What seems like 150 years ago (really only about 10) I lived in downtown Chicago. Right next to the Sear's Tower and the CME. It was the financial district. The building I lived in had a small grocery store on the ground level, but it was mostly just packaged food. They had produce but was pretty horrible. I was a vegetarian at the time, so quality produce was kind of a big deal. To make matters worse the store's true clientele were the folks working in the financial district, so the store was only open till 8 pm during the week, for 6 hours on Saturday, and closed Sunday. If you don't have any food, or they didn't have something you're looking for: the nearest grocery store was 4 El stops and a transfer away (aka a $12 cab ride).
This effectively created what is known as a Food Desert.
I was plenty healthy, and did walk, bike, or take the El to get food, but the round trip was over 2 hours. Taking a cab really weren't an option as the expense quickly became prohibitive. So what did I do?I did what millions of folks who live in similar areas do, I went out to eat. It was horrible. My options were sit-down dinner that I couldn't afford regularly, or fast food. I had to really work to make the right choices (which not everyone is informed enough to do).
Things were far worse as you got away from the money of the financial district. On the other side of the UIC there was absolutely nothing.
This is becoming more of a common problem in large urban areas. As areas have economic problems, businesses close up shop. Some of the first to go are the small neighborhood grocers. It creates entire neighborhoods fed by the fast food industry.
The same phenomena is occurring in rural areas. My grandmother lives in the Ozark mountains of Missouri. In her town there was a 'grocer' really an convenience store that was a little too big for it's britches. It had the basics, some pretty fair produce, and a butcher that came around once a week in a big meat truck. If you wanted something they didn't have, or they were closed the IGA that was about 12 miles up the road was bigger and had more hours. If you needed something really big there was a Super Wal-mart about 50 miles away. This was pretty common: get the basics from the local guy, supplement with a garden/fishing rod/shotgun and if you needed something special you took a "trip to town."
Many of those local stores are closing up (as are some of the intermediate grocers), and that makes the communities less than viable. Young people "move to town" closer to where they can get food, and the old folks who are trying to stick it out either have to make the drive themselves, or someone has to deliver their food.
In some of these areas fast food are some of the only businesses that are financially viable, but how often can people eat like that? Combine this with similar phenomena in poor urban areas is it really any wonder that the obesity rates skyrocket as people's income drops? I don't really have a solution, but I think it's going to get a whole lot worse before it gets any better.