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 upgrades and downgrades -- just in case their reasoning behind the call makes sense.

What: Shares of UnitedHealth (NYSE:UNH) slipped slightly this morning after Cantor Fitzgerald downgraded the health insurer from buy to hold.

So what: Along with the downgrade, analyst Joseph France lowered his price target to $70 (from $75), representing about 2% worth of downside to yesterday's close. While value investors might be attracted to the stock's pullback yesterday -- fueled by a disappointing Q3 -- France believes that the upside remains limited given the several risks that UnitedHealth still faces.

Now what: Cantor lowered its 2013 EPS estimate to $5.45 from $5.50 and its 2014 view to $5.75 from $6.10.

"We are lowering our rating on UnitedHealth from BUY to HOLD to reflect its recent move near our $75 price target," noted Cantor. "Although the company's 3Q:13 results were inline with expectations, and there is no significant change in its guidance (although management was more specific about the outlook than in the past), we see little upside in the stock, given uncertainty about reform, costs and utilization."

With UnitedHealth shares up about 40% from its 52-week lows and trading at a clear P/E premium to its peers, I'd agree that the risk/reward trade-off looks unappealing.

Fool contributor Brian Pacampara has no position in any stocks mentioned. The Motley Fool recommends UnitedHealth Group. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.