NEW YORK — Steve Tarpin can bake a graham cracker crust in his sleep, but explaining why the price for his key lime pies went from $20 to $25 required mastering a thornier topic: global economics.

He recently wrote a letter to his customers and posted it near the cash register listing the factors — dairy prices driven higher by conglomerates buying up milk supplies, heat waves in Europe and California, demand from emerging markets and the weak dollar.

The owner of Steve's Authentic Key Lime Pies in Brooklyn said he didn't want customers thinking he was "jacking up prices because I have a unique product."

"I have to justify it," he said.

The U.S. is wrestling with the worst food inflation in 17 years, and analysts expect new data due tomorrow to show it's getting worse. That's putting the squeeze on poor families and forcing bakeries, bagel shops and delis to explain price increases to their customers.

U.S. food prices rose 4 percent in 2007, compared with an average 2.5 percent annual rise for the last 15 years, according to the U.S. Department of Agriculture. And the agency says 2008 could be worse, with a rise of as much as 4.5 percent.

Analysts are forecasting that tomorrow's Department of Labor report will show the Consumer Price Index rose at a 4 percent annual rate in the first three months of the year, up from last year's overall rise of 2.8 percent.

For the U.S. poor, any increase in food costs sets up an either-or equation: Give something up to pay for food.

"I was talking to people who make $9 an hour, talking about how they might save $5 a week," said Kathleen DiChiara, president and CEO of the Community FoodBank of New Jersey. "They really felt they couldn't. That was before. Now, they have to."

For some, that means adding an extra cup of water to their soup, watering down their milk, or giving their children soda because it's cheaper than milk, DiChiara said.

U.S. households still spend a smaller chunk of their expenses for foods than in any other country — 7.2 percent in 2006, according to the USDA. By contrast, the figure was 22 percent in Poland and more than 40 percent in Egypt and Vietnam.

In Bangladesh, economists estimate 30 million of the country's 150 million people could be going hungry. Haiti's prime minister was ousted over the weekend following food riots there.

Still, the higher U.S. prices seem eye-popping after years of low inflation. Eggs cost 25 percent more in February than they did a year ago, according to the USDA. Milk and other dairy products jumped 13 percent, chicken and other poultry nearly 7 percent.

USDA economist Ephraim Leibtag explained the jumps in a recent presentation to the Food Marketing Institute, starting with the factors everyone knows about: sharply higher commodity costs for wheat, corn, soybeans and milk, plus higher energy and transportation costs.

The other reasons are more complex. Rapid economic growth in China and India has increased demand for meat there, and exports of U.S. products, such as corn, have set records as the weak dollar has made them cheaper. That's lowered the supply of corn available for sale in the U.S., raising prices here.

For some Americans, the resulting increases might be barely perceptible. The Cheesecake Factory raised prices by 1.5 percent at the end of February, Applebee's by 3 percent.

But for the poorest U.S. families, the higher costs may mean going hungry. A family of four is eligible for a maximum $542 a month in food stamps, which never lasted the whole month before, FoodBank of New Jersey's DiChiara said.

"Now food stamps go fewer and fewer days of the month," she said.