“These numbers are a wake-up call for the country,” said Agriculture Secretary Tom Vilsack.

One figure that drew officials’ attention was the number of households, 506,000, in which children faced “very low food security”: up from 323,000 the previous year. President Obama, who has pledged to end childhood hunger by 2015, released a statement while traveling in Asia that called the finding “particularly troubling.”

The ungainly phrase “food insecurity” stems from years of political and academic wrangling over how to measure adequate access to food. In the 1980s, when officials of the Reagan administration denied there was hunger in the United States, the Food Research and Action Center, a Washington advocacy group, began a survey that concluded otherwise. Over time, Congress had the Agriculture Department oversee a similar survey, which the Census Bureau administers.

Though researchers at the Agriculture Department do not use the word “hunger,” Mr. Obama did. “Hunger rose significantly last year,” he said.

Analysts said the main reason for the growth was the rise in the unemployment rate, to 7.2 percent at the end of 2008 from 4.9 percent a year earlier. And since it now stands at 10.2 percent, the survey might in fact understate the number of Americans struggling to get adequate food.

Rising food prices, too, might have played a role.

The food stamp rolls have expanded to record levels, with 36 million Americans now collecting aid, an increase of nearly 40 percent from two years ago. And the American Recovery and Reinvestment Act, passed last winter, raised the average monthly food stamp benefit per person by about 17 percent, to $133. Many states have made it easier for those eligible to apply, but rising applications and staffing cuts have also brought long delays.