This short piece in The New York Times presents an interesting situation.
Using Cuba as a case study, it states that in times of economic crisis, calorie consumption drops, leading to decreased rates of obesity and heart complications. Once the scenario improves, more food is purchased, more calories are eaten, and waistlines expand.
One factor worth pondering, I think, is that nutrition education and awareness could really shift these values.
After all, it is the people with higher expendable incomes -- at least in the United States -- who can afford the gym memberships, the personal chefs, daily meals at expensive health-centered restaurants, etc.
In this country, studies have shown that obesity rates are higher among children and adults of lower socio-economic status.
While I do not believe healthy eating can only be achieved with a lot of money, extra income permits certain luxuries that make healthy living easier.
Not to mention, when it comes to dining out, most unhealthy, highly-caloric foods in this country are available at dirt cheap prices (mainly because of agricultural subsidies that favor corn and wheat, which can be processed into things like high fructose corn syrup and fiberless white flour, rather than fruit).
A dollar can get you chicken nuggets at McDonald's, but you'll need at least one more dollar to score a side of brown rice at a standard restaurant.