Calling all new GNC (GNC) bulls: chill out, this is one company that's far from a healthy investment. 

Shares of the beleaguered vitamin seller exploded 51.6% on Thursday after it shocked Wall Street with an upbeat preliminary fourth-quarter outlook. The company said it sees adjusted earnings in a range of 24 cents to 25 cents a share, up from 7 cents a year ago. Same-store sales for domestic company-owned stores, plus GNC's website, are expected to increase 5.7%. 

The crazy session for GNC pared the stock's two-year decline to about 83%.  

Wall Street has been quick to caution on being too optimistic off GNC's holiday profit surprise. 

Analysts at Barclay's said GNC mostly benefited from an overall improvement in the economy and discounts on vitamins. "We do not believe GNC is yet on a sustainable path despite recent comp sales momentum," said analyst Karen Short. 

But it's Short's price comparison study that should really worry GNC bulls. It hints that GNC will continue to struggle against rivals such as Amazon (AMZN) and Vitamin Shopper (VSI) as its prices are too high. 

GNC's prices were a whopping 11% greater than Amazon and store rival Vitamin Shoppe, according to Short's pricing study of 30 identical branded items at the three retailers. Prices at Vitamin Shoppe and Amazon were below GNC for 90% and 80% of items sampled, respectively. 

Concluded Short, "We believe merchandise margins at GNC will continue to be pressured until this pricing gap narrows."

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