PHILADELPHIA — Some of the fattest people in America are among the poorest.

And with food prices rising, the problem is likely to get worse.

Tianna Gaines, who describes herself as impoverished and obese, knows this. At 5-foot-3 and 242 pounds, she lives on public assistance and eats junk food because it’s cheap and more readily available in her neighborhood than carrots and apples.

Besides, said Gaines, 28, and a mother of three, “I don’t have the money for Bally’s fitness clubs. And I can’t run here. They shoot you.”

More poor people may suffer Gaines’ fate, with the U.S. Department of Agriculture predicting food prices will be up 4.5 percent throughout the year, due to high fuel costs, weather problems, and the growing diversion of corn crops to make ethanol. Globally, prices will rise nearly 50 percent, according to the president’s Council of Economic Advisers.

“The food crisis will make obesity and attendant diabetes even more rampant,” said University of Washington epidemiologist Adam Drewnowski. “Fruits, vegetables and fish are becoming luxury goods completely out of reach of many people. Consumption of cheap food will only grow.

“Obesity is the toxic consequence of a failing economy.”

While more people from every economic background are becoming obese around the world, the poor are still outpacing the better-off.

A recent U.S. Department of Health and Human Services study found that women in poverty were roughly 50 percent more likely to be obese than those with higher socioeconomic status.

In U.S. households making less than $15,000 a year, 31 percent of the women are obese, according to the federal Centers for Disease Control and Prevention. In households with more than $50,000 annually, 17 percent are obese.

University of Pennsylvania epidemiologist Shiriki Kumanyika and other investigators found that poor 15-to-17-year-olds — black or white, male or female — were 50 percent more likely to carry excessive poundage than nonpoor teens.

And a study by Drewnowski last year showed that obesity rates in poor Seattle neighborhoods were 600 percent greater than in rich areas.

Poor people frequently live in “food deserts” — neighborhoods with few supermarkets. They rely on corner stores and convenience marts for groceries, said Carey Morgan, director of the Greater Philadelphia Coalition Against Hunger.

These are great places to buy chips and soda, not so good for asparagus.

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.