It's hard to square this hypothesis with the fact that much of the world (Asia, Africa, etc.) lives on carbohydrates such as rice, and that meat and other dense proteins are too expensive for many people to afford - yet, those people tend not to be overweight.

Countries where people eat bread, pasta and rice on a daily basis have lower obesity rates than the US, so I think it's more likely a problem with lack of food culture. After 15 years I'm still surprised by British adults talking about eating their "5 a day" like an Italian 6 year old would. My experience with American tourists, tells me that the situation there is even worse.