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.

Much of the time, the U.S. stock market is the first to rise or fall when market-moving news hits, leaving foreign stock markets to follow in its wake. Today, though, China asserted its new role as a major global economic powerhouse, with many analysts blaming a poor reading on the country's manufacturing sector for the big drop in the Dow Jones Industrials (DJINDICES:^DJI). The HSBC flash manufacturing purchasing-managers index reading for China came in at 49.6 for January, indicating the potential for contraction in the industry. That sent stocks falling around the world, and the Dow was down 175 points as of 11 a.m. EST.

Declines in the Dow came from across the board, with only a handful of stocks managing to climb. Two of them were telecom giants AT&T (NYSE:T) and Verizon (NYSE:VZ), which respectively gained 0.6% and 0.4%. Both companies have focused almost all of their resources on the U.S. wireless market, and although they do have some international growth prospects, the impact of further sluggishness in the Chinese economy doesn't affect them to nearly the same extent as companies with more direct connections to China.

Some of the worst losses came from companies that arguably have the most to lose from falling Chinese growth. Credit card giants Visa (NYSE:V) and American Express (NYSE:AXP) posted respective losses of 1.6% and 2.3%, weighing the potential fallout on their respective customer bases from weakness in China. For American Express, which relies on higher-end card members for much of its revenue, strong economic growth throughout Asia has fueled growth both from new prospective customers there and from existing customers with travel interests in the region. Similarly, Visa has hoped to cash in on international growth as it aims to hold off its main rival and its greater focus overseas.

It's far too early to overreact to a single reading on the Chinese economy, and the moves in the Dow and some of its worst-performing components today could well prove short-lived. Nevertheless, today's market shifts are a good reminder to investors that you have to pay attention to what's happening outside the U.S. in order to have a complete picture of what could push your portfolio up or down on any given day.