Usana Health Sciences Inc., a marketer of vitamins and nutritional supplements, has set sales records in every one of its past 18 quarters while watching its stock price soar more than 1,600 percent over that time.

But the Salt Lake-based company's unusual sales system is drawing a skeptical review from Barry Minkow, the convicted stock-fraud felon turned private investigator who has bought "put" options on Usana's shares in a bet the price will fall.

Usana stock plummeted Thursday, closing down $8.92, or 15.2 percent, at $49.85 per share on the Nasdaq Stock Market.

Minkow is no ordinary gumshoe. After serving jail time in the 1990s for stock fraud in the ZZZZ Best debacle, he became a Christian pastor, founded a San Diego company that hunts for other potential frauds and has won praise from the Federal Bureau of Investigation. He recently helped expose Pinnacle Development Partners, an Atlanta real-estate marketer whose founder has been indicted.

Usana Health Sciences, like Amway Corp. and other "multilevel marketers," uses at-home distributors, or "associates," to sell products, according to company documents and interviews. It arranges them in a hierarchy. One person is at the top, two distributors sit below him, four below them, eight on the next level and so on. The company pays a distributor a commission of about 8 percent on sales made by those below him in the hierarchy, up to a certain limit. But to be eligible for commissions, distributors must buy or refer to the company $116 in orders each month.

Usana — which disagrees with Minkow's analysis of it — uses the recruitment slogan "True Health and True Wealth," and its Web site offers "high income potential." The company has said it holds 500 recruitment meetings a week around the country. About 86 percent of Usana's revenue comes from sales to its 153,000 associates. Only 14 percent comes from sales to customers unaffiliated with the company. Last year, the company earned $41 million on sales of $374 million.

Usana, which was founded in 1992, was trading Wednesday at about 27 times its 2006 per-share earnings, well ahead of the Standard & Poor's 500-stock index's multiple of 17. Analysts expect strong earnings growth of 20 percent this year.

Minkow says the company's sales model is unsustainable because it requires the constant recruitment of new associates. Eventually, he argues, the company will run out of distributors, who will face long odds selling products or recruiting new disciples. Usana's major product, a multivitamin, is more expensive than rivals.

As of the end of 2005, only 37 percent of Usana's associates had ever earned a commission, according to the company's latest figures. Among those who had been paid, the figures show, 87 percent didn't earn enough to cover the $116 they have to purchase or refer each month to qualify for commissions.