Supermarkets joined with first lady Michelle Obama on Wednesday in a pledge to build stores in poor neighborhoods that have historically lacked access to fresh groceries, part of her signature effort to combat childhood obesity.

Participating retailers include Wal-Mart, the country’s largest grocer, Walgreens and Supervalu and regional supermarkets such as Brown’s Super Stores in Philadelphia and Calhoun Foods in Alabama and Tennessee. Together, they promised to open more than 500 stores that will employ tens of thousands of people.

“The commitments that you all are making today have the potential to be a game changer for our kids and for our communities all across this country,” Obama said during a news conference at the White House.

Traditionally, grocers have been wary of opening stores in low-income areas, creating food deserts in many urban and rural markets. A report released this month by the Food Marketing Institute, a trade group, said the costs of security, transportation and employee training are often higher in these areas. In addition, customers may have difficulty getting to the stores or may be too dispersed to justify the investment.

“We knew the conventional wisdom on this issue,” Obama said. “If we truly wanted to end the epidemic of childhood obesity . . . then we didn’t have a choice. We needed to confront this problem head on.”

Retailers have started to take a second look at these markets as their traditional suburban strongholds have become saturated. Wal-Mart, for example, recently unveiled a new, smaller store model called Wal-Mart Express designed specifically for urban and rural neighborhoods.

As part of its pledge Wednesday, Wal-Mart said it would open or remodel 275 to 300 traditional supermarkets over the next five years, a move it expects will create 40,000 jobs. Although the stores are just part of the hundreds of locations the behemoth retailer will probably build during that time, executives said Obama’s initiative helped focus its selection of sites.

“Her leadership causes companies like ours to go back and ask questions that challenge ourselves,” said Leslie Dach, executive vice president of corporate affairs for Wal-Mart.

Grocery chain Supervalu also committed to opening 250 stores within five years, which it expects will generate 6,000 jobs. Drugstore Walgreens said it planned to convert at least 1,000 stores into “food oases” that sell fresh produce. It estimated that 45 percent of its current locations operate in underserved neighborhoods.

A 2009 report by the Agriculture Department defined a food desert as a low-income neighborhood where the nearest grocery store is at least a mile away, or 10 miles away in rural communities. About 23.5 million people — or 8.4 percent of Americans — live in these areas.

“Today we move past talking about statistics . . . and focus on steps toward a solution,” said James R. Gavin III, chairman of the Partnership for a Healthy America, a nonprofit group founded last year to help implement Obama’s obesity prevention efforts.

Improving access to healthy foods is one of the key components of that campaign. Last month, the first lady helped the Agriculture Department revamp the food pyramid into a chart called MyPlate, which left out desserts. But on Wednesday, Obama said that advising families to eat healthy foods was just the first step.

“If parents can’t buy the food they need to prepare those meals . . . then all of that is just talk,” she said.

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