While Fools should generally take the opinion of Wall Street with a grain of salt, it's not a bad idea to take a closer look at particularly stock-shaking analyst upgrades and downgrades -- just in case their reasoning behind the call makes sense.

What: Shares of GNC Holdings (NYSE:GNC) rallied 2% this morning after Bank of America initiated coverage on the health supplements retailer with a buy rating.

So what: Along with the bullish call, analyst Curtis Nagle planted a price target of $50 on the stock, representing about 36% worth of upside to yesterday's close. So while momentum traders might be turned off by GNC's sharp year-to-date pullback, Nagle's call could reflect a sense on Wall Street that the concerns surrounding the retailer's growth trajectory are becoming overblown.

Now what: Bank of America believes GNC has a good chance to generate sustainable comparable growth, especially after its recent contract with market researcher Dunnhumby. "In our view, this will lead to multiple expansion and could drive upward revision in consensus expectations and ratings (GNC had six downgrades after 4Q13 and 1Q14)," said Nagle. "We also believe that near-record-low short interest could indicate a bottoming of negative sentiment." With the stock still off about 40% from its 52-week high and trading at a PEG of 0.8, the downside certainly seems limited enough to bet on it.