From Slate: Food Deserts in America:

2009 study by the Department of Agriculture found that 2.3 million households do not have access to a car and live more than a mile from a supermarket. Much of the public health debate over rising obesity rates has turned to these “food deserts,” where convenience store fare is more accessible—and more expensive—than healthier options farther away. This map colors each county in America by the percentage of households in food deserts, according to the USDA’s definition. Data is not available for Alaska and Hawaii.

And here’s the map for obesity: