(FORTUNE Magazine) – There are many beautiful bodies in the General Nutrition Center on Seventh Avenue in Manhattan's Greenwich Village. Most of them belong to men. Big, rippling men in tight tank tops, spandex shorts, and combat boots. On an unseasonably warm May afternoon, these Adonises are hungrily shopping for more of the "miracle" powder that helped create their chiseled physiques. It's called creatine, and some call it Nature's steroid. Since its introduction into the U.S. market in 1993 by a small, privately held company called Experimental & Applied Sciences, creatine has become one of the hottest items in the $1.4 billion sports-supplement market, with sales expected to double from $100 million to $200 million in 1998.

Synthetic creatine, which duplicates an amino acid compound produced by the liver, kidneys, and pancreas, owes its fame to two facts: (1) It's legal, and (2) it works. Creatine supplements have been shown to significantly increase muscle mass, strength, and recovery time for athletes by recycling the energy that muscles release during workouts back into energy that they can burn. And while the body naturally produces around one or two grams of creatine every day, the synthetic supplement allows an athlete to simply mix a scoop of powder--grape-flavored if he (or she) likes--in water and chug it. Although no long-term studies have been completed--to the chagrin of critics who worry that its rampant use is premature--the negative side effects of creatine seem restricted to occasional nausea and cramping in some users.

When EAS began distributing creatine in 1993, the supplement was mainly the secret of professional athletes and bodybuilders abroad. For example, British sprinter Linford Christie, who set a world record in the 100-meter sprint at the '92 Olympics in Barcelona, drank creatine before the race. In 1995, EAS owned 90% of what was still a paltry $30 million creatine market. But as stories like the one about Christie began to travel, more professional athletes in the U.S. began to use creatine and hype it publicly--American sprinter Michael Johnson swore by it in the '96 Atlanta Olympics--and the current frenzy was born. Sports Illustrated estimates that a quarter of all professional baseball and basketball players, and half of all professional football players, use creatine. Today EAS's brand is promoted by such stars as the Baltimore Orioles' Brady Anderson and the Denver Broncos' John Elway.

As the market for creatine has grown, so has the competition: Other sports supplement manufacturers like Weider, General Nutrition Centers, and TwinLab have joined the fray. (EAS's market share, meanwhile, fell to 30% last year. Despite all the new competition, however, creatine remains a significant business for EAS: In 1997 the muscle powder generated one-third of the company's $100 million in revenues; EAS's creatine sales continue to grow 15% a year.)

According to Grant Ferrier of the Nutrition Business Journal, for creatine's sales growth to continue, companies will have to broaden the market by concocting creatine bars and sports drinks. But Bill Phillips Sr., president of EAS, recoils at the thought that creatine might someday be peddled to consumers who think that watching ESPN counts as exercising. "This is a supplement designed for use with a strenuous workout," he says.

Phillips' warnings aside, creatine has the potential to become the sports-supplement equivalent of St.-John's-wort, the natural antidepressant that introduced mainstream America to herbal remedies. Already, the creatine craze has accelerated the growth of the rest of EAS's sports-supplement business from 10% a year to 15%, a trend matched in the rest of the industry. "I think customers go into stores to buy creatine, but then they see other products that they would never have considered using, and that has helped our sales," says Phillips. "It also gets us a new kind of customer."

--Shaifali Puri