This is an interesting question and several of the answers I agree with.
Here in rural Brazil, the families with little money walk, work and eat basic foods. They are extremely fit and healthy and they have beautiful white teeth.
The other side of this is, when families get a bit of money, they buy a TV, a motorcycle and coke. This is the decline, their families become fat, inactive, similar to the countries you are referring to.
It isn't only a problem for America, Britain is becoming obese as well.