WASHINGTON — Michelle Obama’s campaign against childhood obesity moved a step forward Wednesday with the announcement that Walmart and other retailers plan over the next five years to open or expand 1,500 stores in areas without easy access to fresh fruit, vegetables and other healthy foods.

“Make no mistake about it. This is a big deal. It is a really big deal,” the first lady said at the White House, where she was joined by executives from the national and regional retailers.

“I think our vice president put it better, but I’m not going to use his words,” she said, drawing laughter from an audience that remembered the profanity Vice President Joe Biden whispered last year as he stressed to President Barack Obama what a big deal it was to sign the health care bill into law.

Mrs. Obama is leading a nationwide effort to lower childhood obesity rates, including by making fresh and healthier foods more widely available. She has urged grocers to expand into so-called “food deserts,” which are rural or poor areas without many grocery stores.

The White House says nearly 24 million people, including 6.5 million children, live in such areas. Studies have shown that limited access to healthy foods like fruits and vegetables can lead to higher levels of obesity and diet-related diseases.

Walmart Stores Inc., the nation’s largest retailer, says it will open 275 to 300 locations in urban and rural areas by 2016, expanding food options in more than 700 food deserts. The new stores will be under the supercenter and Walmart Market formats.

Walmart said it has opened more than 200 stores serving food deserts since 2007, and the roughly 500 or so stores will serve about 1.3 million people living in those areas.

Walgreen Co., a nationwide drug store chain, will convert or open 1,000 stores in areas that need healthier choices over the next five years, said company spokesman Jim Cohn. The new stores will sell whole fruits and vegetables along with pre-cut fruit salads and green salads.