What: Shares of nutritional supplement retailer GNC Holdings (NYSE:GNC) were up 12% at 3:05 p.m. today after its full-year outlook impressed investors.

So what: GNC shares have suffered in recent months on concerns over slumping sales, but in-line guidance for the full year suggests that demand is at least stable. So while GNC's Q2 results happened to miss estimates (EPS of $0.79 on revenue of $678.5 million vs. the consensus of $0.81 and $686.4 million), management's outlook reignites some optimism on Wall Street over GNC's long-term growth trajectory.

Now what: Management now sees full-year 2015 earnings of $3.00 to $3.10, versus the average analyst estimate of $3.06. "Our retail business delivered improving trends as the second quarter progressed, which are a result of the sales recapture initiatives identified and implemented earlier in the quarter," said CEO Mike Archbold. "We expect the benefits of these initiatives to continue to contribute in the second half of 2015." When you couple GNC's still-hefty debt load and intense competitive environment with its suddenly hot stock price, however, I'd wait for a much wider margin of safety before betting on it.