I don't know about America, but here it depends a lot on money (yes, organic is so damn expensive) and social background.
It takes a while to change one's way of eating too. When I was a teen I lived on junk food, but now that I'm used to a healthier diet, I get disgusted by it very quickly (I still get a craving for mcdonald's or for candies sometimes, but it's quite rare).
It's probably quite a lot easier in Europe too, in France the only really big chain is mcdonald's. And I've heard the portions are much smaller.
But lower classes are starting to have weight problems because they go to the cheapest supermarket, drink more coke, not enough greens, etc.
In the past weight was a sign of richess and money, and it's funny comparing a Rubens to the skinny models from today...