 5'5" actress has reportedly dropped an additional 15 lbs. on a high-protein, low-carbohydrate diet and now weighs 110. Yet, surveying the turnout at last month's Emmy Awards, according to columnist Liz Smith, she was overheard to say, "My God, these girls make me look fat!"
     Kathy Kaehler, Aniston's former trainer, recently distanced herself from the Friends star. "Countless times I have been asked to reveal hew [Jennifer] got so skinny, which combination of exercises did the magic trick," Kaehler wrote in September's Women's Sports & Fitness magazine. "The truth is, her new figure did not come from working out with me. She lost body fat (seemingly all of it) by drastically reducing carbohydrates in her diet - a way that's not healthy in my book."
     It's a book going largely unread in Hollywood. Trainer Greg Isaacs, 38, director of the on-site gym at Warner Bros. studios in Burbank and whose clients include Melanie Griffith and the model Vendela, says the need to be pin-thin has "gone to a new extreme. I don't think there was ever a look as skinny as today's. Linda Hamilton's muscular arms or the G.I. Jane look is not what people are looking for." Instead, he says, the image du jour is "very skinny, less muscle tone" and, in some cases, "a bit unhealthily skinny." In order to achieve it, he adds, "there's some serious dysfunction out there."
     True, some stars insist that being thin is in their genes. According to Sally Boyle, her 29-year-old daughter, Lara Flynn Boyle, "has no eating problems. She's Irish and she has that fast metabolism. People would kill for her metabolism." Others, though, admit they work hard at staying skinny. Flockhart (like Helen Hunt) has discovered spinning - vigorous workouts on stationary bikes. "At first it hurts your butt," said the 5'51/2", 100-lb. Ally McBeal star, 34. "But you become addicted to it like a maniac." Elizabeth Hurley, 34, admits to following the book Eat Right for Your [Blood] Type, which (erroneously, according to experts) claims that someone with type O blood, for example, is descended from hunters and therefore should eat meat, vegetables and rice, but no wheat or dairy. "It sounds like absolute nonsense," Hurley told Cosmopolitan, "but [beau Hugh Grant and I] both lost an astounding amount of weight."
     Even more popular - and less medically sound - is the cigarette and coffee diet, "where Altoids become a meal," says Allure editor-in-chief Linda Wells. "I was at a party in L.A.," she recalls, "and one trainer was bemoaning the fact that so many celebrities have become painfully thin. He said they're not doing it to be strong or fit. They're just trying to whittle their bodies down to nothing."
     Only because that's what the industry pressures them to do, say many actresses. "Producers are in the business of getting as many people to watch their show as they can," Party of Five writer-producer Ian Biederman told WHO (PEOPLE's Australian edition). "Thin is the prevalent idea of what is sexy, and you want stuff on your show that's going to get people to watch." It's a message actress Lauren Holly got firsthand recently. After being cast in the upcoming movie Any Given Sunday, Holly, 35, was told by director Oliver Stone that it was "time to get lean." She went on a 1,500-calorie, high-protein diet and upped her exercise regimen to 30 to 45 minutes of cardio plus an hour of weight training each day, six days a week. In five weeks, she lost 10 lbs. "When you're an actress," Holly's trainer, Valerie Waters, said, "you're expected to maintain an ideal body weight based on an unrealistic standard."
     That standard includes single-digit dress sizes. According to stylists, designers often deliver only small-size samples for TV wardrobes. "Think about it. When was the last time you saw a movie where the lead actress was over a size 6?" Augusta (she goes by only one name), a costumer on FOX's new fall TV series Popular, told WHO, "When I go to a rental house, if I'm looking for a [sexy] size 14, they don't exist. They're all matronly."
     In youth-obsessed Hollywood, looking matronly could mean career suicide. But staying schoolgirl slim by following diets that emphasize protein and all but eliminate carbohydrates poses physical dangers. "Some of these low-carb diets would have you eating pork roast two times a day, just as long as you avoid fruits and vegetables," says Dr. James Dillard, clinical instructor of medicine at Columbia University's College of Physicians and Surgeons in Manhattan. "You need fruits, vegetables and grains. Together, they are the best sources of fiber, B complex and E vitamins, which lower cholesterol and have been proven to lower cancer rates." As for high proteien diets, he says, "they can ultimately lead to kidney damage. But people are so shortsighted. They will do anything to lose weight."
     Until, perhaps, something makes them realize how unhealthy skinny can be. "I used to run eight miles, go to lunch and order my salad dressing on the side. I was always tired and hungry," Ally McBeal's Thorne-Smith told Self magazine. Then, at the end of last season, she got the flu. "I lost 10 lbs. On my body that's a ton of weight. It was scary: I could see I was sick. I looked gaunt. But there were some people who came up to me and said, 'How did you do it?' People thought it was good," says Thorne-Smith, who has cut back to running every other day, "and that was terrifying."
     Scarier still are the lengths to which some go. "People won't eat or drink for days, [then] use suppositories and basically keep themselves on their feet with [over the-counter weight-loss drugs] or some type of diet pill," says L.A. personal trainer Peter Seamans. According to New York City fitness guru Radu, whose clients have included Cindy Crawford and Jennifer Lopez, "Everyone wants to have a quick result and less work. They expect miraculous results - and they get into trouble." Like the L.A. model who didn't eat or drink water - and took diuretics and suppositories - for four days leading up to a photo shoot. Or the well-known actress who so starved herself as her wedding day approached that she passed out before the ceremony. ``The combination of cutting calories and dehydration causes a state we call 'browning out,'" says Hollywood trainer Steven Kates. "You've almost fainting."
     Then there's the impact these Hollywood role models have on younger viewers. "We're seeing quite an increase in inquiries [from parents and therapists] about girls 9, 10 and 11 years old trying to emulate their favorite stars," says Adrienne Ressler, body-image specialist at the Renfrew Center in Coconut Creek, Fla. "For adolescents, the ideal for the person they want to be when they grow up is either a movie star, TV actress or supermodel, and the emphasis is very much on external appearance. Our patients would die - and practically do - to look like Calista Flockhart. They say, 'I want to look like her. I want to be her.'"
     Except for a handful of such celebs as Kate Winslet, The Practice's Camryn Manheim, talk how host Rose O'Donnell and Nickelodeon's Linda Ellerbee who are actively working to counteract the extreme dieting message, Hollywood doesn't seem inclined to help change the mind-set. By 1998, says Cybill Shepherd, "seven shows went off the air, all with women stars over 40. Not just Cybill - Murphy Brown, Roseanne, Grace Under Fire, Murder She Wrote, Dr. Quinn, Medicine Woman and Ellen. There were all different sizes and shapes of women. No wonder those actresses are starving themselves - they want to keep working!" Shepherd, 49, who once crash-dieted almost 20 lbs. from her then 155-lb., 5'9" frame for a 1968 Model of the Year contest, believes she has a solution: "We need to get those women off the Rubens paintings and put them in prime time." Joan Rivers, who eats lots of salads and hits the treadmill religiously to keep herself a size 6, doubts even that will work. One of these days, "a big voluptuous woman is going to come along to be the beauty of the moment," she says. "The tragedy is, she won't be able to get a dress for the awards shows."

* Susan Schindehette
* Karen Grigsby Bates, Michelle Caruso and Laura Schiff in Los Angeles, Joanna Blonska and Ellen Tumposky in London and Olivia Abel and Jennifer Longley in New York City