One theory says that the food you eat can affect your set point. High-calorie, strongly flavored foods that are easily digested (which make up most of the modern American diet) raise your set point. Breaking the flavor-calorie association with flavorless calories supposedly lowers your set point.
It does seem that the set point is movable. If you've been overweight for 20 years, there may very well be nothing that you can do about it.
An interesting (but very difficult to conduct) study would be an investigation into this particular rubicon of flab. How fat can you become for how long before it is impossible to go back.
The French have more leisure time than Americans. It's just as much about what going on in your life stress-wise as it is about what you eat.
We have much to learn about life-work balance from the rest of the world. It's unfortunate that some parts of the world look at America and think that they should be emulating us! Please stop that before it's too late.
America needs Mexican siestas and French vacation time in the worst way.
It's also about the rate of absorption into your body, as well as the consistency of flavors (packaged food is very uniform, and creates strong associations)
http://en.wikipedia.org/wiki/The_Shangri-La_Diet