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.

Nervousness about investing has made a comeback recently, with last week's declines reminding investors that stocks do in fact go down sometimes. A report today from Japan signaling weaker second-quarter GDP growth of just 2.6% annualized called into question the island nation's recent economic policies, which have centered on dramatically devaluing the yen against the U.S. dollar and the currencies of other major export markets. Given this and ongoing worries about a possible correction after such an impressive advance for U.S. stocks, the Dow Jones Industrials (^DJI 0.69%) continued their slide from last week by opening lower, but by 10:55 a.m. EDT, the average had recovered to a gain of a few points.

Still, fears also made themselves evident among individual Dow stocks. UnitedHealth (UNH -1.03%) is one of the biggest decliners in the Dow this morning, falling about 1.1% as shareholders continue to argue about what impact Obamacare and other health care reform efforts will have on the major insurance company. With social media starting to provide an outlet for repeal efforts, it's almost impossible to know whether the law will ever even take full effect, let alone remain in its currently anticipated form. The resulting uncertainty leaves UnitedHealth struggling to figure out how to respond to the changing cross-currents in the health care industry.

Chevron (CVX 0.57%) has dropped almost 1% as it faces the problem of how best to make natural gas a more portable commodity in world markets. Given plentiful natural-gas production in the U.S. and other concentrated areas of the world, the challenge is how to get that gas to the markets that need it the most. Chevron has been trying to ramp up its liquefied-natural-gas capacity in an effort to meet what most see as rising long-term demand, but worries over how well the oil giant can nimbly respond to changing conditions have to worry shareholders who have nevertheless seen the stock hit all-time highs as recently as last month.

Finally, earnings continue to make waves across the market. This morning, Sysco (SYY -0.47%) has plunged 4.8% after reporting that its quarterly profit dropped by 8.5% compared to last year. With its leadership position within the food-services market, the company has managed to keep sales growth at reasonable levels, posting gains of 5% and topping revenue estimates. But even with the company's CEO arguing that the company should see gradual improvement, Sysco still shows that even well-established companies have to stay in front of changing trends to avoid the earnings misses that shareholders fear most.