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

Stocks have held back from big movements Monday as the Dow Jones Industrial Average (^DJI 0.34%) has hung around the red throughout most of the trading day. As of 2 p.m. EST, the Dow had dropped more than six points, although its blue-chip member stocks were split evenly between risers and losers. Insurer UnitedHealth (UNH 0.65%) has dropped 2% to lead the Dow's cadre of losers, while McDonald's (MCD -0.41%) is facing stock trouble of its own after a sales disappointment in January. Let's catch up on what you need to know.

McDonald's feels the chill
McDonald's hit a snag today as it announced its January same-store-sales in the United States plunged 3.3%, more than doubling the 1.6% loss analysts had projected. The frigid temperatures nationwide contributed to the disappointing results, but with the American economy still in recovery and the jobs picture unclear, consumers are not spending enough on fast food to encourage growth at the Golden Arches. Tough competition from hard-charging rivals looking to bite into McDonald's market leadership in the U.S. hasn't helped, and the stock has fallen around 1% so far today.

To compensate, McDonald's has fired up growth overseas. Europe has performed well for the company despite the region's ongoing economic slump, as McDonald's racked up 2% year-over-year growth in the continent in January. The fast-food giant won big elsewhere, with sales in its Asia, Middle East, and Africa group climbing 5.4%. McDonald's recently opened its first store in Vietnam, and while the company needs to focus on getting its U.S. growth back on track, its performance across the Pacific is an encouraging sign for investors frustrated with the company's lack of progress lately.

UnitedHealth's loss comes despite a lack of market-shaking news from America's largest publicly traded health insurer. The company continues to deal with Obamacare's ongoing rollout nationwide which has prompted a mix of insurance enrollment and headaches. The company's recently reported fourth quarter -- its first under the new health law -- paid off in a big way, with 15% year-over-year earnings growth.

Investors need to keep an eye on how UnitedHealth drives enrollment growth through 2014 under the health insurance market's new landscape. Cuts to Medicare will mean it'll be tougher for UnitedHealth to rely on the Medicare market to promote subscriber growth. However, rival Humana recently released optimistic news for UnitedHealth and other insurers as it noted that Obamacare enrollment points to young people signing up for insurance, a critical measure needed to help keep costs down for in the industry. UnitedHealth will need to keep those costs in check as it deals with Medicare cuts and other earnings pressures ahead, particularly as the company's medical cost ratio stands at 81.5% as of its most recent quarter.