For four decades, Yves Saint Laurent and Karl Lagerfeld have been the reigning kings of fashion, internationally lionized designers celebrated for their good taste and impeccable style. But according to a new book, their lives have not been quite as glamorous as advertised. Alicia Drake's sprawling biography, The Beautiful Fall: Lagerfeld, Saint Laurent and Glorious Excess in 1970s Paris, exposes the true depths of the bitter rivalry between the living legends and the inner demons that plagued them.

When Lagerfeld recently obtained a copy, he fired off an angry letter to Drake, accusing her of making over 100 mistakes in the book's description of his childhood. "There is this fantasy he has created and he has become that fantasy," Drake tells Radar. "He disputes some details of his childhood and he's upset that his fantasy has been exposed."

According to Drake, even before his starvation diet, the famously insecure Lagerfeld had intense body issues that resulted in dramatic weight fluctuations and odd dietary ticks. At one point, he drank 15 bottles of Coke a day and constantly gorged on chocolates and cheese slices, which he kept in a mountainous pile on his desk at all times. In his thirties, he began working out four times a week in his private gym but eventually got "bored to death" and quit exercising altogether. Instead, he disguised his ballooning figure with an adjustable "over-blouse" he created for himself. According to Drake, his weight wasn't the only thing he tried to disguise. While Lagerfeld claims he was born in 1928, the author unearthed proof he was born in 1933, though it's unclear why he would inflate his age by five years.

In contrast, Saint Laurent maintained a trim figure for most of his life, thanks to a serious drug habit. His dalliance with heroin began in Marrakesh where he and his neighbor, billionaire J. Paul Getty's wife, Talitha, threw extravagant hash- and opium-fueled parties that drew the cream of seventies Europe's pop and fashion crop. At one particularly wild bash, Saint Laurent got so high after eating a block of hash he redecorated his entire house in a single afternoon.

By 1976, Saint Laurent was downing two liters of whiskey a day, and his prolonged absence from the fashion scene fueled persistent rumors of his death, which grew louder in 1985 when he was hospitalized after falling into an alcohol-induced coma.

According to the author, their enmity was professional as well as personal. Envious of Saint Laurent's place in the fashion firmament, and furious over his attempt to woo his boyfriend away with flowers and passionate letters, Lagerfeld took to demeaning his older mentor at every turn—a habit that drove mutual friends to dub him "Katty Karl." (According to the book, Paloma Picasso grew so tired of Lagerfeld's acerbic outburts she fired him from her label.)

Saint Laurent could be equally bitchy. Asked for his opinion of Lagerfeld's work at Chanel, he said, "That poor woman [Coco Chanel] must be turning in her grave." (Not that he reserved his opprobrium for Lagerfeld. When Tom Ford declared that his first shows were an homage to Saint Laurent, the designer sniped, "I think he has a lot of talent, for—what do you call it?—marketing...He does what he can, poor guy.")

Though they're in their seventies now and their reputations are well-established, both men refuse to put the past behind them. For the moment, at least, Lagerfeld seems to be relishing the limelight that eluded him until his more famous rival finally retired in 2002, traveling the world attending celebrity studded parties in his trademark black jeans and fingerless gloves. Saint Laurent, meanwhile, remains a virtual recluse, holed up in his home on the rue de Babylone, obsessively watching reality TV and rarely venturing out, except to walk his dog.

Archival photos: (from top) Antonio Lopez, Guy Marineau