Wal-Mart sneezed today. Will the economy catch a cold?

The company, the nation’s largest retailer and a bellwether for the economy, warned that its profit would be lower than expected for the year, citing higher energy costs and interest rates, which left its low-income shoppers with less money to spend.

“Many customers are running out of money at the end of the month,” said H. Lee Scott Jr., Wal-Mart’s chief executive.

The sobering forecast sent Wal-Mart’s shares down $2.30, or 5 percent, as of 12 p.m., and appeared to spook the broader market. The Dow Jones industrial average and the Standard & Poor’s 500-stock index each dropped about 0.8 percent in midday trading, with the Dow losing more than 100 points.

Given how jumpy investors have been lately, “Any forward-looking guidance from companies or from industries will take on more importance than has been the case in the past,” said Liz Ann Sonders, chief investment strategist for Charles Schwab. Referring to consumer sentiment, she added, “Wal-Mart is as good of an indicator on that as any other.”

But Wal-Mart also blamed itself, for poor clothing and home décor products, making it difficult to discern how much of the trouble was self-inflicted, and how much was tied to larger economic forces.

For the second quarter, which ended Aug. 3, Wal-Mart missed its profit estimates and those of Wall Street analysts, a rarity for the company, whose finances are generally the envy of the industry. Earnings per share from continuing operations were 72 cents, below the company forecast of at least 75 cents. Even so, sales rose 8.8 percent, to $92 million.

Net income rose 49 percent, to $3.1 billion, or 76 cents a share. But that figure included 4 cents per share worth of one-time gains like lower workers’ compensation claims. (Excluding those gains, earnings per share were 72 cents.)

“Although some people will report that Wal-Mart has had record sales and earnings, our underlying operating performance this quarter was not what we expect of ourselves, and not what our shareholders expect of us,” Mr. Scott said.

For the year, Wal-Mart said it would earn $3.05 to $3.13 a share from continuing operations, lower than its original forecast of $3.15 to $3.23.

Mr. Scott, the chief executive, said Wal-Mart’s shoppers, who generally earn less than $40,000 a household, are “under difficult pressure economically.”

He added that “the paycheck cycle is more pronounced now than ever before,” meaning customers are left with little cash by the end of the month.

Wal-Mart has lowered prices across its stores to appeal to such customers, but the strategy has hurt its profit margins.

Sales of grocery items, electronics and pharmacy items rose considerably during the second quarter, but clothing and home products — which sell at a high profit margin — did not. Eduardo Castro Wright, who runs Wal-Mart’s United States stores division, said, “We see pressure in all areas of apparel.”

Charles Grom, an analyst at J. P. Morgan, downgraded Wal-Mart stock this morning, saying its second-quarter earnings “ is creating a slippery slope for Wal-Mart to climb.”

“Wal-Mart’s lowered outlook,” Mr. Grom wrote, “is more than just resetting the bar this morning. Rather, the company and this management has suffered a credibility blow that will take time to overcome.”