The experiment I'd like to see would be for some country with an obesity problem to make it so that prepared food must be priced proportionally to its calories.

I think one of the reasons for obesity is that it is really really easy to get a ridiculous amount of calories without realizing it. Most people would not eat half a large pizza for lunch every day--they would be aghast at eating that many calories. Yet a large combo from a fast food burger joint can easily have more calories than half a large pizza, and many people will eat that every day. (And I'm not talking about getting the biggest, most decked out burger offered...I'm talking middle of the line, such as a double at a place that offers singles, doubles, and triples).

I realize there is much more to healthy eating than just watching calories, but for most people in obese first-world countries the problems from obesity dwarf any other nutritional problem they are likely to have, so first things first.