PARIS — “It’s the only game you can play where you win when you lose.”

That’s how Karl Lagerfeld describes his adventures in dieting, which famously allowed him to shed more than 90 pounds in a year and is the subject of a hotly anticipated book, “3-D,” due out early next month.

Written by Lagerfeld’s doctor, Jean-Claude Houdret, with a foreword by the designer, the 250-page, soft-cover tome is destined to become the fashion diet.

But Lagerfeld’s most crucial advice to those who want to lose weight is free: to approach the task, pardon the pun, lightly. Lagerfeld has made no bones about the fact that he transformed his physique only for fashion: to wear the stick-to-the-ribs tailoring being popularized by Hedi Slimane, the designer of Dior Homme.

“You have to do it for bad and superficial reasons,” he said in an interview this week over a lunch of steamed fish and vegetables, with cinnamon-flecked fromage blanc for dessert. “I did it because I wanted to be a hanger for a different kind of clothes.”

By contrast, if a person is dieting for a “deep” reason like improving self-esteem, then it’s a recipe for disaster. “If you have to lose weight and the results are not immediate, then depression waits for you,” he said. “Do it for something superficial and the rest will improve along with it.”

For Lagerfeld, no setbacks derailed his quest to become a slim man. He dipped below 150 pounds last December and he now hovers around 135, what he weighed at age 18. His narrow and nipped Dior Homme suit in gray cashmere flannel fits like a dream.

For that, he can thank Houdret, a general practitioner, nutritionist and a disciple of the French physician renowned for discovering and articulating the role of essential elements like gold and zinc in human health.

Lagerfeld discovered Houdret simply because he took over the practice of his former physician, who had retired. The designer stated his desire to lose weight to Houdret, who worked out his low-fat, low-low carb regime, rounded out with protein powders and vitamin and mineral supplements.
Lagerfeld stressed that the diet requires that a person be in good health and of strong discipline. “I listened to the doctor,” he said. “I had no personal opinion.”

Plenty of would-be dieters like to continue drinking alcohol, smoking or eating sweets. For Lagerfeld, who neither drinks nor smokes and whose principal indulgence is Diet Coke, clean living has paid off.

Houdret is vacationing and could not be reached for comment. But Lagerfeld assures that his physician has been swamped with requests to get on the Karl Plan.

Astounded by his dramatic new silhouette, some people have accused Lagerfeld of taking drugs or even having liposuction. He said both accusations are false; he just fortunately has a body that responds well to diet and one that still remembers all the sports he played as a youth. Lagerfeld said he doesn’t exercise that much now to avoid bulking up. Indeed, the only chemical aid he employed was a Clarins anticellulite cream to speed the slimming of his hands. A favorite ring, now loose around his finger, is a constant reminder of his accomplishment.

“You can hardly pinch me,” he said. “I’m a piece of wood.”