Feb. 15 (Bloomberg) — Weight Watchers International Inc.'s disappointing profit forecast provided further evidence that new mobile applications and bracelets that track calories are hurting traditional diet companies.

Weight Watchers, endorsed by celebrities like Jessica Simpson, reported its fourth straight quarterly sales decline after the close on Feb. 13 as fewer people attended meetings and bought its products. The company also projected earnings that trailed analysts' estimates, sending the stock plunging 28 percent to $22.10 yesterday, the worst one-day drop since its initial public offering more than 12 years ago.

The five-decade-old company — founded as a home business by a housewife in Queens — is struggling to adjust to the Internet era. While Weight Watchers has a strong brand name and reputation as a diet that works, it now has to compete against free alternatives online, said Kurt Frederick, a Los Angeles- based analyst at Wedbush Securities Inc. That's hurt the company both on the Internet and off. Weight Watchers' website saw its online users decline 6.7 percent last quarter.