Low-income America is rife with food deserts, where supermarkets are scarce and good food so rare that people have little choice but to shop in corner stores, whose processed and highly caloric foods contribute to obesity.

Build a decent supermarket with good, fresh produce, social scientists have said, and residents will flock to the oasis, their neighborhood a desert no more and their health much improved.

That kind of thinking inspired the creation of a Fresh Grocer store in North Philadelphia, opened to great fanfare - including an appearance by Michelle Obama - on North Broad Street near Temple University in 2009.

Has health in the neighborhood improved?

A landmark study released this month - the first of its kind in U.S. history - says: not really.

Though the market has made people more aware of the availability of fruits and vegetables, shoppers haven't been eating any more produce, and their body-mass index (a combination of height and weight) hasn't declined, according to the study, published in the journal Health Affairs.

Before anyone blames North Philadelphians for failing to get healthier, experts say the mere presence of a 46,000-square-foot supermarket cannot alter human behavior.

"You shouldn't expect a new store to change consumption," said Stephen A. Matthews, a Pennsylvania State University sociologist and one of three authors of the study. "It's naive to think that intervening at one level will be a magic bullet. Obesity is so complex."

The $15 million Fresh Grocer was developed by, among other entities, the Pennsylvania Fresh Food Initiative, a public-private partnership established to increase the number of supermarkets in underserved communities across the state.

Since 2004, the initiative, in partnership with the state and two Philadelphia nonprofits - the Reinvestment Fund and the Food Trust - has helped establish groceries statewide. The work has inspired similar efforts across the United States.

State Rep. Dwight Evans (D., Phila.), who helped launch the initiative, said the Health Affairs study should not be read as the definitive judgment on the effort.

"The fruits and vegetables are something new for many shoppers," he said. "And they're competing with the McDonald's dollar meals, which are cheap and convenient. Think how long it took Americans to accept seat belts in cars, and to understand the dangers of smoking."

Hannah Lawman, an obesity researcher at Temple, agreed. "This is very slow stuff, changing people's eating and shopping patterns," she said.

And judging low-income people for not eating enough fruits and vegetables makes little sense in a country that practically celebrates the chance to eat poorly, said Shiriki Kumanyika, an epidemiologist and obesity expert at the University of Pennsylvania.

"If supermarkets alone could cure unhealthy diets, what Americans eat would look very different," she said. "The choices people make in supermarkets are bad if you look at the overall American diet. And that's regardless of income."

Aside from what people eat, the study teased out an interesting fact about where people shop.

Of the more than 300 people surveyed who lived within a mile or so of the Fresh Grocer, 83 had adopted it as their main store within six months of its opening. Most people continued to shop elsewhere - as many as 40 other stores, according to Matthews.

Some shopped as far away as the Reading Terminal Market in Center City, which has long been a favorite of low-income shoppers because of easy transit access, antihunger experts say.

Matthews said people are still loyal to their old stores. And for many, Fresh Grocer - though less pricey than the corner stores - still is more expensive than other surrounding venues, local food experts say.

Obviously, smart shopping alone cannot end obesity, Lawman said. Though childhood obesity in Philadelphia decreased from 21.5 percent to 20.5 percent between 2006 and 2010, health officials say, it's still a severe problem for children and adults.

Three factors drive people to eat unhealthful foods, said Kelly Brownell, dean of the Sanford School of Public Policy at Duke University: advertising, especially to children; ubiquitous access to such foods; and attractive pricing.

"Any effort this nation undertakes regarding obesity must include decreasing unhealthy foods," Brownell said. "That's why we work on taxes on soda, and decreasing the barrage of ads for unhealthy foods that is aimed at children."

Though healthy food options are important, the availability of kale and apples won't mean much if low-income families can't pay for them, said Julie Zaebst, policy manager of the Greater Philadelphia Coalition Against Hunger.

Recent cuts to food stamps by the federal government have the potential "to worsen the health outcomes of families in our state by making it much harder for them to afford the food they need," she said.