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 Kellogg (NYSE:K) slipped 2% in premarket trading Tuesday after Goldman Sachs downgraded the cereal giant from neutral to sell.

So what: Along with the downgrade, analyst Jason English lowered his price target to $59 (from $64), representing about 11% worth of downside to yesterday's close. So while contrarians might be attracted to Kellogg's price weakness since early June, English's call could reflect a sense on Wall Street that its growth prospects are just too limited to trigger a significant rebound.

Now what: Goldman lowered its 2014, 2015, and 2016 EPS estimates by 4%, 6%, and 6%, respectively. "[T]op-line rejuvenation efforts are falling flat and the portfolio is coming under what we believe is a retail lead SKU rationalization knife," said English. When you couple that downbeat view with Kellogg's hefty debt load and forward P/E of 15, it's tough to disagree with Goldman's bearish stance.