Although we don't believe in timing the market or panicking over market movements, we do like to keep an eye on big changes -- just in case they're material to our investing thesis.

What: Shares of GNC Holdings (GNC) plunged 13% today after the health supplements retailer's quarterly results and outlook disappointed Wall Street.

So what: The stock has pulled back in recent months on concerns over slowing growth, and today's fourth-quarter miss -- adjusted earnings per share of $0.63 on revenue of $613.7 million versus consensus of $0.64 and $631.5 million -- coupled with downbeat guidance only reinforces those worries. In fact, operating margin during the quarter slipped about 140 basis points from the year-ago period, suggesting that intensifying competition is forcing GNC into heavy discounting.

Now what: Management now sees full-year EPS of $3.18-$3.24, significantly lower than the average analyst estimate of $3.46. "Despite the challenging retail environment, our business performed well, generating solid top and bottom line growth in the quarter," Chairman and CEO Joe Fortunato reassured investors. "This culminated in a strong year where we delivered a 22.3% increase in adjusted earnings per share and returned more than $350 million to shareholders, all the while making significant investments in the business which allows us to maintain growth momentum and to capitalize on our industry growth, optimize our customer base, and position the Company for new growth opportunities." With the stock off about 25% from its 52-week highs and trading at a forward P/E in the low double digits, now might be a good time to buy into those prospects.