Whenever American fast food (McDonald's, KFC) has established itself, the health in that country has gotten worse. This is not just by chance. I tell all my Chinese friends to stay away from such places as much as possible. Sometimes they are hard to avoid, but they shouldn't be a large or even significant part of a person's diet.
As for microwave, I think they have changed to composition of the food so that it can cook in a microwave better. What they seem to sacrifice is taste - most of it tastes like wood pulp.