Concentrating on filling their stomachs, poor, hungry people go for high-fat, high-sugar foods. "They're not thinking about health — just getting through the day," said Mariana Chilton, a hunger expert at the Drexel University School of Public Health, and principal investigator for the Philadelphia GROW Project, which deals with nutrition and physical development among poor children.

Derek Felton, a community organizer with the Coalition Against Hunger and a former poor, obese person, agreed.

"I was the oldest of seven, with a lifetime of no breakfasts to eat," said Felton, who is 5-foot-9 and went from 247 pounds to 185. "When we had the chance to eat, we ate white bread to feel full."

All that corner-store processed food is relatively inexpensive — artificially so. Researchers say that many junk foods contain high-fructose corn syrup, made from government-subsidized corn crops. Federal help keeps the cost of syrup-containing foods such as sodas, fries and even burgers down. Drewnowski said that healthful, unsubsidized foods like spinach cost five times more per calorie to produce, thus driving up the price.

Food stamps are supposed to help. But Chilton's research shows that the allotments families in Philadelphia receive are not accounting for higher food prices.

As a result, families often run out of food stamps by the second or third week of the month, Chilton said.

The hunger can be excruciating, said Gaines, who lets her three children under age 4 eat whatever food is left after the stamps are gone.

It makes her all the more voracious at the beginning of each month, when the new stamps arrive.

"You go without eating, then gorge," Gaines said. "Then you go to sleep with a full stomach. That's how the weight picks up."

It works that way for lots of people. And with the current food inflation, even cheap foods are getting more expensive.

"What choices can poor people afford now?" asked Stella Volpe, a nutritionist at the University of Pennsylvania's School of Nursing. "Will their diets get even worse, and will hard times contribute to more obesity?"

Interventions to improve the eating habits of the poor in Philadelphia have "failed miserably," according to Terri Lipman, a colleague of Volpe's at the nursing school.

"We've told kids to eat fruit and walk," she said. But fruits are scarce and walking in poor neighborhoods can be dangerous, Lipman added.

Current school programs, like dance aerobics, are making some inroads toward health, she said. And Students Run Philly Style, which teaches poor kids to run in supervised settings, is creating good results, said Heather McDanel, director of the National Nursing Centers Consortium project, which works with Students Run.

"BMIs are down and kids' self-esteem is up," she said.