Instapundit links to a Forbes article that discusses a study linking access to Wally World with weight loss. Areas that have access to Wal-Mart have populations that weigh less than areas without such access, in brief. They discuss many reasons for such a correlation.
I can think of two more. First, with the poor, who are the focus of the study, most poverty stricken areas do not have what the rest of us would consider to be 'real grocery stores'. Because of the increased risk of loss due to theft, vandalism or other factors, merchants in such areas charge more per item. They also have much smaller inventories. They therefore tend to stock junk foods, which have longer shelf lives than fresh meats, fresh fruits or fresh vegetables.
The second, less serious, reason is that Wal-Marts are frickin' huge. If you go there to buy a gallon of milk, a loaf of bread, some socks and some touch up paint, you're going to walk all over the planet to get those items. Whereas if there is no Wal-Mart, you're going to drive to A&P for the milk and bread (typically A & P's are fairly small), then to Belks (also small), then to Ace Hardware (ditto). That's a lot of driving, but very little walking.
Does the second reason mean that in addition to helping us lose weight, Wal-Mart is reducing greenhouse gases?